The last two articles walked you through strategies and approaches to High performance Housing and High Performance New Construction. This article dives into energy conservation in existing houses and deep energy retrofits that lead to high performance in existing houses.
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LEVELS OF ENERGY RETROFITS
There is a renovation pyramid that we can reference when looking at energy conservation measures (ECMs). The pyramid bottom is low cost or no cost energy savings measures, and typically relates to the ‘non-permanent’ aspects of the house:
1. Smart thermostats, motion detectors, power bars and kill switches that deal with lifestyle choices made by the occupants
2. Compact fluorescents, LED lights, and conserver fixtures
3. Improved performance appliances (dishwasher, laundry equipment, refrigerator, freezer, dehumidifier)
Often, these are the ECMs that homeowners or occupants can carry out themselves, and are part of a rebate or incentive program.
Appliances, lighting, and lifestyle choices. You, the renovation contractor, have zero control over the energy usage of these areas. Where you can have the most impact is on the permanent aspects of the house. First, with the building envelope materials, with 25-50 year lifespans, then the mechanicals, then the renewables. When looking at energy retrofits, it serves the homeowner best if you approach the energy conservation measures in phased steps that don’t short-circuit the long-term goals.
Cost and complexity of ECMS start to increase with the building envelope, and continue as we go up the pyramid:
4. Draftproofing/Airsealing in the attic, at the foundation, replacing or improving caulking and weatherstripping at doors and windows
5. Increasing insulation levels in the attic, walls, and foundation
The deeper the reduction in air leakage, the more important it becomes to verify that the house has adequate mechanical ventilation to maintain a healthy indoor environment and avoid high indoor humidity levels.
Once ECMs have been taken for the envelope, then mechanical systems can be improved.
6. Space conditioning (heating and cooling) equipment and delivery systems need to be sized and designed to match the heat loss/heat gain required by the house
7. Water heating is a challenge, because it cannot be limited to a specific amount. Hot water usage can’t be controlled by renovation, but the renovation can improve the performance of the system and reduce system losses
Sometimes windows and siding need to be replaced.
8. When windows are replaced, the units purchased should be the best level of performance that the budget allows. Window inserts and retrofits, as well as interior storm windows, can be a way to improve occupant comfort cost effectively
9. When siding is replaced, a layer of exterior insulation should be added before the new siding is installed. The thickness depends on the climate zone, but in most of Canada, a 2” layer, taped and sealed, will keep the sheathing warm and stop moisture migration in the wall.
The most cost effective way to improve the house from the exterior is to replace windows and siding at the same time.
At the top of the renovation pyramid is:
10. Adding on-site renewable energy to bring the house to Net Zero or near-Net Zero Energy.
Working from the lower part of the pyramid up, typical ECMs aim to reduce the energy consumption for space conditioning and water heating by 20 to 30 percent. This reduction level can be achieved by improving the envelope only, but in many cases, it can be achieved by installing high performance space conditioning equipment. While the equipment works more efficiently and reduces the energy costs for the homeowner, it does not permanently improve the energy efficiency of the house, and will have to be replaced within a 15 to 20 year timeframe, depending on the equipment. To counteract this, many rebate and incentive programs have shifted to focussing their incentives on envelope improvements, providing permanent energy conservation for the house, no matter what space conditioning equipment is in place.
A deep energy retrofit (DER) focusses on the envelope and targets energy reduction measures that drop the space conditioning and water heating loads by 50 to 90 percent. The aim is to optimize the building envelope and minimize the mechanical systems. If the target is a net zero energy retrofit, reducing overall energy loads allows for smaller renewable energy systems as well. This approach optimizes energy security and resiliency for the owner and/or occupants.
A DER can be made more sustainable by also taking into account barrier-free layouts and user friendly details, or aging in place strategies.
WHAT’S THE BIG DEAL ABOUT DERS?
It’s easy to think of building a new house to a net zero energy target – we have the technical capacity to do it, and some builders have mastered the learning curve and are actually doing it. But existing homes are a challenge. So why focus on deep energy retrofits?
In Canada, housing represents about 15% of all energy use and 15% of greenhouse gas emissions.
There are ± 110,000 new homes built per year in Canada.
There are 12.6 million existing houses.
More than 80% of housing stock that will be standing in 2050 is already built. A good portion of it is already over 20 years old.
There is a clear economic benefit to improving housing stock. Investing in energy efficiency puts money into the pockets of local business, back into the pockets of homeowners, and has the effect of multiplying economic benefits. Every dollar invested in energy efficient renovations generates between $5 and $7 dollars of other economic activity, so energy efficient renovations lead to local economic growth. The jobs created cannot be outsourced, and there is a constant supply of houses that will age into requiring renovations and replacing equipment. Skilled, qualified contractors and trades are needed to keep up with the demand for improvements. Energy efficient renovations, especially those that happen over time, keep money in our communities.
We are at the point where there are fewer skilled contractors and tradespeople out there, we’re losing highly skilled people to retirement, and not enough new folks are coming into the industry. The building codes are increasing the need for understanding of building science and advances in energy efficiency targets. You need to be on top of your game so the industry can deliver what the homeowner and homebuyer want, and what the code requires.
The first step in a deep energy retrofit that focusses as much as possible on reducing energy consumption by improving the building envelope. As the energy consumption is reduced, then the mechanical systems can be improved or replaced with properly-sized equipment. The ideal HVAC system includes right-sized equipment that feeds properly designed delivery systems to maintain the comfort and health of the occupants. The mechanical system is to be of the highest efficiency possible so that the amount of energy required for the house to get to Net Zero Energy (NZE), or NZE-ready, is minimized. Ideally, the amount of energy required to provide comfort, hot water, and occupant driven loads like lighting and appliances, is roughly the same as the potential output of a rooftop PV system. ‘Net Zero’ means the house produces as much energy as it consumes over the course of a year.
HOUSE AS A SYSTEM: RENOVATION
Change in one area changes how whole house works. All improvements to any house must be based on the house as a system approach. All of the different components of a house interact and react with each other in a variety of ways. When considering a house as a candidate for an energy efficient upgrade, you need to know how to recognize, analyse and anticipate the impact of a single change on all of these components. The deeper the retrofit, the more you will affect how the house performs as a system made up of many subsystems.
In an energy retrofit, especially a deep energy retrofit, the envelope improvements change the ways that heat, air, and moisture flows interact with each other. This can lead to higher levels of humidity, resulting in condensation and possible mold growth, more noticeable odours and higher levels of pollutants, including radon.
There is also a higher possibility of the house being depressurized when exhaust appliances are operating. In houses with combustion appliances, this can lead to combustion spillage and backdrafting.
The same measures need to be taken with renovations as with new houses: eliminate the sources of pollutants, ventilate the house and filter the air.
SEQUENCING & SCOPE
As the house is improved, with a focus on improving the envelope, the end use patterns change, from an emphasis on meeting space heating requirements to minimizing appliance loads.
Tightening up the envelope affects the indoor air quality, in good ways and in bad ways. Mechanical ventilation is the solution to maintaining a healthy indoor environment. In new houses, this looks like whole house ventilation, but in existing houses, sometimes that’s not a possibility, so there’s a work around using spot ventilation.
As the envelope improves, internal gains (such as standing losses from DHW, and heat generated by appliances and lighting) can all be considered as part of the space heating regime, especially in all-electric houses.
As DHW load becomes more significant compared to space heating, and the space heating load drops, integrated space and water heating systems become a more viable option than two separate systems.
Not all houses are good candidates for deep energy retrofits, and there are very few homeowners who could undertake a deep energy retrofit in one go, financially. So understanding how to sequence a retrofit project that includes ECMs is important. You don’t want to short-circuit future improvements by locking the homeowner into a limited path towards energy security.
Ultimately the goal of any energy retrofit, whether it has 20 percent or 90 percent reduction target, is to improve occupant comfort and health. So understanding building science and how the ECMs impact the dynamics of heat, air, and moisture flows is key to a successful project.
An ideal candidate house for a DER would check off all of these points:
√ Simple house shape
√ Up for renewal of exterior finishes
√ Simple roof shape
√ Southerly roof face
√ Pitch that matches latitude
√ No significant upgrades
√ Minimal deferred maintenance/hazards
Besides the checklist, there needs to be an assessment of:
The performance of the house
The Indoor Air Quality (IAQ)
The level of mold mitigation
Radon testing/mitigation strategy
Presence of hazards like asbestos, or knob and tube wiring
In addition to all of the physical factors that must be considered before starting an energy retrofit, there’s the juggling of costs and savings. This depends on the homeowner’s cash flow and financing options. The best timing for a DER is often later in the fall, as the homeowner will see the energy reductions and lower utility bills around the same time that their financing charges start. If the project is completed in the spring or early summer, the homeowner will see little in the way of lower utility bills, but their repayment period will start.
Ideally, the Deep Energy Retrofit reduces utility bills to the point where the financing costs plus the utility bill is lower than the initial utility bill, or at least equal to it. The cost of deeper ECMs that focus on the envelope are higher, but the savings last for the lifetime of the house. Having a known period of time to pay off a set of upgrades means that you can map out savings for the homeowner over a long period of time.
DERs often work best as a series of phased projects that are laid out for the homeowner in a roadmap. A roadmap is how you get from the current state of something, like a house, to the desired end state (50% less space conditioning energy, for example). So we start with the house as is, and we do a gap analysis (the energy assessment and house walk-through, and homeowner interview). The gap analysis shows what has to be changed, improved or added to bring the house to the end state.
The next step is the strategy for closing the gap, recommendations on improving the envelope and the mechanicals.
Once the strategy is in place, then there are sequences of action, in most DERs this will be phased improvements, that bring you to the desired end state.
Regardless of how many phases there are in the roadmap, the first phase must include deferred maintenance and repairs that are done prior to any air sealing or insulation work. This is crucial. All water and moisture issues, structural and other known problems must be dealt with before the house is sealed up, otherwise, issues that were static or manageable before the initial envelope upgrades will become massive problems, quickly.
Some good reference documents for DERs in cold climates:
This Deep Energy Retrofit Manual from Building Science Corporation and MassSaves
This video of a presentation by Greg Pedrick about a NYSERDA program he lead in 2010.
Deferred maintenance the term used to describe postponed maintenance or repairs on a building. It could be done in order to save costs, or because the homeowner is saving up to do the work, or waiting to refinance a mortgage or a loan. Continued deferred maintenance may result in higher costs, asset failure, and in some cases, health and safety implications.
It can impact whether a renovation can be financed, and at what rate. A real estate appraiser considers several factors when determining the market value of a house, including deferred maintenance, which can cause the appraiser to lower the value of a house.
Lenders look at three primary classes of deferred maintenance ‘items’ that are reported by appraisers:
Safety Items: anything that could jeopardize the health and safety of property users. Must be dealt with immediately, or financing of the property could be stopped until they are dealt with.
Priority 1 Items: immediate repairs that affect the integrity of the property, and therefore affect the value of the property and the lender’s security. Should be completed within the next 12 months.
Priority 2 Items: need to be replaced or refurbished in the next two or more years. Most lenders look at a period of 10 years out from the set-up of the mortgage and will monitor the state and condition of Priority 2 items over the life of the loan. These items are most likely to require an undertaking to complete at some point during the term of the mortgage.
Undertakings and holdbacks are the lender’s way to enforce completion. When refinancing existing mortgages, an owner should be aware of any of these types of deferred maintenance items and address them accordingly.
Lenders can require that a borrower obtain an engineering report to address the severity of the deferred maintenance. Holdbacks on construction loans or mortgages can be significant. If the cost to repair an item is 10 percent or greater than the loan amount, a holdback will force the completion of the item. This means the owner pays the costs out of pocket but would be reimbursed from the holdback once repairs are complete. In extreme cases, lenders can decline loans if there are extensive safety and priority one items.
While deferred maintenance items can become a problem, if addressed properly, including ECMs that improve and enhance the durability and value of a house can result in a much better house.
WHOLE HOUSE RETROFIT
Current business models for whole house energy retrofits are lacking. The General Contractor + Individual Trades model creates silos where there needs to be a more collaborative or team-based approach to ensure that whole-house performance is always at top of focus when bids and work plans are being developed. Building America has identified two distinct business model, both of which require whole-house performance metrics for contractors and homeowners to minimize risk and optimize homeowner health and comfort. The following two descriptions are taken from the US Department of Energy Technologies Office report: The Next Step Toward Widespread Deep Energy Retrofits.
THE ALLIANCE BUSINESS MODEL
A group of contractors with building science training agree to work as a team to assess, implement, and manage the project. The contractors maintain independent businesses and interface with the client individually; however, one contractor acts as the project manager and takes responsibility for verifying whole-house performance metrics. This is an important distinction from the concept of a homeowner acting as general contractor. There is lower bias potential because an independent auditor formulates the recommendations, but has the benefit of real quotes for cost-effectiveness projections.
THE EXPANSION BUSINESS MODEL
An energy-related trade contractor continues the main line of business, but expands in-house knowledge and capabilities to assess and implement common deep retrofit elements. The scope of activity may warrant hiring a trade contractor with more expertise in a specific area, but the client interfaces with a single contractor who manages the entire project. All work is driven by performance specifications that the in-house project manager develops. A final audit is used to verify the achievement of whole-house performance metrics. The deep retrofit business line provides “off season” activity for the main line of business, and the main line of business ensures the lag time between audit and deep retrofit sale is not idle.
TRADES AND SUBTRADES
Renovations bring together a diverse group of experts across professions – coordinating this diverse group is the key to a successful renovation.
A renovation contractor is someone who provides materials, labor, equipment, and other necessary services on a renovation project. The general renovation contractor may also hire sub-contractors to do specialized work when needed. Contractors work with reliable trades (also known as subcontractors) with the aim of having consistent quality and a good working relationship. Each trade knows what they are responsible for, and what is expected of them.
An energy retrofit requires some understanding on the part of the trades and subtrades about the ‘why’ of the project. If building science is not understood or recognized by the people carrying out some parts of the project, the potential for the project to meet energy targets can easily be compromised.
Some aspects of an energy retrofit require different sequencing of trades as well.
General contractors (GCs) or renovators typically oversee large or extensive projects, especially ones that include repairs and maintenance. In this case, they typically oversee everything from design, sub-contracting tradespeople, managing permits and inspections, as well as obtaining building products and materials. In an energy retrofit, they also need to oversee or accommodate an energy evaluation or audit, and ensure that work being done is in line with energy targets.
National retail chains offer contract installation services for roofing, windows, and flooring, as well as kitchens and other cabinetry. With this service, the homeowner deals with the retailer for all aspects of the work and payment – the retailer provides the qualified tradespeople and warrants their work. Energy reduction targets and performance testing are not typically within the scope of installation services.
Trade contractors specializing in electrical, mechanical, heating, and plumbing are hired as subcontractors to the general contractor or renovator if the GC doesn’t have them on staff. Typically, contractors in these areas must be licensed to operate in a province, territory, or state. Many utility companies offer repair services for heating and cooling systems, which can make the process simpler.
Scheduling is the most complex aspect of any renovation. It’s like conducting an orchestra, each trade has to come in at the right time, and be coordinated with other trades, product availability, permits and inspections.
A short list of the possible trades and specialists involved in a whole house energy evaluation and retrofit:
Air Seal/Insulation Installer
Gas or Oil Fitter
Electrician - PV
HEALTH & SAFETY ISSUES
Many products containing asbestos have been used in construction, mainly because asbestos has qualities that make products strong, long-lasting and fire-resistant.
Prior to 1990, asbestos was used for insulation, noise abatement, and fireproofing in houses. Products that were used in assemblies or materials found in houses include:
Insulation for hot water pipes
Cement and plaster
Floor and ceiling tiles
Tar paper for roofing
Spackle, putty and caulking
Paints and sealants
Asbestos, when it is disturbed, is a known health risk, causing:
Asbestosis: a scarring of the lungs, which makes it difficult to breathe
Mesothelioma: a rare cancer of the lining of the chest or abdominal cavity
Lung cancer: smoking can greatly increase this risk
There are no significant health risks when asbestos-containing materials and products are in good condition or sealed in wall cavities, in unused attic spaces, or otherwise left undisturbed. When people commonly get exposed to asbestos, is when a home or building is being renovated or demolished. This is when small asbestos fibres can be released into the air.
Reduce risk of exposure to yourself and clients by hiring a professional to test for asbestos before starting a renovation, demolition or addition. Vermiculite-based insulation, often found in attics, may contain asbestos. To avoid exposure to asbestos fibres, do not disturb vermiculite-based attic insulation in any way or attempt to remove it yourself.
If asbestos is found, hire a qualified asbestos removal specialist to get rid of it before beginning work. Check with your provincial and territorial workplace safety authorities to find out the qualifications or certifications needed in your area.
If vermiculite insulation is present in the attic, make sure all cracks and holes in the ceiling of the rooms below the insulation are sealed. Caulk around window and door frames, along baseboards and electrical outlets as well as light fixtures and the attic hatch to prevent insulation from falling through.
Health Canada’s webpage on Asbestos in houses.
Canadian Cancer Society’s webpage on Asbestos
Lead is a significant health hazard that can be present in older homes. Lead poisoning can cause anaemia (a deficiency of red blood cells) as well as brain and nervous system damage. Children are at greatest risk, as their growing bodies absorb lead easily. Unborn children are also at risk if the mother-to-be is exposed to, or consumes, lead. Currently there is no known safe level of lead exposure.
The biggest risk in the house is through lead-based paint. If the paint is in good shape, the lead paint is usually not a problem.
It is when the paint is being removed, repaired or otherwise disturbed that exposure happens. However, normal wear-and-tear on surfaces that children can chew (such as paint chipping off doors, windows, stairs and railings) can expose occupants to lead. Lead in household dust results from indoor sources such as deteriorating lead-based paint but it can also be tracked into the home from contaminated soil. Exterior sources include deteriorated exterior lead-based paint, industrial pollution, and past use of leaded gasoline.
Another source of lead exposure in houses is plumbing. Lead pipes and lead solder were commonly used until 1986. Lead can leach, or enter the water, as water flows through the plumbing.
Houses built before 1960 likely have lead-based paint on the interior. Houses built between 1960 and 1990 could have lead in the exterior paint, and some small amounts of lead in interior paint. Any house built after 1990 will likely not contain lead, as by this time, all consumer paints produced in Canada and the US were lead-free.
To find out if lead is in the paint in a renovation project, paint chip samples can be sent to a lab, or a lead abatement contractor can be hired to use specialized equipment to detect lead in painted surfaces.
Health Canada’s page on Lead Based Paint in Houses
Health Canada’s webpage on Lead Exposure
Knob & Tube Wiring
Old knob and tube (K&T) wiring was in common use in North America from about 1880 to the early 1940s. K&T wiring doesn't have a ground wire (meaning you can’t use any appliances that have 3-pronged plugs). Because of this, K&T doesn’t meet the current electrical code. While there's nothing in the Canadian code that requires K&T be removed from existing homes, it is an obsolete system, and can't be used in any new construction. Many insurance companies will declare the homeowner's policy null and void until K&T is replaced, while others require a licensed electrician to inspect it and sign off.
If you find K&T wiring, you MUST report it to the homeowner and wait on work until a licenced electrician signs off on it. Reporting the presence of K&T is your professional due diligence, ensuring that you, the contractor, are not to blame if the homeowner takes no action.
You can recognize K&T by these characteristics:
There are only two wires, a black one and a white one.
The wires run separately, as opposed to modern wiring, where three wires are enclosed in a single cable insulated with plastic.
K&T wires are are typically insulated with a rubberized cloth fabric.
The wires run through porcelain cylinders or tubes inserted in holes in floor joists.
Porcelain knobs keep the wires secure and from touching wood.
K&T is not inherently dangerous. However, knob and tube wiring requires open space to keep cool. This means it should never run through insulation, especially blown-in insulation. Any insulation surrounding the wiring can cause serious problems. That’s also why it becomes a great concern when improving the energy performance of the building envelope.
Some of the risks associated with K&T:
High demand on old circuits designed for a few electrical appliances vs. modern electronic and appliance loads
Knob and tube wiring is easily accessed in the basement, leading to unsafe alterations over the years by people other than licensed electricians.
Damage: Serious problems can occur when this type of wiring is damaged.
Wear: The rubberized cloth insulation on k&t wiring becomes brittle over time, and can flake off.
Insulating walls, floor or attic spaces where K&T wiring is present creates a fire risk, as the insulation does not allow heat to dissipate.
Here’s a video from a Winnipeg electrician that discusses K&T.
Envelope improvements vary widely depending on the age and condition of the house you are renovating. For example, if the house is ready to have the cladding and windows replaced, the cost of improving the building envelope with a layer of exterior insulation is only a premium on the deferred maintenance costs.
First, you need to know what you’re working with. When working with existing houses, the construction types and methods you uncover will vary. Residential construction primarily consists of light-frame structures that use standard dimensional lumber (2x4s or 2x6s). Light-frame is an economical building method, using minimal structural material to enclose a large area cost-effectively. It also allows for a wide variety of architectural styles. Current light-frame construction practices use sheet materials (plywood or other manufactured wood panel) for rigidity and strength. Prior to sheet materials being commonplace, diagonal bracing provided rigidity and strength to the wall system. Special framing techniques are used to create shear walls to meet earthquake and wind loading.
Common residential construction methods include:
Platform Frame: Most current light-frame residential construction is platform framed, that is the foundation is installed, then the main floor platform is built and the main floor walls are built on that platform, then the next floor platform is built, and so on. Platform framing is typically limited to four floors, although some building codes call for six floors with additional fire protection. In Canada, houses and small buildings that come under Part 9 of the building code are limited to three storeys.
Balloon Frame: Another form of light-frame residential construction, common from the 19th century until the mid-1950s. Balloon framing uses 2x4s for the exterior walls that extend uninterrupted from the sill on top of the foundation to the roof. Like platform framing, it makes use of many lightweight wall studs rather than fewer, heavier posts used in older construction styles like timber frame or post and beam. The heights of window sills, headers, floor heights are marked on the studs with a story pole, and intermediate floor structures are let into and nailed to the studs, or carried on a ledger sill. Blocking, short pieces (blocks) of dimensional lumber, acts to stabilize the wall system as well as reduce the ‘chimney’ effect in wall cavities in case of fire.
Timber Frame: Timber framing and post-and-beam construction are traditional methods of building with heavy timbers, used in most wooden buildings constructed in the 19th century and earlier. Posts, beams, and rails form the structural frame that transmits all vertical and horizontal loads to the foundations. Heavy frame or timber frame comes from working directly from logs and trees rather than pre-cut dimensional lumber. The timber was shaped by hand, and required a significant amount of skill and expertise. The main difference between timber frame and post and beam construction is the method of joining the parts. Traditionally, timber frames were made with solid wood with mortise-and-tenon connections secured with wood pegs. Post and beam uses half lap joinery with hidden fasteners, and decorative metal braces some of the time. Builders are now using engineered wood such as glulams or glued laminated timber (a type of structural engineered wood) and metal connectors.
Solid Masonry: Solid masonry construction is also called ‘Solid Brick’, or ‘Double Wythe’ (a layer of brick is called a ‘wythe’). In its most common form, a solid masonry wall consists of an outer wythe and an inner wythe. While the outer wythe is brick, the inner wythe, which will never be seen, can be brick, concrete or cinder block. Header bricks hold the wythes together. From the outside, header bricks look like short bricks. In reality, they are a normal brick installed sideways to act as a bridge between the outer wythe and the inner wythe. Most solid masonry walls have header bricks in every 6th row or course. Sometimes, every brick in the course is a header brick. There are a number of common patterns. When bricks are in the wall longways, they are called stretchers. When bricks are installed vertically, for instance, above windows, they are called soldiers. When metal ties are used to hold the two wythes together, no head bricks are visible, and the wall can be mistaken for a brick veneer wall. A brick veneer house is a wood frame house where the cavity between the studs in the wall can be insulated. In a brick veneer house, metal ties are nailed onto the wood frame wall, bent so that they are horizontal and embedded in the mortar joints as the brick veneer wall is constructed. The brick veneer is built on the outer edge of the foundation wall so that an air space remains between the brick veneer and the sheathing.
A quick recap of building science:
Heat, air, and moisture flows always work together in a building, but which type of flow is driving the others depends on many variables.
Temperature differences can drive air flow in a building, but at the same time, that air flow rate will go up in relation to the speed of the wind outside.
Moisture usually goes along for the ride, as water vapour in air.
While heat and air flows lead to unwanted energy loss, moisture flows usually lead to durability problems in buildings, especially when the water condenses out of the air and finds a home in the structure of the building.
Building science helps us to understand how the performance of a building will be affected by heat, air and moisture.
This in turn helps us understand how to maintain good standards within the building for occupant health, structural durability and energy performance.
While energy performance is often the main selling point of an insulation installation, the primary goal of retrofit project is actually occupant comfort and health.
Well insulated and air sealed, a durable and energy efficient building envelope creates the base conditions needed for a healthy indoor environment when combined with controllable mechanical ventilation.
Foundations are often the most challenging part of an energy retrofit.
An uninsulated foundation can account for anywhere from 10% to 40% of the total heat loss of a house.
While the temperature below ground does not fluctuate as much as seasonal air temperatures above grade, there is constant and consistent lower temperature on the exterior, leading to heat loss.
Most heat loss happens at the top of the wall that is above grade or close to grade, but heat loss occurs throughout the basement enclosure.
The rate of heat loss at the wall slows down as the foundation wall gets deeper below grade. This is why buildings codes used to allow partial insulation on walls. As the science behind energy efficiency has improved, and the issue of moisture transfer has become better understood, full-height foundation wall insulation has become a part of many building codes in cold climates.
The foundation of a house can be a major source of moisture problems, air leakage and energy waste. In older houses with poor air sealing, the stack effect can bring moisture and pollutants into the living space above. This affects the indoor environment and occupant health. Moisture problems and sources need to be addressed before any energy conservation measures are taken. The potential for existing problems to become worse and harder to fix is high.
How to apply building science:
In retrofit situations, focusing on reducing air leakage at the foundation will go a long way to improving the performance of the house.
In most houses, a stack effect is created because warm air rises. This induces a negative pressure on the basement and draws moist air in through any cracks or openings in the foundation including open sump pits. For this reason, sumps should have an airtight cover. With a concrete block foundation, moist air is drawn through the block cores, especially if they are left open at the top course.
Stopping air leakage at the foundation minimizes the potential infiltration areas at the bottom of the house that drive the stack effect. If air leakage at the ceiling or roof is also addressed, the stack effect can be brought under control and thus, air flow and heat loss due to air flow are controlled.
Vapor diffusion is the movement of moisture in the vapor state through a material. It's dependent on the permeability of the material and the driving force of vapor pressure differential. In a basement, vapor can diffuse from the wetter ground through concrete walls and floors toward the dryer basement interior. Vapor retarders such as foundation waterproofing and polyethylene slow down this process.
Basements are increasingly used as living space. Where old houses used to have a foundation or a cellar, new homes are expected to have a high quality living space. Retrofits are also expected to have high quality living space in the basement, but in many older houses these were never meant to be living spaces.
There are three typical foundations that can benefit greatly from energy conservation measures:
· Cellars typically have a dirt floor, rubble or stone walls, lots of moisture, and horrible air quality. Access is often from outside, so there is no intentional opening in living space.
· Basements in older houses are a step up from a cellar, and are attached to the living space via a stairwell of some sort. The construction ranges from rubble and stone with a dirt floor to concrete block to poured concrete and insulating concrete forms (ICF) with a floor slab. Some regions may also see pressure treated wood (PTW) foundations.
· Crawlspaces range from damp cellar-like conditions to dry and pristine encapsulated spaces. A crawlspace is essentially a short basement. The construction range is the same as for basements.
If the cellar, crawlspace, or basement has a dirt floor and no moisture barrier, adding one is the first order of business. The minimum barrier should be 0.10 mm (4 mil) polyethylene. The sheets need to be overlapped, caulked and taped at the seams. The moisture barrier needs to extend about 12 inches up, and then be mechanically fastened to the foundation walls, as well as any plumbing, structural posts, or other vertical obstructions. If you can find white or coloured polyethylene, it will show any leakage areas and signs of vermin activity better than the clear product.
If the foundation space is going to be used for a living space, or will experience regular traffic (for example, laundry, storage, and mechanical system access), the floor will need protection. The best solution from an energy efficient perspective is at least 2” of foam insulation and a 4” minimum concrete slab over the polyethylene vapour barrier. Another possible solution (depending on code requirements in your region) is a layer of insulation covered with ¾” concrete board.
When a house was built with a cellar as the foundation, it was constructed to keep the living space above the grade, to provide stability for the above grade structure, and was probably used as storage for wood or coal for heating. Without any upgrades to the thermal envelope, it likely functioned fine in that role.
Energy conservation measures and upgrades are made to the living space (air sealing and insulation) lead to changes in how the house performs. More of the moisture generated by appliances, people, and exposed soil/damp foundation walls is now kept inside the house. The empty walls or high air leakage rate allowed the house to dry out over the heating season. An unintended consequence of above grade energy conservation measures could be moisture problems or indoor air quality problems that are directly related to the foundation. The house should also have upgrades to the cellar that control the amount of moisture generated (moisture barrier on a dirt floor, for example). Air sealing work at the rim joist and floor header areas is another key upgrade.
A basement can be as roughly built as a cellar, or an encapsulated space completely below grade. It can also be ‘raised’, as in half-in and half-out of the ground, or it can be a ‘walkout’ or ‘daylight’ basement on a sloping site.
A livable basement includes the expected components: walls, a floor slab, and stairs to the floor above. It is designed and constructed so that the exterior completely separates the interior from water, soil, and air intrusion. This ensures the basement living space is free of mold, moisture problems and radon.
In addition, the envelope components must support heavy lateral loads and control heat loss/gain (they must be insulated). Finally, a livable basement is easily finished on the interior with readily available materials and products, like gypsum board and paint.
There are two ways to deal with a crawlspace: make it part of the warm side of the house, or make it a partially heated area.
To bring the crawlspace completely into the conditioned space: Close off any venting, insulate the walls, and seal off the floor if there is no slab. Then condition the space. This can be done easily with a forced air system by installing a heating supply duct and a cold air return to balance the air exchange. This requires (prior to commencing the energy conservation work) confirmation from an HVAC technician to ensure that the heating system has the capacity to service the crawlspace as well as the rest of the house. In a dry crawlspace, a minimum 2” closed cell spray foam will act as both vapour barrier and insulation on the interior. A damp space will benefit from having insulation on the exterior to keep the moisture from wicking through the concrete into the space. Insulating the walls and bringing the crawlspace completely inside the envelope protects all plumbing and heating distribution systems.
Partially heated crawlspace: Insulate the underside of the floor. To avoid problems with rot at the joist ends, this approach works best combined with a dry crawlspace, a sealed floor, and exterior foundation wall insulation. Ductwork and plumbing that runs through the crawlspace needs to be insulated as well to avoid freezing pipes.
Below grade walls
Basement walls can be insulated from the interior or the exterior. There are pros and cons to both approaches, and some flags for different types of foundation construction.
Some authorities have expressed concern about the possibility of frost action and structural damage when foundations are insulated from the inside. The concern is that frost will penetrate deeper down the outside of the foundation wall. Research has found that this is not a problem. Under some circumstances, such as in soils that are particularly frost-susceptible in extreme climates, there could be a problem caused by some construction techniques.
There are potential problems however with insulating rubble or stone foundations from the exterior. The way that the wall was constructed can result in the structural stability of the wall relying on the weight of the soil. This approach needs a geotechnical expert and possibly a structural engineer to ensure the foundation is not made unstable by excavation.
• Incorporate into renovation plans
• No seasonal constraints
• Insulate full wall
• Higher R-values
• Landscaping/driveway not disturbed
• No excavation costs
• Moisture problems will get worse if not fixed
• Foundation walls colder – can lead to condensation w/o air barrier
• Obstructions make work more difficult
• Continuous wall/no obstructions easier to insulate
• Correct moisture or structural problems
• Install drainage system
• No household disruption
• Freeze-thaw stress eliminated
• Excavation disruptive
• Soil conditions can limit
• Where to store dirt?
• Non-removable steps, carports etc.
• Rubble foundations could rely on soil
Energy conservation upgrades should also include radon testing before and after the upgrades, followed up by mitigation if needed. Radon mitigation in existing houses can be accomplished 3 ways:
Extended Rough-in system: Is a completely plumbed rough-in where the piping extends fully through the envelope of the building and penetrates to the outside. It does not have a fan to actively move the air, however it is ready for a fan to be added once a radon measurement shows that activation is necessary.
Passive System: Is a completely plumbed rough-in where the piping extends fully through the envelope of the building, through the attic and penetrates to the outside, ideally above the highest roof line. The pipe extends vertically with all offsets made with 22 ½ degree fittings; and no horizontal runs. This system relies on air movement due to convection air movement (due to the thermal stack effect) from below the slab; It does not have a fan to actively move the air; is a non-powered radon mitigation measure, however it is ready for a fan to be added once a radon measurement shows that activation is necessary.
Active system: Similar to passive system in the fact that the system is already piped through the interior of the building, except an active system relies on a fan to create negative pressure from below the slab relative to the dwelling. This reduces the concentration of gas under the slab and exhausts gas into the atmosphere. An active system can also be called an active depressurization system, sub slab depressurization, or sub-membrane depressurization.
ABOVE GRADE WALLS
Determine the presence of insulation (and the extent and quality of the installation) using a thermal camera on a cool day so that the temperature difference between the inside and outside is pronounced.
Inspect the attic space for wet areas at the eaves (lift back any existing insulation to check the tops of the walls. As with foundation upgrades, deal with all moisture issues first, then look at air sealing and increasing the insulation levels.
In wood framed houses, empty wall and exposed floor cavities can be filled without too much inconvenience and cost with dense pack blown-in cellulose. Dense pack also helps to reduce air leakage. Houses built using masonry (double-wythe brick walls, concrete block) are more challenging to insulate. Often this type of construction is quite airtight, and while the mass of the wall does retain and radiate some heat back into the living space, there is still significant heat transfer. Don’t be tempted to insulate the airspace between the two layers of brick or between the concrete block and the brick – this is where the wall drains any water.
If the retrofit includes stripping out walls from the inside, there are several options for improving the insulation levels. In a framed house, the wall can be strapped out so that it makes a thicker wall (the interior will be a few inches smaller – carefully measure what impact this has around hallways and doorways!) and then insulate. You could:
· Fill the expanded cavity with fibrous insulation and install a continuous air and vapour barrier to the inside face of the walls
· Install a couple of inches of Type IV (extruded polystyrene) rigid board insulation, and use an ‘airtight drywall’ approach to the air and vapour barrier requirement
· Install at least two inches of medium or high density spray foam as insulation, air and vapour barrier
At R5 to R6 per inch for foam insulation, there’s nearly double the insulation value of fibrous insulations, so less living space is sacrificed for the same amount of insulation. However, most foam insulation is more expensive than fibrous insulation. This approach would work well with an all-masonry house, as the foam insulation doesn’t absorb moisture.
Most people are not going to gut their whole house to improve energy performance. A better all-over solution is to look at the outside of the house. The most cost-effective time to increase the insulation levels and reduce thermal bridging in a house is when the cladding needs to be replaced. Adding a thick layer (2 to 3 inches) of rigid board insulation to the exterior is a good way to create a continuous air barrier on the outside of the house. It has to be sealed at all joints and edges to be a continuous air barrier. In addition, it has to be thick enough to keep the ‘dewpoint’ (where moisture will condense) out of the wall cavity. This is because the foam blocks moisture movement through the wall to the outside, potentially trapping moisture in the framing cavity and causing rot and mold growth. This varies across the country, but a rule of thumb for all regions except the Far North of Canada, is a minimum of 2” of Type IV (expanded polystyrene) or medium density foam on the exterior.
Another way of increasing insulation to the exterior of the house is to build a ‘Larsen Truss’ system. These lightweight, ladder-like trusses are attached to the existing structure and are filled with fibrous insulation (fiberglass, cellulose or mineral wool, typically). A Larsen Truss system may not be allowed if it encroaches on the side yard restrictions in your neighbourhood, as some jurisdictions consider it to be an extension of the overall structure on the property. Check with the local building inspection or development by-law administration office. There is not as much concern about where the dewpoint lands in this wall, because the moisture can move through the whole wall assembly to the outside.
When upgrading walls, regardless of the methods and materials used, the area that often gets overlooked is the header or rim joist area. If this area is left uninsulated, it can feel like there is more heat loss at the floor. The header area is difficult to insulate once the walls and ceilings are finished. In retrofits that do not include stripping out walls or ceilings, the best option is to add insulation on the exterior wall when replacing the siding. Depending on the material and method chosen, the insulation product can do double-duty as insulation and as an air barrier.
Exposed floors are best insulated from the exterior, unless there is rot or damage to the subfloor. This avoids pulling up finished floors. There are several ways to approach this. One is to combine a 2 inch layer of high-density spray foam insulation in the top of the floor cavity (the underside of the subfloor) and fill the rest of the cavity with a fibrous insulation before closing in the cavity with insulated sheathing and the exterior finish. Another method is to add a layer of rigid foam insulation to the exterior of the floor cavity. The thickness of this layer is dependent on your location, the existing insulation level and the rigid board product you choose.
In all cases, you want to make sure that the exposed floor cavity is protected from rising damp. In addition, if the exposed floor is connected to a garage (ie, it is the floor of a bonus room), making sure that the garage and the bonus room are completely separated from each other with an air barrier is key to maintaining good indoor air quality, as fumes from the garage can be dangerous, whether they are fumes from exhaust or stored chemicals.
WINDOWS & DOORS
If windows are not rotting out of the sills, put “replace windows” in the lower priority energy efficiency measure list. Windows and doors are constantly exposed to the weather and the wear and tear of everyday use. Weatherstripping, hardware, door materials, glazing units, frame materials need to be inspected regularly for damage. Repairs can be inexpensive, but may not last. Replacement is costly, but will provide cost savings in energy use, make the house more comfortable, and add to the resale value.
In most homes, there is typically significant air leakage at the rough opening – the part of the wall construction the window fits into – and the frame of the window. This can be dealt with in several ways. But what to do with the window itself? There are three options, with a few variations on a theme that revolve around what is in place.
1. Repair existing windows and storms
2. Install inserts
3. Replace with new window units
Repairing windows can be an interim solution, until inserts or replacement windows are in the homeowner’s budget, or it can be a long-term solution. Heritage house owners may have repair as their only option. When repair is the solution – interim or long-term – the homeowner has to commit to annual inspection and maintenance to ensure their energy dollars are not flying out of damaged windows. Older wood-frame windows, crafted from the rot-resistant heartwood of slow-growing trees, can last for many decades, especially if they are protected from the weather and regularly painted.
Single pane windows require a well-fitted storm window with drainage. A storm window is installed on the exterior side of a window. Usually, a storm window is screwed into place on the outside trim. Note that the original window must be tightly sealed. If not, condensation will occur between the panes, and over time will damage the paint, encourage the growth of mold and mildew, and rot window sills. Storm windows are offered with low-e glass.
Existing double-pane windows that have lost their seals end up with a foggy film on the interior face of the glass, but there’s not much that can be done to fix it. The fog shows up when the seal between the two pieces of glass deteriorates and moisture infiltrates the airspace. Some companies offer to ‘reseal’ units. What they do is drill a couple of holes in the foggy window, clean out the buildup and then seal up all the holes but one. A one-way valve is placed in the remaining hole, allowing the moisture-laden air that infiltrates at the perished seal to escape. This can be a successful interim solution in some cases, but typically, the glazing unit itself (not necessarily the window frame and hardware) needs to be replaced.
Existing windows that have solid frames can be fitted with inserts. The old operating and stationary sashes and hardware are removed, and a new window unit with a low-profile frame is inserted into the original frame. The process leaves you with a new, energy efficient window without alteration to interior or exterior finishes. This option can be a very cost-effective solution where there is no rot or other damage to the existing frame.
In most jurisdictions in Canada, local building codes require windows in new houses to meet a certain standard. Depending on the extent of the renovation, code requirements may also apply to energy conservation projects that include replacement windows. However, you can use these requirements, and the national window rating system they are based on, as a guide to what you should be looking at for a minimum replacement unit.
Visit this link to find out more: oee.nrcan.gc.ca/residential/personal/windows-doors/buying.cfm
Doors used to be made and hung specifically for the house they were being installed in, and a sign of a good carpenter is still one who can hang a door perfectly. Replacement doors now come pre-hung in a weatherstripped frame and you don't have to be a master carpenter – or fret about hiring one – to get a first-class job.
Replacing doors requires draftproofing between the door and the rough stud opening around it, proper seating of weatherstripping and will require new trim work inside and out. For energy efficiency purposes, all new door units should be insulated metal with magnetic weatherstripping. These have a better R-value and stronger seal against cold winter winds than solid wood doors. Install a deadbolt so the door can be pulled snug to the weatherstripping.
Building Science Corporation has something to say about Windows in DERs.
ATTICS & ROOFS
Ceilings and attics differ widely between house types. In houses with flat-ceilings and no liveable space in the attic, improving insulation levels is fairly straightforward. The key to a good job is to first make sure that the penetrations through the ceiling into the attic are well-sealed. Reducing air leakage at the top of the building ensures that the stack effect is minimized. Along with additional insulation at the eaves, air sealing work also minimizes or eliminates ice damming.
After the attic has been sealed off from the house completely, more insulation can be added. The amount that you end up with is dependent on where you live, with a range of R40 to R80 being most common. This translates into 14 to 20 inches of fibrous insulation. The areas around the eaves in most older homes will not be able to accommodate this much insulation, and stuffing the equivalent amount into the area will not give the R-value needed. This is one of the weakest points in the building envelope, where several pieces of wood join and intersect. In addition, heat rises, so the ceiling is typically warmer than the floor area. This can be more noticeable in a two storey house than in a one storey house.
At the eaves, use a high-density foam, either in layers of rigid board glued together, or a high-density spray foam product. Ventilation space from the soffit vent to the ridge vent needs to be allowed for, further reducing the amount of insulation that can be installed at the eaves. If the eave area is only 6 or 8 inches deep, and you need 2” deep ventilation channels (typical), you will get at least R20 over the top of the walls this way. The high-density foam (board or spray) only needs to extend to the point where the appropriate depth of fibrous insulation can be installed.
Houses with liveable attic space (like a 1-½ storey house or an older 2 storey house with a staircase into the attic) offer up a whole set of challenges. What approach will work best for your house is dependent on what already exists (finishes, insulation, roof structure), what you use the space for now and what you expect to use the space for.
MECHANICAL SYSTEM UPGRADES
The extent that you can upgrade and/or replace mechanical systems depends on the age of the equipment, the condition of the delivery system, and what you want to replace it with. For renewables, you are much more tied into the existing roof line than you are with a new build.
Before the mechanical equipment in an existing house is changed out, there are some reasonably inexpensive ways that some existing equipment can be upgraded as the building envelope is improved. This can often be one of those make-or-break costs of a renovation project
When the existing equipment is too far gone, and needs to be replaced, what will the upgrade look like?
At the very least, you need to be legal. The good news is that no furnace or boiler can be imported or sold in Canada that does not meet the current building code minimum efficiencies. To do better, look for ENERGY STAR labels on any equipment that will be replaced. This label shows that the product is in the top 30% of its category for energy efficiency. Be sure when you are looking at ENERGY STAR labels that you are comparing units that are in the same category.
Just as in a new house, a full F-280 heat loss calculation should be done. This is very important as you improve the building envelope, not only to let you know the right size for any new heating equipment, but also to have a map for the delivery system, and to know how much heat needs to go to each room. The distribution system may need some upgrading as well to match lower loads.
Then the installed delivery system needs to be tested to find out how much heat it is actually delivering.
Distribution and Controls
When renovating or replacing an existing boiler, it is important to consider the different temperature requirements of the radiators you have, or are considering installing. Newer, high efficiency boilers work better on systems that require lower temperatures.
Outdoor reset controls and output modulation capabilities work to keep the water temperature cooler when less heat is required, dramatically increasing overall efficiency and comfort.
While it’s more common for hydronic systems to be zoned, creating zones for forced air systems can save anywhere from 10 to 20 percent.
Smart controls are available to run the heating system efficiently.
SPACE HEATING & COOLING
Gas Furnaces and boilers
Replace mid-efficiency gas furnaces and boilers with highest efficiency units available for the budget.
Condensing gas furnaces have seasonal efficiencies between 90% and 96%. The combustion gases are cooled to the point where the water vapor condenses, releasing additional heat into the home. The resulting liquids (condensate) are piped to a floor drain. Because the flue gas temperature is low, plastic piping can be used for venting out the side wall of the house.
Condensing gas-fired boilers can sometimes not meet the rated performance, because the return water temperature from the distribution system is above the dew point of the flue gases. By installing a water-to-water heat exchanger and storage tank upstream of the boiler, the return water temperature can be brought below the dew point, flue gases will condense, and efficiency will improve.
While most existing furnaces serve a low-velocity ductwork, high velocity systems can be worked into renovations. Watch this video.
Oil Boilers and Furnaces
In some areas, many older homes have oil boilers and hot water baseboards, or oil-fired forced air systems. Homeowners want to get away from the risks of on-site oil storage, and will often want to jump right into a heat pump. But doing that short-circuits the benefits of an envelope upgrade. Where the boiler or furnace is in decent shape, and the tank is not on its last legs, a few more heating seasons can be coaxed out at a higher efficiency by retrofitting it with a new nozzle, for example. Some pump motors can be changed out for electronically coupled motors, which are highly efficient.
If the boiler or furnace is fairly new and high efficiency, it may be more cost-effective to see instead if the delivery system can be changed out as the envelope improves, installing smaller radiators or convectors, or swapping out large units for low temp, high efficiency units.
Significant energy savings and better overall performance for the whole system can be found in control replacements or updates. Hydronic systems should have an outdoor reset controller, making the system more responsive to the heating needs of the house.
Many existing hot water heating systems can be easily zoned, providing an added level of control over your home heating. Zone control is most effective when large areas of the home are not used often or are used on a different schedule than other parts of the home. Automatic valves on the hot-water radiator piping, controlled by thermostats in each zone of the house provide independent control of the heat in that area. Using programmable thermostats will allow you to automatically heat and cool off parts of the home to match usage patterns.
ENERGY STAR boilers start at 84% Annual Fuel Utilization Efficiency (AFUE). This means that a baseline ENERGY STAR boiler produces 84 Btus of useful heating out of every 100 Btus it consumes. The rest is wasted heat in the exhaust.
The baseline for an ENERGY STAR oil-fired furnace is 78% AFUE.
As you go up the ENERGY STAR ratings, you get to units that have power vents, and then to condensing boilers and furnaces. These units vent through a sidewall, allowing you to get rid of the chimney and eliminate combustion spillage concerns at the same time.
Condensing units eliminate the ventilation and mechanical system effect and reduce the stack effect when a furnace or boiler is replaced. The original flue must be sealed off to ensure no air leakage at the top of the flue.
Sealed combustion or direct vent also eliminate air flow due to the combustion process and can prevent backdrafting--hazardous flue gas spillage--caused by exhaust fans or conventional fireplaces in airtight homes. It can also prevent depressurization of the house caused by the furnace itself. A sealed combustion or direct vent unit uses outside air, brought in through piping directly to the burner.
In the case where original equipment shared a flue (a gas-fired furnace and a hot water tank, for example), and new direct vent or condensing equipment is installed, the technician will need to test the flue to ensure that the ‘orphaned’ appliance does not cause combustion spillage.
Renovations often see older oil-fired appliances headed out the door and being replaced with electric heating equipment because of risks associated with on-site oil leakage and site contamination, as well as higher insurance costs.
While gas is very inexpensive in many areas, homeowner interest in heat pumps is very high.
Going from oil or gas to electricity typically means moving to a heat pump. If the house is going to have a furnace replaced by a central heat pump, then the duct sizing needs to be addressed. Heat pumps push air at lower temperatures than furnaces, so they need bigger ducts.
The happy news is that there’s a really good chance that a decent amount of envelope improvements will drop the energy load enough that the existing ducts are now the right size for the smaller capacity heat pump.
Whole house ventilation is a challenge in existing houses without a forced air system to feed the HRV or ERV into. If that’s the case, and you cannot install a whole house system, or have to wait for a future phase of renovation to install it, then spot ventilation - meaning bath fans and modest range hoods - can make up the ventilation requirements.
Choose ENERGY STAR units with a low sone rating to ensure that they operate quietly and so will be used.
Control bathroom fans with humidistats with a manual override. This ensures that the fan will go on when there is a moisture-producing event like a shower or bath, and stay on until the humidity is cleared from the house.
How many cfms of spot ventilation do you need in a house? Not a lot, it turns out.
Enough ventilation to meet the building code requirement of 0.3 natural air changes per hour. That can be translated into CFM by multiplying the volume of the house by 0.005.
To give you a sense of how many cfm an average house might need: a 2 storey house with a 1500 s.f. footprint has a volume of 24,000 cubic feet. Using the formula, the required ventilation rate is 120 cfm. That’s about the equivalent of a single bathroom fan. A typical 2 storey house built in the last 20 or 30 years would likely have at least 2 bathrooms, plus a range hood.
For houses where an HRV/ERV can be installed (or already exists), spot ventilation can lead to better performance, clearing odour and moisture events quickly without over ventilating the whole house. Most HRV/ERV units have call buttons in the kitchen, bathrooms, and other ‘wet’ spaces like laundry rooms. Pushing the call button moves the HRV/ERV from low speed to high speed to clear the odour or moisture. However, this action affects the whole-house, and typically for a set period of time, for example 20 minutes. Keeping the HRV/ERV on low speed and using a quiet and effective fan in the immediate area can be a better solution for fast clearing and minimal energy use.
A case study and video from Green Energy Futures on a Net Zero Energy Retrofit by Calgary contractor, Peter Darlington