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May 7, 2019
The last article introduced the concept of high performance housing, discussing a high-level view of strategies and approaches to achieving high performance houses. This article focuses on how to bring high performance concepts to new construction. The last article in this series will focus on improving existing houses through deep energy retrofits.
Here’s the evaluation form, so you can rate yourself on your existing understanding of New High Performance Houses. The rating is from 0 (I know nothing about this topic) to 4 (I’m an expert in this topic and can handle complex tasks on the daily). You don’t have to share this with anyone else, so rate yourself honestly!
Highly Efficient Buildings
Highly efficient buildings are all about performance and understanding how the house works as a system. There are three physical components that every house is going to be affected by: heat, air, and moisture flow. These three components are driven by the laws of physics, and cannot be avoided. Ignoring how heat, air, and moisture flows behave can lead to disastrous consequences.
Tied into heat, air, and moisture flows is the challenge of maintaining a healthy indoor environment.
When designing a new house, best practices will lead to you a building envelope that controls heat, air, and moisture flow by creating strong thermal and pressure boundaries. The house shape and size will also impact the energy use.
An envelope that controls heat, air, and moisture flows determines the size of the space conditioning system required. Ventilation needs to be provided. Here’s a great graphic from BCHousing that points out six key strategies for creating a high performance house.
Watch this video of Dr. John Straube, talking about the ‘why’ behind building performance, and how energy conservation and high performance construction affects new builds and retrofits.
Canadians have always been at the forefront of efficient cold climate housing design. Before the R2000 program, there was the Saskatchewan Conservation House. This house, and the features in it, set the precedent for not only the R2000 program, but also for Passive House and Net Zero Energy energy targets. The R2000 program came into being in 1986, and has been one of the drivers behind other energy efficient housing programs, as well as building code improvements that focus on energy efficiency measures.
Natural Resources Canada is helping to keep Canadian builders at the forefront of energy efficient construction for new houses. LEEP, the Local Energy Efficiency Partnerships, is a program that brings together builders, energy advisors, material and equipment manufacturers and suppliers.
BC Housing has a series of videos that feature builders who have participated in the LEEP program:
This video discusses the important role of an energy advisor
This video discusses the importance of ventilation and indoor air quality
Building Codes in Canada are moving towards requiring that homes are Net Zero Energy. By 2032, this will be the Model energy code. BC implemented an Energy Step Code in 2017 to help builders improve the performance requirements for code compliance over a period of years.
A Net Zero Energy house, by Natural Resources Canada’s definition, is one that produces as much energy in a year as it consumes. There is a clear strategy for getting a house to Net Zero: reduce the energy consumption for space and water heating first, then optimize the mechanical systems, and then add renewable energy.
This is important to note, because we have a small window of opportunity to increase the capacity of the industry and get the learning curve under our belts before code changes require us to make a huge leap in skillsets.
Here is a Design Guide for the BC Energy Step Code.
Here is a Builder Guide for the BC Energy Step Code.
There are three main control layers for heat, air, and moisture flows.
The building envelope, the skin and bones of the house, is where we have the opportunity to reduce energy consumption for both heating and cooling. The building envelope is the permanent part of the house. Choices about insulation levels, air sealing techniques, window and door characteristics, and exterior finishes are crucial to the durability, longevity, and long-term affordability of a house.
Building performance is about controlling heat, air, and moisture flows to provide comfort, safety and health for occupants in buildings.
From a building science perspective, the building envelope needs to have three key items to perform well:
A vapour barrier, or vapour diffusion retarder, to keep moisture vapour in the house from entering the wall cavity.
An air barrier, to stop air.
A thermal blanket to reduce the flow of heat transfer from inside to outside.
These can be three separate materials, or a single material that functions as two, or even all three, control layers.
The design stage is where all the decisions are made in regard to energy conservation measures. This is the least expensive and most cost-effective part of the process. Decisions can be revisited based on energy modelling and costing before construction with the final result being a package that works for the builder and for the homeowner.
Reducing space conditioning loads is key to new high performance construction. Understanding the control layers and the various approaches to wall, ceiling, and floor assemblies is key to success. Understanding how high performance assemblies impact first costs for construction or renovation is also a key element to creating a successful project
This is the stage where an Energy Advisor comes in. Using software to model the energy use of the building allows you to see a variety of options that would lead to an energy reduction target. Some common energy reduction targets are:
20% better than code - this is equivalent to the ENERGY STAR for New Houses program
50% better than code - this is equivalent to the R-2000 program
Passive House reaches for 90% reduction in energy use
BC Energy Step Code has 5 steps from a code-compliant energy model (a ‘reference house’)
Step 1: Performance testing (energy modelling and blower door test) to ensure the building meets minimum requirements
Step 2: 10% better than code reference house
Step 3: 20% better than code reference house
Step 4: 40% better than code reference house
Step 5: Net Zero ready, up to 80% better than code reference house
On the path to high performance housing, construction quality management becomes one of the keys to success in project management and project delivery. According to this article from esub.com: Good construction quality management can reduce the number of mistakes and rework in a project. This can help projects come in on time and budget, and helps contractors maintain a good relationship and reputation.
An article in Professional Builder (by Susan Bady, Contributing Editor | November 7, 2010) offers up a handy list of 14 Best Practices for High Performance Housing:
Get the input of architects, trade partners, suppliers, and other decision-makers while the project is still in the design phase.
Whenever possible, site the home to take advantage of southern exposure on the rear, with fewer penetrations on the north side. Use roof overhangs and deep porches to help reduce solar gain.
Design according to a 2-foot module to accommodate common sheet-good sizes.
Over-designed foundations are a waste of concrete and wood. Determine the correct number and size of footings ahead of time.
Apply advanced framing techniques to reduce the amount of wood in the house. Don’t use 10 studs when two will do.
Right-size HVAC equipment. Many systems are much larger than they need to be.
Take advantage of prevailing breezes and cross-ventilation to reduce the load on mechanical cooling systems.
Choose the most energy-efficient windows your budget allows. Make sure they’re flashed and sealed properly.
Seal all ductwork and openings in the building envelope.
Maximize the insulating capacity of the thermal envelope. In other words, get the best R-value you can for your money.
Avoid ductwork in unconditioned spaces, such as attics.
Reuse and recycle building materials.
Test your homes regularly to monitor energy performance and identify opportunities for improvement.
Ensure that the home is ready for alternative technologies that the homeowner may want to add in the future. For example, pre-wire the home for a solar photovoltaic system.
On top of construction quality management and tips on what to include in a high performance construction project, success depends on the capacity of construction crews and subtrades to carry out the work. This requires continuing training, skills improvement, and keeping up to date with current technologies, materials, and building science. How well the insulation and air barrier are installed is reflected directly in the results of performance tests like blower doors and thermal imaging.
Commissioning is done regularly on large commercial and institutional construction projects. It is the process of assuring that all systems and components of a building or industrial plant are designed, installed, tested, operated, and maintained according to the operational requirements of the owner or final client.
Currently, houses are not typically designed and built with an overarching or standardized commissioning phase. Meeting building code requirements for safety and structure has been the conventional way to ‘commission’ a building, at least in regard to the fire safety and structural requirements. However, as building envelopes improve energy conservation, and mechanical systems for space conditioning become more refined, other commissioning processes that ensure building performance will become industry standards across the board.
Often, houses do not perform optimally or even as predicted by code requirements. This is due in large part to the evolution of residential construction:
Houses are typically field assembled
Houses are built on a component-based approach with separate trades responsible for pieces of the house
No consistent process exists to identify problems or to correct them
This is not to say that commissioning doesn’t exist in residential construction. Some forms of commissioning are in place for various programs, and within some local building codes. Residential commissioning includes processes currently carried out by home energy advisors, home inspectors, auditors, and weatherization contractors.
Why do we care about commissioning?
Building science shows that the various building envelope and mechanical equipment components of a house interact to form a complex system. Occupant health, safety, and comfort is impacted directly by the dynamic relationship between all of the components that make up the house. Commissioning based on the house as a system needs to be specific to either new or existing houses. As we move towards high-performance housing, commissioning must include diagnostic/performance testing such as blower doors and thermal imaging, as well as balancing ventilation systems and testing space conditioning equipment and distribution systems.
Air, heat, and moisture flows all affect occupant safety, health, and comfort, as well as building durability. If a component is installed incorrectly, or if a component is modified by the occupants or renovator, or when it degrades over time, there is an impact on whole-house performance and the occupants because of changes to the air, heat, and moisture flows.
Commissioning is part of a quality management program that can help improve the bottom line of a construction business, while at the same time helping the business to tease out potential innovations and improvements that can be made to new construction or renovation projects or processes. Commercial buildings are commonly commissioned by an independent third party (Commissioning Authority). This third-party structure (like the Energy Advisor program) means that the Commissioning Authority stays neutral and avoids conflicts of interest. The cost of commissioning can be reduced if it is folded into other programs, or as part of a quality management process. Integrating commissioning (see integrated design, next section) at the start of the construction process is important to avoid scheduling conflicts and unnecessary cost.
A whole house commissioning process should account for both energy and non-energy opportunities for improving building performance, related to both the building envelope and the HVAC systems, including:
Insulation levels and installation quality
Space conditioning distribution strategies
Combustion appliance backdrafting with spillage
Indoor Air Quality
Quantifying how well a house or building performs is done through metrics. Metrics are ways to measure, assess, compare, and track performance or production. There are three areas where metrics can be applied to commissioning a safe, healthy, comfortable, and affordable house that has a minimal adverse impact on the external environment.
Energy Performance Metrics:
Indoor Environment Metrics:
Indoor air quality
Integrated Design Approach
An energy advisor working with a builder or renovator is a key team player when moving towards high performance targets. In the same vein, builders and developers can benefit greatly by incorporating an integrated design approach into their processes, because of the importance it places on building science, energy modelling, and proper costing data. Integrated design brings many specialists to the same table in the design stage of a project. Often, building design, sustainability, structural engineering, building science, HVAC systems, and other specialties like lifecycle costing, universal design or building information modelling (BIM) are considered separately. The conventional approach is to ‘silo’ these specialties, while integrated design brings the whole team together.
This is important when working towards successful high performance projects, as each specialist brings specific skills and perspectives to the table. Input from the whole team leads to new insights in the design and construction processes to create a solution that is well-designed for the client, cost-effective, secure/safe, sustainable, accessible, and functional/operational, as well as being energy efficient and low carbon.
The goal of integrated design is to improve and streamline the construction process, saving time and money, but also to provide quality, comfort, and improved environmental profile for the whole lifecycle of the building. This video is a quick overview of the benefits of integrated design.
An integrated design process is driven by the project leader. There must be a commitment from the lead to bring together the multiple disciplines in a series of steps that can provide an orderly flow to dialogue and participation. This type of process must have buy-in from all members of the design and delivery team to be successful, as the opportunities to make changes to the design and construction are most cost-effective in the first stages of design.
CMHC created a guide to integrated design for use in large construction projects that incorporate green building practices. Much of it is relevant to Part 9 construction, especially for those working with large developers and high-volume builders.
Here is what the CMHC guide lists as the stages of an integrated design process, as identified by Bill Reed of the Integrative Design collaborative:
PREDESIGN 1 – STAGING THE PROJECT (The Foundation)
Client involvement in the design decision process
Design Problem Setting
Identifying Base Conditions
PREDESIGN 2 – MANAGEMENT MAPPING AND GOALS (The Foundation Dialogue)
Includes all participants—including the main decision maker
Alignment of Expectations and Core Purposes
Addressing the Mindset
Goal Setting of Environmental Metrics and Benchmarks
Creating a Project Specific Systems Map and Schedule
DESIGN PROCESS – SYSTEM OPTIMIZATION
Contractor or Cost Estimator Engagement
CONSTRUCTION AND OPERATIONS – REALIZING THE OBJECTIVES
Follow-through in Construction Process
Maintenance and Monitoring
High Performance Envelopes
The building envelope is where most of the building science knowledge rests. The building envelope needs to minimize heat and air flows and block moisture flows. The performance of the house depends on air tightness and insulation levels. So how do we know how much insulation to use?
Using the BC Energy Step Code (because it creates a ladder of efficiency, and BC encompasses all of the Canadian climate zones), here is a table of expected effective R-values for above grade walls per climate zone. Each one has a range that the builder can choose as trade-offs are made in other areas. The further we go down the path to Net Zero Ready, the more important it becomes to use the performance path. This gives builders an opportunity to make low-cost decisions that improve energy efficiency.
Most of Canada’s population lives in Zones 4, 5, and 6.
Step 1 (improved from code)
Step 2 (10% better than code)
Step 3 (20% better than code)
Step 4 (40% better than code)
Step 5 (Net Zero Ready, up to 80% better than code)
So, back to air barriers and vapour barriers, two of three key control layers in a house. We need both. We also need a thermal blanket.
Here’s a common analogy that we can all relate to: how to dress for Canadian weather (substitute ‘snow’ for ‘rain’ for 5 months of the year in most of the country).
If it’s chilly outside, you put on a sweater (insulation). Insulation doesn’t stop the wind, so you put on a windbreaker (air barrier). A classic windbreaker doesn’t hold the rain out. In wet weather, a waterproof rainslicker over the sweater will protect you from the rain (vapour barrier), but will make you all hot and sweaty, soaking your insulating sweater and defeating the purpose of it.
To stop us from getting all hot and sweaty in the rain, some clever folks invented fabrics that are Waterproof, Windproof and Breathable all at the same time. Now, a good raincoat will protect you from the rain and keep the wind from chilling you off, while allowing moisture you generate to move through the material.
It’s the same with building materials. Insulation controls heat loss, air barriers control air flow, and vapour barriers control moisture flow. And clever folks are inventing new materials that do one or more of these jobs better than conventional or traditional materials. So we have smart vapour retarders that allow moisture to move in or out of an assembly. We have insulation that can act as all three control layers. We have products that can supply all three of these control layers while also acting as the weather resistance barrier (WRB).
Out of these three layers, the one that is key to the success of a wall assembly is the air barrier, as it controls air flow, and by doing that, controls water vapour movement. According to the U.S. Department of Energy, "air movement accounts for more than 98% of all water vapor movement in building cavities.” Stop the air flow, and you stop most of the moisture flow.
We focus on vapour barriers and air barriers in part because of the dewpoint - where the drop in temperature causes water vapour to turn to liquid and condense out in the wall. The warmer the air inside the wall, the more moisture it can hold. The dewpoint in the wall is determined by the difference in temperature from the conditioned space to the exterior, as well as by the relative humidity of the air. While the dew point can technically fall anywhere in the cavity, the water vapour will only condense out onto the first solid surface it hits once the dewpoint has been passed. Typically, this means the back of the sheathing.
Watch this video from RDH Building Science for a quick refresher on Relative Humidity and dewpoints
Many materials can act as an air barrier but do not, on their own, create a vapour barrier. For example, gypsum board. Water vapour can diffuse through gypsum board, but air cannot pass through it (this is the premise of the airtight drywall approach, or ADA). However, the surface of gypsum board can be finished with a latex vapour retardant to create a vapour barrier if the standard 6 mil poly vapour barrier is not present.
The role of the vapour barrier is to prevent warm humid air from condensing out when it meets a cool surface. It is in place primarily to counteract moisture diffusion, water vapour that moves through materials. The vapour barrier is not required to be taped and sealed. In fact, it’s usually in fairly bad shape by the time strapping, drywall, and services are installed. The key aspect of the vapour barrier is where it sits in a building assembly. In cold climates, the default position is ‘on the warm side of the insulation’, or with at least two-thirds of the insulation outside of the vapour barrier.
Smart Vapour Retarders
Houses in cold and mixed climates, like those in Canada, have changing RH and temperature conditions. This is especially pronounced where air conditioning is common. The standard rule of ‘vapour barrier on the warm side’ means that many new and existing houses will see a season where the vapour barrier will be on the wrong side, because it’s colder inside than outside. This can lead to solar vapour drive, especially in houses with masonry or brick cladding. To counteract the seasonal changes, instead of using 6 mil poly as a vapour barrier on the warm side of the wall or ceiling, we can now use smart vapour retarders. These materials have a vapour permeance that changes based on humidity conditions. A smart vapour retarder (not a barrier!) is formulated so that it has low permeance in the winter when RH is low, but moisture flow needs to be blocked, and high permeance in the summer when humidity is high. This allows the wall to dry to both the interior and the exterior.
The first smart vapour retarder was kraft paper, often used as a facing on fiberglass batts. Paper is made from wood, and wood absorbs moisture, becoming more permeable in the summer, and less in the winter when the humidity drops. Paper vapour retarders, made from recycled paper with fibreglass reinforcement are now on the market, as are new smart vapour retarders made of materials such as polyamide or nylon.
Framed Walls Above Grade
The rule for placement of the vapour barrier changes somewhat as we move towards highly insulated walls. The change is determined by what materials are being used, and where they are in the wall assembly.
As we push towards high performance walls, there is a concern about materials creating double vapour barriers, essentially sandwiching moisture into the wall. Again, this is not an issue if air movement (and water vapour movement) is highly controlled by the air barrier. All high performance walls must control air and moisture movement as well as heat loss.
There are two primary categories of high performance walls:
Split insulation, cavity insulation and outboard insulation
Double stud, cavity insulation between two parallel framed walls
There are also two primary categories of insulation:
Permeable, allowing moisture movement
Non-permeable, severely limiting moisture movement.
A common way to build out a high performance wall is to fill the wall cavity with fibrous insulation & exterior insulation. Wall cavity insulation is often called ‘inboard’ insulation, and exterior insulation is often called ‘outboard’.
A split insulation wall can have an exterior air barrier, but you must keep in mind the inboard/outboard ratio, and total insulation level as well as what type of exterior (or outboard) insulation is present. Rigid board to the exterior also gives full coverage, a blanket of insulation to the exterior of the framing eliminates thermal bridging.
Boards must be taped and sealed at the seams to create a continuous drainage plane
Where brick is commonly used as a cladding material, adding rigid board to the exterior may require a thicker foundation wall to act as a brick ledge, or commercial style fasteners.
When using exterior insulation, there are a few issues that come up:
Insulation material choices
Moisture concerns (double vapour barrier)
Cladding attachment issues
Permeable Insulation vs. Impermeable Insulation
All of the other issues listed above are predicated on the material choice and thickness:
Expanded Polystyrene (EPS, Type I or II)
Extruded Polystyrene (XPS, Type III or IV)
Polyisocyanurate (1 or 2 sides foil faced)
Semi-rigid mineral wool (1 or 2 sides paper faced)
The insulation type and the ratio of inboard to outboard matter because, during the winter, Canadian houses deal with increased water vapour flows due to high indoor RH and cold exterior temperatures. In a typical wood framed wall with cavity insulation, vapour flow occurs outwards during the winter. Adding any type of exterior insulation warms up the sheathing, lowering the potential for condensation. Permeable exterior insulation, like mineral wool, allows vapour to flow through to the exterior. In this case, standard 6 mil poly is acceptable as a vapour barrier. When a wall system uses impermeable types of insulation such as extruded polystyrene or foil-faced polyisocyanurate that restrict drying to the exterior, they can benefit from a smart vapour retarder on the interior, or rely on a latex vapour retarder paint over the drywall.
When a high ratio of outboard insulation is used on the wall, it doesn’t matter what type of insulation is used. The sheathing will be above the interior dew point, so it’s not at risk of vapour diffusion or air leakage condensation. These high-performance walls allow for greater flexibility in design.
Materials and Moisture Issues
Testing has shown that exterior insulation actually reduces condensation inside walls, primarily because the dewpoint is moved outside of the wall cavity. This means there are fewer hours per year where the backside of the sheathing gets cold enough for the water vapour to condense out. Exterior insulation can also slow outward drying, so we need to install rainscreens to allow for proper drainage and ventilation to the outside of the wall.
To minimize the risk of condensation, we need to install enough outboard insulation to keep the back surface of the sheathing warm. In most of Canada, this translates into 2 inches (50mm) or more of outboard insulation. To reach Net Zero performance levels (close to R40 nominal for a wall assembly), 3 inches (75mm) of outboard insulation is often used in Climate Zones 4 through 6.
The type of sheathing installed will dictate the ability of the wall to dry to the outside. We need to know the permeance (a measure of how readily water vapor can pass through) of materials. Low permeable sheathing reduces outward drying, so it can actually increase the risk of a double vapour barrier. The lower the permeance of the sheathing material, the greater the R-value needed to the exterior.
Recall that there are a wide number of materials available to create effective vapour barriers, including:
Polyethylene plastic sheet
Asphalt-coated Kraft paper
Vapour retarder paints
Extruded polystyrene or foil-faced foam board insulations
Exterior grade plywoods
Sheet-type roofing membranes
Glass and metal sheets
In the US, a ‘perm’ is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury. The metric perm (not an SI unit) is defined as 1 gram of water vapor per day, per square meter, per millimeter of mercury. The equivalent SI measure is the nanogram per second per square meter per pascal (ng/S-m2*Pa); this is the unit referenced in the National Building Code.
Conversions for Vapour Permeance
ng/Pa•s•m2 (METRIC SI)
In the US, materials are rated as Class I, Class II and Class III vapour barriers and vapour retarders. In Canada, any material that allows less than 60ng/S-m2*Pa under specific conditions, is considered a vapour barrier by the National Building Code under Part 9, Residential and Small Buildings.
Materials can be classed as an impermeable ‘vapour barrier’ or a semi-permeable ‘vapour retarder’ depending on how much water vapour passes through the material under specific conditions. There are vapour retarder primers on the market with a permeance of 30 to 36 ng/S-m2*Pa, well below the 60 ng/S-m2*Pa allowed by code. Below is a table showing US vapour control classifications. It’s helpful to be able to understand how the classes, perms and descriptions relate to each other.
Less than 0.1
Less than 6
Less than 5.72
0.1 to 1.0
6 to 60
5.72 to 57.2
1 to 10
60 to 600
57.2 to 572
A note about Canadian vapour barriers. The US has a far wider range of climate zones than Canada. Canadian code only deals with two types or classes of vapour barriers:
Type I (US Class I) Vapour Barrier: not allowing more moisture passage than 15 ng/(Pa.s.m2) Perms (SI) or 0.26 Perms (US).
Type II (US Class II) Vapour Barrier: not allowing more moisture passage than 0.75 Perms (US) before aging, and not allowing more than 60 ng/(Pa.s.m2) Perms (SI) or 1 Perm (US) after aging
Refer to the National Building Code Subsection 9.25.4 for details on Vapour Barrier Conditions.
When working with highly insulated walls of any sort, we need to know the permeance of the various materials being used. We also need to know the permeance of the insulation being installed to the exterior, as this will also impact the ability of the wall assembly to dry to the outside. Perm/nanogram ratings can vary by manufacturer (and also by material thickness), but here are some groupings to help identify where various materials fall in the spectrum of permeance. Sources: buildingscience.com and greenbuildingadvisor.com
Expanded OR extruded polystyrene with polypropylene facings
Unfaced expanded OR extruded polystyrene
30 lb asphalt coated paper
Bitumen coated Kraft paper
House wrap (check mfr specs)
Coated/faced thin profile structural sheathing
Some vapour retarder primer
¼” exterior plywood
4” Brick veneer
Fiberglass insulation (unfaced)
15 lb asphalt coated paper
House wrap (check mfr specs)
Some vapour retarder primer
Asphalt impregnated felt
What do some common wall high-performance wall assemblies look like in terms of permeance? A key takeaway from this discussion of permeance is whether the wall dries to the outside or the inside. If it can dry to the outside (semi-permeable) then a 6 mil poly vapour barrier is required. If it cannot dry to the outside (impermeable, semi impermeable), then the vapour barrier is to the exterior, and you must rely on air sealing to ensure that moisture transfer via air flow is minimized or eliminated. When in doubt, a smart vapour retarder can be installed on the warm side.
Foil faced polyisocyanurate taped and sealed, under any type of cladding
Type II Vapour barrier insulation (rigid board) taped and sealed, OSB or Plywood covered with building paper or house wrap
EIFS (External Insulation Finish System) over OSB or plywood
Double stud construction with OSB or Plywood and building paper/house wrap
Most builders are using wood furring when there is a 2” (50 mm) depth of outboard insulation. This is an affordable way to ensure you don’t need to use longer, more expensive screws to attach the cladding, you can still use regular siding nails. You need more expensive screws to attach the furring, but fewer of them than for the cladding.
Won’t my cladding fall off or sag down if it’s outboard of the wall and only held in place by screws and furring?
Short answer: No.
Longer answer: long-term trials and tests done by Dr. John Straube and his team at the University of Waterloo have shown that there is no issue with cladding when it is put in place over thicker depths of outboard insulation. In fact, the wall is made stronger, as the screws act in the same manner as the tension chord in a truss. What they also found was that the cladding is held in place by the compressive resistance of the insulation material itself (friction between the materials also plays a part). The reports show that deflection in vinyl and wood claddings in 1/100ths of an inch. There is a slight bit more movement with heavier claddings, such as stuccos and adhered veneers, up to 15/100ths of an inch. To put that in perspective, the seasonal moisture movement in a rim joist is at least ⅛ of an inch (that’s roughly 12/100ths of an inch).
Closed Cell Foam
Another option for high performance wall is a 2x6 cavity filled with medium or high-density spray polyurethane foam = RSI5.8 to 6.3/R-33 to R-36. Closed cell spray foam has a higher insulation value than fibrous insulations, and, as an added benefit, 50mm (2 inches) of medium or high density spray foam is required to create a air barrier and VDR. Note: low-density or open cell foam does not meet these requirements.
Spraying inside the cavity does not resolve issues with thermal bridging. Some contractors use spray foam to the exterior of walls, with or without strapping or bridging. The success of this approach is determined by the skill of the applicator.
Double Stud Walls
Double stud wall systems were pioneered in the 1970s by Canadians such as Harold Orr and Rob Dumont from the Saskatchewan Research Council. This is a reasonably low-tech approach to a high performance wall, relying on standard carpentry skills and common materials.
A double stud, or staggered stud, wall is built with a secondary interior wall parallel the primary load bearing wall. Most often, the secondary wall is not structural, allowing for wider spacings and smaller framing lumber, but some designers prefer to use the interior wall as the structural element. The two walls are offset from each other, giving a wide cavity (anywhere from 9 to 12 inches) for insulation (R-35 to R45). The two walls don’t touch, so there is no thermal bridging, however, there could be bridging at the sills, top plates, and window or door openings.
Commonly, double stud walls are insulated with dense-pack or sprayed cellulose. Other fibrous insulations (fibreglass or mineral wool) are also used.
A typical double stud assembly for a house with a basement or crawlspace foundation, would have these layers (from outside to inside):
2x4 structural frame
4.5” wide (or better) gap
2x3 non-structural frame
Here is a quick video showing a typical double stud wall assembly.
The wall could also have sheathing on the exterior face of the interior wall.
The size of the framing can vary, depending on the type of foundation and floor system. On a foundation wall, the stud wall sits on the floor framing, so the assembly listed above can be used. However, with a slab on grade, a 2x6 wall could be used as the exterior load bearing portion, with the plate offset from the slab edge to line up with vertical rigid board insulation on the frost wall or turn down of the slab.
Separate walls with individual sill plates are easier to construct, with less thermal bridging and higher insulation at the top and bottom of the wall. In addition, separate walls allow for the use of small dimension lumber. The studs in the walls can be staggered or aligned, but they must line up at the openings.
The primary risk to double walls built with fibrous insulations is water, either through rainwater penetration or via condensation from air leakage due to a compromised air barrier. Cellulose is a possible good choice because the material itself can store and redistribute a certain amount of moisture, but is also treated with borates against mould growth. Drying is controlled by other vapour impermeable components such as plywood or OSB sheathing. As installing a 6 mil poly vapour barrier on both sides of the wall would seal any moisture into the cavity, a smart vapour retarder should be installed on the warm side of the insulation. As the wall typically relies on highly air-permeable insulation, the air barrier must ensure that through-the-cavity air leakage is minimized. In many cases, the primary air barrier is a layer of plywood or OSB sheathing, but airtight drywall approach is also successful in this assembly.
Code may require fireblocking for the assembly.
If the floor and roof loads are stacked on the load-bearing walls, and the assemblies meet design criteria, you can take advantage of 24” o.c. and other advanced framing methods.
Some designers specify that the inner wall will have another layer of framing installed after an interior air barrier layer, but before the drywall. This creates a service cavity, which provides room for services to be installed without damaging the airtight layers. If the service cavity is insulated, it adds an additional thermal element to the wall.
Peter Amerongen, of Habitat + has built double wall Net Zero and Passive House projects for several years. VIDEO
Dealing with openings - Innies, Outies or in the Middles?
There are pros and cons to placement of windows in thick walls, either with split insulation or double wall construction. The location of the window or door affects air sealing, flashing, drainage plane, and finishing details. The aesthetics of each type of installation play a part in the decision. Other components to the decision to line up the window with the exterior edge, or the middle of the wall are:
Quality Control - how much do you trust the skills of your trades in sealing the drainage plane and installing flashing?
Comfort - The closer the window is to the exterior, and the thicker the wall, the more chance there is of creating a cold ‘pocket’ in the window opening on the interior. There is risk of a colder glass surface, which could lead to occupant discomfort, depending on where the window is located, but there is also risk of the cold pocket causing condensation if air movement is restricted by the depth of the wall.
Aligning the window with the drainage plane/weather barrier reduces the number of horizontal elements, making water management simpler. It also simplifies the finishing detail on the exterior. Doors on the outside edge of a thick wall can’t swing past 90 to 105 degrees without bevelling the opening, or thinning the wall on the hinge side for a small section. Some designs also specify expandable offset door hinges. These special hinges are designed to swing the door clear of the opening adding about 2" additional clearance for wheelchairs and walkers. The hinges wrap around the door trim (at least 3" is required between the inside of the door jam and adjoining wall for the hinges to fit).
Another solution is a thermally-broken, single-stud opening within a larger double-stud opening.
Outie installation on a double stud wall is essentially the same as a standard install. Thickening the wall with outboard insulation requires a window buck (for example 50mm (1/2 inch) plywood) to extend from the inside edge of the stud cavity to the outside edge of the rigid board.
Window sizes must take into account this reduction in RSO.
Screw the buck into the studs, wrap edges with waterproof membrane, and then install window to exterior edge.
Cost saving measure: use drywall with radiused corner bead for interior casings. The deep reveals with outie windows and doors give an opportunity to highlight interior finish details, and create great placement for plants, or window seats.
For split insulation walls, keeping the window inline with the framing requires extra jamb extensions (made from trim or siding material).
For double stud walls, innie positioning requires the inner and outer rough opening may be sized differently. In most assemblies, the headers go in the outer framed wall. In some designs, the structural load is carried by the interior wall. If the load is carried by both walls, headers may be required in both walls.
The deeper the window moves into the wall, the more accentuated the depth of the wall is from the exterior.
A window that sits ‘inside’ the drainage plane will need careful water management detailing and installation. The window needs to be connected to the exterior sheathing to create a fully continuous drainage plane. On the other hand, mid-wall or ‘innie’ placement provides water protection to the window itself and a small degree of shading, which can result in smaller overhangs.
Roofs and Ceilings
High heel trusses are often used for aesthetic or decorative purposes (making the house look taller from the outside). They also have a very practical purpose: they allow for full insulation coverage at the weakest point of the ceiling: the junction between the top of the wall and the eaves.
Continuity of the air barrier at the junction between the exterior wall and the ceiling is crucial to minimizing heat loss due to air flow.
For flat ceilings, a good option for air sealing is to spray a minimum of 50 mm (2 inches) of high-density polyurethane foam on the floor of the attic (not applicable for sloped ceilings or flat ceilings with parallel chord trusses or rafters).
In foundations, walls can be insulated from the inside, the outside, or on both sides. When insulating on the inside, always assume that moisture will be a problem and either use moisture-impervious materials or isolate all wood and fibrous materials from masonry/concrete.
Insulation on the exterior keeps the whole foundation wall system warmer and drier. Rigid foam insulation, or semi-rigid with a drainage plane, acts as the vapour barrier as well as the water control layer. Insulation to the exterior also helps keep the wall assemblies dry by keeping the wall above the dew point. This video from Construction instruction goes over the benefits of insulating a foundation from the exterior.
When insulating from the interior, materials must be chosen carefully, as poor drying capacity of the assembly causes moisture from the soil and concrete to penetrate into the insulated wall cavity.
This video discusses an interior insulation, air and vapour barrier system on the foundation of a Net Zero build in Ontario.
Insulating from the inside OR the outside requires a tie-in with the above grade walls, as shown in the video.
Insulating Concrete Forms, ICFs, create a sandwich of expanded polystyrene and concrete with steel reinforcing. ICFs are a good way to minimize moisture issues in below grade applications, as the manufacturer typically requires a proprietary waterproofing on the outside.
Like any new endeavour, there is a learning curve when you change your construction assembly.
Assembly Issues include changes to the shape or size of such items as brick ledges, fasteners, window bucks and RSO details, or the drainage plane.
Trades scheduling issues arise (examples: ceilings need to be boarded in a different order than remainder of house if spray foam is used on walls; conflicts between services installation and ICF process).
Inspectors don’t always agree on equivalencies in code (example: high-density foam is not accepted as an air barrier in all jurisdictions).
High Performance Mechanical Systems
Only after all viable improvements are made to the building envelope should you turn to the mechanical systems. Mechanical systems are relatively short-lived parts of the house; equipment typically has a lifespan of between 15 and 25 years with annual maintenance required. The building envelope components are permanent aspects of the house, typically in place with little or no maintenance over 25 to 50 years, depending on the materials chosen. This means there is one shot at creating a high performance envelope.
As the building envelope is improved, the energy end use patterns change, moving the emphasis from meeting space heating requirements to minimizing appliance loads.
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.
When the DHW load becomes more significant compared to space heating, and the space heating load drops, the biggest challenge becomes meeting the DHW load and matching or integrating space and water heating equipment.
Dropping hot water loads in the construction phase includes low-flow fixtures and drain water heat recovery (DWHR) units. In addition, plumbing runs from water source to point of use should be kept to absolute minimum lengths. Where long runs are unavoidable, circulating pumps can help reduce the amount of energy loss associated with them.
Space Heating & Cooling
Heating and cooling systems are designed to provide thermal comfort. At the same time, some types, such as furnaces and forced air systems, can also provide air flow and filtering as well as humidity control.
Typically a heating or cooling system will have a source – furnace, boiler, heat pump, air conditioner – and a delivery system – ductwork, hydronic radiators, convectors or tubing, panels, or other.
As space conditioning loads drop, integrated space and water heating systems become a more viable option than two separate systems, especially when the space heating can rely on low-temperature delivery strategies.
Where pumps, blowers, or fans are used, they can be optimized by specifying ECMs (Electronically Commutated Motors). These are ultra high efficiency programmable brushless DC motors. However, they can push up the price of an air handler or a pump, and that premium might be more valuable to another budget line item.
Using a less-efficient, small-capacity air handler with a heating coil does not incur an energy penalty as the house moves closer to NZE because the space heating load has been decreased by 40 to 60 percent or more from a code-compliant house.
Code-compliant house ‘A’ uses a 55,000 Btu 90% AFUE furnace, and has an annual load of 100,000 MJ. This means 10,000 MJ are ‘lost’ in a year through the operation of the furnace.
NZE house ‘A’ uses a 35,000 Btu 86% AFUE air handler, and has an annual load of 40,000MJ. This means 5,600 MJ are ‘lost’ in a year through the operation of the air handler.
Heat pumps are an excellent choice for getting to Net Zero because they don’t rely on electric resistance to supply heat to the house. A heat pump transfers energy from the outside air to the inside air. In the past, heat pumps have not performed well in cold climates. The performance of a heat pump, the COP, diminishes as the temperature drops. However, cold-climate heat pumps are now available. These units can extract more energy from colder air (down to 18°C) by using variable speed and multiple compressors. A properly sized cold climate heat pump can meet 70 to 90, or sometimes even 100 percent of the the heating load of a high performance house.
A quick recap:
Air to air heat pumps transfer energy from outside to inside.
Ductless mini-splits have an indoor and an outdoor unit (that’s the split). They can be recognized by the ‘head’ or cassette that is typically hung from the wall or fitted in the ceiling. They can be one zone or multi zone. Each cassette or head typically heats what it can ‘see’ - meaning it will provide heat to a single space. In other words, a ductless system does not act like a central forced air heating system, more like strip electric/baseboard space heaters
Ducted mini-splits have heads that are mini air handlers with ducting behind the drywall. Ducted mini-splits have lower efficiencies than ductless units because they are pushing air through the duct system. This type of system can make sense in houses with smaller rooms, where it would be too costly to install a ductless head in each room.
Centrally ducted heat pumps act like a low-temperature furnace with slower moving air.
A variable refrigerant flow, VRF, system acts in the same manner as a heat pump, but has the capacity to deliver more or less heat or cooling, depending on the outdoor temperature. Heating and cooling systems are designed to ‘peak’ or ‘design’ temperatures, to accommodate the comfort needs of the occupants at the worst-case temperature scenarios. In fact, the house only sees those design temperatures for either 2.5% or 1% of the heating or cooling season. VRF gives the opportunity to vary the speed of the compressor, somewhere between 15 and 100 percent of the capacity of the compressor. This helps the house meet ‘part loads’ throughout the heating season in an efficient manner.
Air to water heat pumps are not common in residential construction in North America yet, but are proven technology in Europe, providing space and water heating by transferring energy from air to water. This allows for energy to be stored for later use.
Geothermal systems use coils filled with liquid to transfer heat from the ground into the home. They can be used with forced air or hydronic systems and can also provide domestic hot water.
High Velocity Air Distribution
Typically, forced air systems are classed as low velocity, and require large ducts to distribute a certain volume of heated air to each supply port in the house. A high velocity, or Small Duct High Velocity (SDHV) system works on the principle of air pressure instead of volume. The supply ducts, as small as 2” diameter, are sometimes known as ‘mini ducts’. These can be easily installed in wall cavities, reducing the need for bulkheads and chases.
The system moves air at a high velocity from a port located high on a wall, throwing the air into the room. The high velocity causes turbulence in the room and mixing the supply air with the room air. High velocity systems ‘stir’ the air in a similar way to how a waterjet works in a whirlpool tub, causing turbulence and even temperature distribution.
Right-sizing Heating Equipment
Heating systems for houses are often casually sized, using a rule of thumb that says you need 50 Btu of heat for each square foot of living space. What is this based on? Primarily the fact that people don’t complain about being too comfortable. They might complain about heating bills and noisy equipment that short cycles, but they won’t necessarily be uncomfortable.
There is a standard to calculate heat loss in houses, known as F-280 that is referenced in the building code. As materials and technologies have changed, the F-280 standard has changed as well. The 2012 version takes care of the oversizing that the original standard was causing in new homes.
As we move down the path towards net zero energy, right-sizing the heating system and optimizing the delivery become key aspects of both comfort control and cost control.
Right-sizing a heat pump, for example, keeps the unit operating at low speed to optimize energy use. The unit will reach the set point faster, but will cycle properly so that there is a minimal temperature swing. When the unit is oversized, it switches off and then comes back on at high speed to compensate for the a wide temperature swing in the building between cycles, reducing its efficiency.
The importance of proper load calculations is underscored by many benefits to various players:
Homeowner: improve comfort and better home performance
Builders and Renovators: reduced number of callbacks, improved relations with trades, better final product
Suppliers & Manufacturers: equipment should operate closer to design intent with increased longevity
Designers: confidence that aesthetics and functionality are better integrated
HVAC Contractor: Confidence that HVAC equipment meets load requirements for home, which brings us back to 1.
F-280 provides important information about equipment size and the distribution system requirements
Calculates whole house load - capacity of heating/cooling equipment. Considers the factors below in the calculation, while the amount of heat loss/gain is based on the climate zone the house is located in.
Room-by-room breakdown indicates how much heating or cooling must be delivered to each space, creating controlling factor for the design of the delivery system. Heating or cooling can be delivered to the house in any form.
System (effective) R-values
People and Appliance Heat Gain
The challenge to an airtight envelope is that very little moisture can escape without ventilation, but the key is not to have a leaky envelope.
The house doesn’t need to breathe, the occupants do. Providing controlled ventilation with filtration in an airtight home is much more cost effective and healthful than a house that is full of allergens and dust that have come in through cracks and holes in the building envelope.
Ventilation systems can be integrated into forced air systems, or stand-alone, low-volume ductwork. A ventilation system is dedicated to providing air flow, filtration, and humidity control, improving indoor air quality for the occupants.
As the building envelope tightens, a properly sized and designed HRV system is key to the health and comfort of the occupants. In Canada, CSA F-326 tells us how to size a balanced ventilation system. It doesn’t tell us how to design the system. Ductwork is sized according to established requirements for effective length and static pressure, but where do supply and exhaust ports go for a balanced ventilation system that has dedicated low-volume ductwork?
The exhaust ports get placed first, in the kitchen, bathrooms, and sometimes the laundry room. Then the supply ports need to be placed strategically, away from the exhaust ports so the fresh air supply reaches as much of the space it is intended for as possible. It’s crucial for good distribution that the supply of fresh air to a room is not ‘short circuited’ by the exhaust. The design goal is to have the fresh air pulled through each living space by the exhaust. This means supply ports need to be positioned:
On opposite walls to exhaust ports
On opposite sides of rooms
Diagonally opposite of doors leading to exhaust ports
We use the first law of thermodynamics (energy can neither be created or destroyed, but can be transformed from one form to another) to evaluate the energy performance of heating and cooling equipment. For example: a certain volume of gas has to be burned by an 85% efficient burner, to provide so many Btus of heat per hour. If you paid $100 for that energy, then $15 of your dollars were lost to combustion processes in your chimney. The other $85 kept you warm, but only for the time it took for the heat to be lost to the exterior conditions. So, it was transformed but never destroyed.
This analysis by itself does not show the whole picture in regards to energy utilization. The gas in the furnace or boiler burns at a much higher temperature (1500°C/2700°F) than the supply requires (room temperature is about 20°C/72°F), so there is a mismatch between supply temperature and demand temperature. Generally speaking the higher the source temperature the higher the quality and vice versa. To reduce the use of high quality fuels, we should aim for ‘exergy’ efficiency: equipment and systems where the supply and demand temperatures are closely matched. As buildings move towards net zero, we can more easily match supply to demand temperatures through space conditioning systems such as heat pumps. A solar thermal system or an air-to-water heat pump would be a better match for low-temperature hydronic in-floor systems than a boiler, for example. Exergy efficiency becomes an easier target because the ambient temperatures of the internal surfaces of high-performance walls and windows are closer to room temperature. In a code-compliant house, those surfaces are far more influenced by the exterior temperature, and so occupants feel discomfort because they are radiating body heat to the cooler surfaces around them.
In a high-performance building envelope, with higher ambient surface temperatures, and less radiant heat loss from occupants, we can supply lower temperature heat for more consistent comfort levels.