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January 30, 2019
This article will walk you through renewable energy systems in housing, starting with the general principles of renewable energy. This article has a focus on passive solar design. Passive solar allows you to optimize the ‘free’ heat from the sun in a house, while avoiding such challenges as summertime overheating. Understanding the basics of passive solar gives a good background for working with active energy systems, like solar thermal and photovoltaics.
Here’s the evaluation form, so you can rate yourself on your existing understanding of General Principles of Renewable Energy. 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!
If you find General Principles of Renewable Energy a challenge, read on. There are links to free resources in this article.
If you’ve got this part of the competency down, tune in next for the next part of High Performance Housing, Understanding On-Site Energy Systems.
Passive Solar: A Primer on Design
Passive solar design uses the sun’s energy for the heating and cooling of living spaces.The building relies on the physics of thermodynamics - the energy characteristics of materials and fluids (like air) - that stem from solar gain. The key elements of passive solar are simple: too collect, store and distribute the sun’s energy. Windows that open, thermal mass (materials like masonry and water that can store heat energy) are two common elements found in passive design. Windows that open allow for natural ventilation. Thermal mass absorbs excess heat and slows down rapid temperature fluctuations.
Passive solar can contribute anywhere from 30 to 70 percent of the heating load of a fairly typically constructed house.
Design for passive solar aims to keep out summer sun and let in winter sun while ensuring the building’s overall thermal performance retains that heat in winter but excludes it and allows it to escape in summer. Passive Solar is:
low cost when designed into a new home or addition
readily incorporated into any construction method
appropriate for all heating climates
In order to maximise winter heat gain, minimises winter heat loss and minimize summer heat gain, passive solar heating requires the following passive design features:
southerly orientation of daytime living areas
appropriate areas of glass on south elevations
selection of appropriate glazing
passive shading of glass for summer
thermal mass for storing heat
a high performance thermal envelope
floor plan design to address heating needs including zoning
Passive Solar Principles
To optimize the free heat from the sun, the passive solar house must do three things:
Collect Solar Gain
Store Solar Gain
Distribute Solar Gain
Solar radiation is collected when it is trapped by the greenhouse action of correctly oriented glass areas exposed to full sun. Window orientation, shading, frames and glazing type have a significant effect on the efficiency of this process.
The solar heat is absorbed and stored by materials with high thermal mass (usually masonry) inside the house, to be re-released at night when it is needed to offset heat losses to lower outdoor temperatures.
Re-radiated heat is distributed to where it is needed through good design of air flow and convection. Direct re-radiation from thermal mass exposed to solar gain is most effective but heat is also conducted through building materials and distributed by air movement. Floor plans should be designed to ensure that the most important rooms (usually day-use living areas) face south and receive the best winter solar access.
A passive solar house can utilize low mass or high mass construction. Which type of construction is appropriate depends on the amount of solar gain possible for the site, the orientation and slope of the site, and the homeowner requirements for the floor plan. More on this later.
To make the most of the heat gained from the sun, it’s important that a house have a high-performance envelope. Heat loss is minimised with well-insulated walls, ceilings and walls, with the best performing windows the budget will allow. Thermal mass (the storage system) must be insulated to be effective. Air infiltration is minimised with airlock entries airtight construction detailing and high-performance windows and doors.
In the design stage, it is crucial to design the appropriate house shape and room layout to minimise heat loss. In cool and cold climates, compact shapes that minimise roof and external wall area are more efficient. In warmer climates, shapes that create more external wall than roof area are appropriate, to allow for better cross-ventilation.
Successful passive solar design also incorporates shading, which allows maximum winter solar gain and prevents summer overheating. This is most simply achieved with southerly orientation of appropriate areas of glass and well-designed eaves overhangs, and minimizing west-facing windows to avoid overheating in the summer afternoon to evening. Shading features can be incorporated into the house design to control the entry of sun during the summer months.
Collecting Solar Gain
The first principle of passive design is to collect the available solar gain. This happens through the glazing on the house when sunlight strikes it directly.
The sun travels through the southern half of the sky in North America, and has a seasonal pattern of being higher in the sky in the summer and lower in the sky in winter. The effect of the seasonal pattern longer daylight hours in the summer and shorter daylight hours in the winter. This pattern and its effect becomes more exaggerated the further north you go from the equator.
The orientation of a passive solar house is important. The bulk of the glazing should face south, for optimal solar gain. This doesn’t take into consideration views that might be in other directions.
The Greenhouse effect
Passive design relies on greenhouse principles to trap solar radiation. Windows act as collectors. When the sunlight strikes the glazing, solar energy passes through. Heat is gained when short wave radiation from the sun passes through glass, where it is absorbed by building elements and furnishings and re-radiated as long wave radiation. Long wave radiation cannot pass back through glass as easily. High-performance glazing has special coatings that help to ‘bounce’ more of the heat back into the living space.
When incorporating passive solar design into a house, it is important to understand that solar gain is a diurnal, or daily, phenomenon as well as a seasonal one. Solar gain happens during the day when the sun is striking windows (it also happens on cloudy days, because there is a temperature differential component). When the sun is not striking windows, at night, there is no solar gain.
Solar Heat Gain
Heat flow through any building element is directly proportional to the temperature differential on either side. Glazing is a critical element of the building envelope, it is where most heat is lost and gained.
Heat is lost through glass (and other building materials) by conduction and radiation, particularly at night. Windows are particularly good conductors, and don’t care which direction heat flows, so when there is solar gain during the day, the heat will be transferred to the interior of the house. When there isn’t solar gain, and it’s cold out, the window will transfer heat from the warmer interior of the house to the exterior. Conductive and radiant loss can be controlled advanced glazing technology, creating higher comfort levels.
Balancing heat gain during the day with heat loss during the night is a challenge and requires the proper selection of both glazing types and sizes for windows. In winter, when there is the highest need of heat gain, there are typically fewer than five hours of peak solar gain. The rest of each 24 hour cycle is heat loss.
Views are an important consideration and are often the cause of over-glazing or inappropriate orientation and shading. Avoid over-glazing — excessive areas of glass can be an enormous energy liability.
Characteristics of High Performance Windows
Specify windows with double or triple panes, a low-e rating and insulating spacers.
You are looking for units that have a low conductivity rating (U-value) and high solar heat gain coefficients (SHGC).
Window frames also conduct heat. Wood, vinyl or fibreglass frames are designed to minimize heat loss.
Solar Heat Gain Coefficient
The Solar Heat Gain Coefficient (SHGC) describes the amount of solar radiation admitted through the glazing of a window on an average clear day. It tells you how well the glazing blocks heat caused by sunlight on a scale of 0 to 1. The lower the SHGC, the less solar heat the window transmits.
SHGC is influenced by the glazing type, the number of panes, and any coatings. One layer of uncoated clear glass is typically about 0.8; a typical double-pane window without low-e sits around 0.7. Adding a low-e coating decreases this value slightly. The SHGC for the whole window is affected by shading from the frame and the ratio of glazing to frame. The frame itself has a very low SHGC.
Visible Transmittance (VT) is a measure of the amount of visible light the window lets through. This is not the same as the amount of solar heat gain. VT is also measured on as scale of 0 to 1. The higher the VT, the more light you see.
VT is influenced by the glazing type, the number of panes and any coatings. VT for a single pane window with clear glass can be around 0.9, while a clear double pane window can have a VT of about .78, with low-e coatings typically reducing this slightly. However, new low-solar-gain low-e coatings have made it possible to reduce solar heat gain without reducing visible transmittance."
Windows are essentially thermal holes in the building envelope, they conduct heat out of the house most of the time, so instead of having an R-value that tells us how well windows insulate, that is, how much they slow down the rate of heat transfer, they have a U-factor that tells us how well windows conduct heat. The U-factor is the R-value turned on its head – the inverse of ‘R’.
R-value is how well something (like fiberglass batting) insulates. Higher R-value is good.
U-factor is how well something (like a window) conducts. Lower U-factor is good
Windows have three distinct areas of heat loss that act differently from each other. One is at the edge of the glazing, one is at the centre of the glazing and the other is at the window frame and sash. Yet the window is given one U-value, which makes calculating its contribution to the overall energy efficiency of the building envelope much easier. But, to make it a little more complicated, U-factors are noted by manufacturers in two ways: centre of glass and overall.
It is important then to make sure that when you are comparing windows from different manufacturers, that you are looking at the same type of U-factor.
The centre-of-glass U-factor is just that: it is a measure of the heat transfer through the glazing and doesn’t consider the impact of the frame edge effects and material. While the glazing characteristics – number of panes, the size of the glazing, the gas fill within the cavities, the types of coatings on specific glazing surfaces – are definitely the main indicators of the U-factor, windows are complex, three dimensional objects, with major differences in materials that can improve or reduce the effectiveness of the window as a whole. For example, in a double-glazed unit, metal spacers have a much higher heat flow than the center of glass and cause an increase in heat loss along the outer edge of the glass that wouldn’t be so dramatic if an insulating spacer were in place.
The Overall U-factor combines the total window assembly: the insulating value of the glazing itself, the edge effects, and the window frame and sash. Edge effects become more important as the U value of the entire unit improves, and also in smaller units, where there is more edge than center of glass.
Layers of Panes
Double and triple glazed windows have come into widespread residential use in recent decades. A typical double window has a ½ inch space between the two panes, and a triple unit has a tighter space, between ½ inch and ⅜ inch. A drawback of triple glazed windows, both for transportation and installation, is their weight – especially with large units. Some manufacturers have overcome this by offering a window that sandwiches a suspended film between two layers of glass.
Some triple pane windows have a transparent suspended film sandwiched between two layers of glass instead of a third pane. Heat Mirror® is one such product. The three layer system has two independent air spaces, and weighs the same as a double glazing unit.
Insulating glass made with Heat Mirror technology can have an R-value from 6 to 20. The thermal insulation is matched with visible light transmission and solar control for use in different climate regimes.
Window units with higher insulation ratings have a coating on both sides of the film. This type of film is used where passive solar heating is useful. Other specifications reduce IR transmittance while minimal reflection and maximum light transmittance for good insulation and heat gain control.
Low-emissivity, or low-E, coatings are very thin, virtually invisible metallic layers applied to glazing surfaces to reduce the window’s U-factor by limiting radiant heat flow. In effect, low-E coatings are mirrors – not for visible light but for infrared (or heat generating) light, reflecting it away from or back into our living spaces. The specific location of a window’s low-E coating depends on whether the goal is to keep the heat outside or inside the house. In colder climates like ours, the priority is usually keeping the heat in.
So, where does the low-E coating go in windows for cold climates? The surfaces of the glazings are identified by number, starting on the outside. In cold climates, low-E coatings usually face the insulating airspace. For a double-glazed window, this means a coating on surface 3 – for a triple-glazed window, it means a coating on surface 3 and another on surface 5.
Filling the spaces between glazings with a gas that is heavier, and therefore slower moving, than air minimizes the convection currents within the spaces and conduction through the unit. This reduces the overall transfer of heat between the inside and outside. The gases used – usually argon, sometimes krypton – are known as inert, or noble, gases because they don’t break down or chemically react with other substances. Argon is often used because it is relatively inexpensive and it reduces the conductivity between the panes of glass by two-thirds, compared to plain air. Krypton is a better insulator, but is more expensive. Argon is most often found in double pane windows, while Krypton is more commonly used in triple pane windows, which have smaller gaps between the glazings than double panes, which offsets the higher cost somewhat.
Older style double and triple pane windows often have metal spacers between the glazings, which conduct heat, promote moisture condensation, and reduce the benefit of having multiple layers of glass. Newer multi-glazed windows have spacers made of non-metallic materials that are poor conductors of heat – like foam, vinyl, and fibreglass – which are often used in combination with special sealing and drying materials.
Window frames are available in several materials, with vinyl, fiberglass and wood being common. For many homeowners, one of the plusses of vinyl or fibreglass frames is that there is very little maintenance required to the interior or exterior surfaces. Vinyl and fiberglass frames are hollow but are constructed with thermal breaks or filled with insulation to reduce conduction. Wood is more prone to degrading, but wood frames can be clad with aluminum or other weather-resistant materials to improve their durability and reduce their maintenance requirements.
The material used to manufacture the frame not only governs the physical characteristics of the window – such as frame thickness, weight, and durability – but also has a major impact on its thermal characteristics. The unit’s U-factor reflects the thermal properties of the frame as well as the glazing – and, since the sash and frame represent 10 to 30 percent of a window’s total area, the frame’s properties will significantly influence the unit’s total performance.
Reducing/eliminating southwest and west-facing windows will eliminate the likelihood of summer overheating. However, that is not feasible for many lots and house designs. Where windows cannot be eliminated, shading strategies need to be considered. Often the eave overhang can be designed to sufficient to shade a window. Permanently shaded glass at the top of the window is a significant source of heat loss with no solar gains to offset it. To avoid this, the distance between the top of glazing and underside of eaves or other horizontal projection should be 50% of overhang or 30% of window height where possible.
Here’s a good old article from Build Green Canada on calculating your overhang width.
The eaves must be designed to allow winter sun access, but no summer access to the south. Eastern shading is not as crucial in cold climates as in warm climates. Occupants are often happier with more eastern light and early-day warmth from the sun, especially in northern latitudes with shorter winter days. Where southwest and western windows exist, overhangs would have to extend several feet to compensate for the shading requirements of long summer days in northern regions. Here are some other shading options:
Fixed horizontal shading devices above windows and glass doors can optimise solar access to south-facing glass throughout the year, without requiring any user effort. Fixed shading above openings excludes high angle summer sun but admits lower angle winter sun. As noted above, correctly designed eaves are the simplest and least expensive shading method for southern elevations.
Consider adjustable shading to regulate solar access on other elevations. This is particularly important for variable spring and autumn conditions and allows more flexible responses to climate change.
Other options for shading on western elevations are pergolas or other open frame structures that are planted with fast-growing perennial vines like hops that grow 30+ feet a year and die down in early fall.
Here is an excellent article from Green Solar Design on orientation and south-facing windows.
Storing Solar Gain
Thermal mass is used to store heat from the sun during the day and re-release it when it is required, to offset heat loss to colder night-time temperatures. It effectively ‘evens out’ day and night (diurnal) temperature variations,significantly increases comfort, and reduce energy consumption.
Thermal mass must be externally insulated (so the stored heat is not lost) and internally exposed (so solar heat can flow easily into the material).
Typically, stick-frame construction is considered ‘low mass’. It doesn’t have thermal mass to absorb heat and re-radiate it after the sun is gone. Low mass passive solar design requires smaller window to floor area ratios to avoid overheating. Low mass solutions with high insulation levels work well in mild climates with low diurnal temperature ranges. Low mass construction also gives faster response times to auxiliary heating.
High mass construction can be high-performance stick-framed assemblies but with an insulated concrete slab-on-grade as thermal mass, perhaps with interior thermal mass walls as well. Examples of high mass wall assemblies include double brick, rammed earth and reverse brick veneer. These high mass systems take advantage of the thermal lag to slow heat flow on a day−night basis.
Thermal lag is the amount of time taken for a material to absorb and then re-release heat, or for heat to be conducted through the material. In a passive solar design, adequate levels of exposed internal thermal mass mean that temperatures remain comfortable all night and, possibly through a few successive cloudy or rainy days.
Thermal lag times are influenced by material qualities:
conductivity and density
texture, colour and surface coatings
As well as environmental aspects:
exposure to air movement and air speed
temperature differentials between the interior and exterior faces
The rate of heat flow is proportional to the temperature differential between the interior and exterior face of the material. The colder climate, the greater the temperature differential, the more insulation required to ensure the material re-radiates the stored heat to the interior.
The useful thickness of thermal mass how much heat it can absorb and release over a diurnal cycle. For most common building materials, like stone or masonry, this is 50−150mm (2 to 6 inches).
The amount of thermal mass used should be proportional to the diurnal temperature range. Higher diurnal ranges require more mass. Where there are lower diurnal ranges (less than 6 to 8°C between daytime highs and nighttime lows), low thermal mass construction performs better.
Houses built to Net Zero/Net Zero Ready or Passive House standards have very small heating loads and smaller temperature fluctuations, meaning they don’t need as much thermal mass, or as much passive solar gain as a conventional stick framed house.
Glass To Mass Ratio
The ratio of solar exposed glass to exposed thermal mass in a room is critical and varies significantly between climate zones, floor plans, house designs, and construction assemblies.
If there is more thermal mass than the solar gain available, the thermal mass acts a heat sink and increases auxiliary heating needs. On the other hand, if there is not enough thermal mass to absorb the available solar gain, there will be daytime overheating and rapid heat loss at night.
The area of south-facing glass with solar access should range between 15% and up to 25% of the area of exposed thermal mass in a room.
Distributing Solar Gain
Convection currents are created when warmer air rises to the ceiling and air cooled by windows and external walls is drawn back along the floor to the heat source. With careful design, convective air movement can be controlled and used to great benefit but with poor design can be a major source of thermal discomfort. Minimise ‘accidental’ convective air movement in the heating season with an air-sealed, well insulated building envelope. Controlled convection can be used to warm rooms not directly exposed to heat sources; it can also reduce unwanted heat loss from rooms that do not require heating. Opening or closing doors controls the return air flow but impacts on privacy. Consider vents between spaces that can be opened or sealed. Openable panels (louvres or transom windows) over doors promote and control movement of the warmest air at ceiling level while retaining visual privacy but not auditory privacy.
Design the house so that thermal mass and the main heating sources are at lower levels.
Use high insulation levels and lower (or no) thermal mass at upper levels.
Ensure upper levels can be closed off to stop heat rising in winter and overheating in summer.
Use stairs and return air ducts to direct cool air drafts back to heat sources, located away from sitting areas.
Avoid open rails on stairwells and loft/balcony areas. They allow cool air to fall like a waterfall into spaces below.
Use ceiling fans to push warm air back to lower levels.
Minimise window areas at upper levels and use the highest performance window the budget will allow.
Maximise the openable area of upper level windows for summer ventilation.
Locate bedrooms upstairs in cold climates so they are warmed by rising air.
Implementing Passive Solar
Passive Solar Design in Action
For best passive heating performance, daytime living areas should face south. Ideal orientation is true south but orientations of up to 20° west of south and 30° east of south still allow good passive sun control.
Where solar access is limited, as is often the case in urban areas, energy efficiency can still be achieved with careful design. Homes on poorly orientated or narrow blocks with limited solar access can employ alternative passive solutions to increase comfort and reduce heating costs.
Active solar heating systems that use roof mounted, solar exposed panels to collect heat and pump it to where it is needed are a viable solution where solar exposure of glass for passive heating can’t be achieved.
Living areas and the kitchen are usually the most important locations for passive heating as they are used day and evening. Bedrooms generally require less heating, especially during the day. Bedrooms can be classified as living areas if considerable hours are spent there (for example, a child’s room, or a studio space).
Group living areas along the south elevation and bedrooms along the north or east elevation.
Utility and service areas (bathrooms, laundries and garages) are used for shorter periods, require smaller windows, and can be functional at cooler temperatures. Locate these areas as follows:
to the west or north-west, to act as a buffer to hot afternoon sun and winter winds
to the east and north-east, except where this is the direction of cooling breezes
Locate a detached garage to the east and west to create protected south-facing courtyards
Internal Thermal Mass
Internal thermal mass walls that are out of direct solar gain can also provide an ideal location for heaters, especially radiant units such as wood stoves or hydronic heating panels. The thermal lag will transfer heat to adjoining spaces over extended periods. Heating units (woodstoves, etc.) can create convection loops when operating. Where possible, locate them where they can draw cooled air back through traffic areas rather than living areas.
Air movement creates a cooling effect by increasing the evaporation of perspiration from exposed skin. Drafts increase the perception of feeling cold. Even air moving at 0.5m/s (barely enough to move a sheet of paper) creates a cooling effect equivalent to a 3°C drop in temperature. This is essentially what the ‘wind chill’ rating is on a sub-zero day. The air temperature could be -2°C, but with a strong wind blowing, the wind chill makes the air feel like it’s -15°C, for example.
While we want air movement in the house to ensure there is no stratification (hot air at the top of the house, cooler air in the bottom), it needs to be controlled. As an addition to controlled air flows, design floor plans to minimize the effect of convection currents that can set up naturally between windows and external walls and thermal mass sources, heating units, or occupants. This is done by ensuring the cool air that is flowing down the surface of the windows is directed through traffic areas (hallways and stairs), and not areas that will be occupied for long periods of time. Open plan designs, the ones that work best with passive solar, can also feature nooks for sitting, dining and sleeping.
Warm air can be circulated via a ceiling fan, a variable speed furnace fan, or via a dedicated balanced mechanical ventilation system (HRV or ERV) with a recirculation mode.
Locating thermal mass
Thermal mass is used to best advantage where it is exposed to direct solar radiation. Insulated concrete slabs make ideal thermal mass storage for solar heat gains, as do interior masonry walls. Other options are water filled containers and phase change materials, but they don’t fit quite so easily into standard construction or home design practices.
Thermal mass also absorbs reflected radiant heat. Thermal mass walls between southern living areas and northern sleeping areas are ideally located as thermal lag radiates daytime solar gains into sleeping areas at night and provides acoustic separation. Locate additional thermal mass predominantly in the southern half of the house where it absorbs most passive solar heat.
Ensure that woodstoves and other heating units are not going to ‘short circuit’ the thermal mass. Heating units should be linked to thermal mass that is not in direct sunlight, or on the northern side of a mass wall that is exposed to direct sunlight.
In cold climates, competely insulate under areas of slab-on-ground that are exposed to direct solar radiation.
Use low thermal mass materials and high levels of insulation in north-facing rooms.
Consider the balance between heating and cooling requirements when designing the house. Air movement within the house heats or cools thermal mass. In cold climates, locate mass away from sources of cold drafts like the main door, but close to warmer areas, or areas with warm air transfer, such as interior hallways.
Natural Ventilation in Passive Solar Design
A passive solar design that uses natural ventilation relies on the wind and the stack effect to keep the house cool in the summer where cool nights and regular breezes are the the norm. The stack effect is caused by convection. When using natural ventilation as a cooling strategy, the convection current is set up when the cool night air enters the house through open windows on the lower level, absorbs heat, rises and exits through upper level windows. When the opening area of windows is larger at the top than the bottom, a partial vacuum is created, pulling more air in through lower-level windows. The effect works best in open plan designs.
Natural ventilation can be controlled through landscaping. A fence, hedge, or row of trees that blocks the wind can be used force air either into or away from nearby windows.
Passive heating in renovations
Opportunities for improving or adding passive solar design features when renovating an existing home include the following:
Increase existing insulation levels and insulate any previously uninsulated ceilings, walls, and floors during re-cladding or re-roofing.
Design additions to allow passive solar gain and movement of passive heat gains to other parts of the house.
Relocate or resize windows to improve solar gain, restrict summer overheating, and minimize losses to the north.
Use high performance windows and glazing for all new windows and doors where possible.
Airseal existing windows and external doors, and replace warped or poorly fitted doors.
Consider switching the location of bedroom/living areas. Can the living space be shifted to the south side of the house without increasing the scale of the renovations?
Create airlock entries in cold climates.
Add interior doors and walls to create zones with similar heating needs.
Consider adding a solar porch or greenhouse/conservatory. Ensure the solar gain can be distributed to the house during the day and sealed off from the house at night.
Cool, cold and temperate climates all require varying degrees of passive cooling.
Use casement windows that allow maximum opening area.