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Tomorrow's Technology Today
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| In some cases, new windows will be required. Many
frames are so inefficient, you may wish to replace them too.
2.2 Should You Reglaze, Retrofit, or Replace? If you're thinking of upgrading existing windows in your home, you have essentially three options. In option 1, let's say you have old, single-pane, double-hung windows in wood frames and sashes. The sashes (the part that moves) and frames (the part the sashes slide in) are in good condition, and you want to keep them. So, you decide to reglaze with double-paned, insulated glazing (IG) units which are custom made to fit into the individual openings (Fig. 1). If the sash isn't thick enough, you may not be able to follow this option. |
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I In option 2, suppose the sashes are in poor condition, but the perimeter frame is OK. In this case, you decide to retrofit the glazing and sash while keeping the perimeter frame and retaining the same window style Fig 2).
In option 3, let's assume the old double-hung window is in such bad shape that nothing is worth saving; even the perimeter frame and trim are in bad shape. In this case, you elect to replace the whole window as a unit glazing, sashes, frames and all. This gives you the option of changing the style of window, say, from double-hung to vertical casement Fig 3)
Given these three options, make sure that you and your supplier are talking about the same type of installation.
2.3 Designing With Windows and Doors
The sun's energy is free. Solar energy can improve the lighting and comfort of your home, and it can cut your fuel bills; it can also provide too much warmth and cause overheating, both in summer and winter. Decisions you make at the planning stage about the number of windows, their size and location particularly in relation to the sun's orientation at different times of the year will be as important a consideration in determining your window requirements as their insulative properties or how they look.
For example, increasing window areas on the south side of a building can increase the contribution that the sun makes to heating the home in the winter, which may offset your heating costs. But be sure to design-in sufficient eave overhang to shade these south-facing windows in the summer months to prevent unwanted solar gain.
Until recently, an established rule of thumb in window placement was to install fewer and fixed windows on the north side of a home, to prevent excessive heat losses in the winter. Another rule of thumb recommended keeping east and west exposures to a minimum except when needed for aesthetics and daylight, in order to prevent excessive solar heat gain in the summer.
But over the past five years the rules for window placement have been turned on their head by advances in window technology. The arrival of high-performance windows (see Section 7) has given consumers more choice in window selection whether it's for an existing home renovation or for specifying in a new home.
Remember the following rule of thumb: keep the ratio of window area to floor area at about 1:10. That is, for every square metre of window area, make sure you have at least ten square metres of floor area. This will prevent overheating in the living space due to too much solar gain. There are also code requirements in many areas for kitchens, dining and living rooms.
If you combine traditional passive solar design principles (described in Section 4) with high performance windows (described in Section 7) you can get much greater net solar gain while reducing your energy consumption for space heating and cooling.
And remember, doors also play an important part in the energy performance of the home, especially if you have a lot of patio doors. Depending on the condition of existing doors, energy-efficient replacement units may contribute to increased comfort and reduced energy bills. An added bonus of energy-efficient doors is that they tend to be heavier and more robust than conventional doors, thus enhancing the security of the home.
2.4 What to Look for in Windows and Doors
The wide variety of windows and doors available on the market can make the task of selecting appropriate units confusing. Price may not always be the determining factor. The cheapest units you can find may not perform at the level you want. However, some inexpensive units may perform as well as, or even better than, higher-priced models. The best advice to keep in mind is that price is not always an indication of quality or performance.
The cost of energy-efficient, high-performance windows can be 10 to 15 per cent more than the standard double-glazed unit. However, many window manufacturers are switching over their entire production line to produce only high-performance units so, in effect, there is no price differential as far as they are concerned. Today, the high-performance window is becoming the industry standard. Read about it in Sections 7 and 8.
But it can still be confusing. For example, some window manufacturers include low-E coatings in their standard windows, with gas fills as an upgrade, at higher cost, while others offer gas fills and special coatings as an upgrade. (See pages 29 to 30 for an explanation of low-E coatings and gas fills.)
Once you've done your homework by reading this guide, you'll be better prepared to ask the right questions when you shop for windows and, equally important, know when you're getting the right answers.
The key is to select windows or doors that are as energy efficient as possible, given your needs and budget. Remember that some super high-performance windows may cost considerably more than normal high-performance windows. The energy rating (ER) system described in Section 6 will give you an indication of the energy benefits. In most cases, the more efficient unit will probably offer other advantages, such as better comfort and resistance to condensation in very cold climates.
Don't forget to consider the advantages and disadvantages of framing materials, as well as the maintenance required and the durability of hardware. Windows are a long-term investment.
Inspect samples before making a decision, taking into account the following basic features described in greater detail in this publication:
- Window type may be fixed or operable.
- Glazing type affects energy efficiency and the amount of light which passes through the window.
- Frame materials affect the insulation value, strength, maintenance requirements and longevity of the window.
- Energy performance (ER)—there may be a trade-off between efficiency and price.
- Warranties differ from supplier to supplier and window to window; compare before you purchase.
Section 3 Understanding Basic Terms
3.1 Window and Door types There are two types of windows: those that open and those that don't called operable and fed, respectively
(Fig. 4). Use as many fixed windows as codes allow, keeping in mind that floors with bedrooms need at least one operable window for emergency exit. Fixed windows are more efficient because of their better air tightness characteristics. They also offer the most safety and security.
Of the operable units, there are many forms: awning, casement, hopper, horizontal slider, vertical slider (either single- or double hung) and sum-and-tilt (fig. 5).
There are two ways of sealing operable windows to minimize air leakage: with a compression or a sliding seal. Windows with compression seals are generally the more airtight of operable types and should be the window of choice whenever possible. Casement, awning, hopper and turn-and-tilt windows, for example, should have a closure/locking mechanism that pulls the unit tight against the seal (Fig. 6). Make sure the gasket is a compression, neoprene rubber type. Doors are a little less complicated. They are either solid, solid with an insulated core, solid with window(s), or solid with an insulated core and window(s). Patio doors operate like a large horizontal sliding window. Hinged
French doors, with a solid centre post to close against, or rolling doors with a compression-fit like an aircraft door, are more energy efficient (Fig. 7).
Some materials reduce heat flow better than others. Solid wood doors, for example, are not as good as metal-clad, insulated core doors, depending on the style of door and insulation material used to fill it (Fig. 8).
Otherwise, doors have a frame, sill, optional glazing, and rough frame opening in a wall as do windows (Fig. 9). Like windows, some doors are even installed in the frame and sill system while still at the factory.
3.2 Glazing Parts Glazing is the generic term for the transparent or sometimes translucent material in a window or door. A window may be divided into one or more sashes, some of which may move and others which may be fixed. For example, a double-hung window generally has two move able sashes, while a single hung window has just one move able sash (Fig. 10).
A sash may be divided into two or more lights (panes of glass) held in place by mullions and muntins (Fig. 10).
3.3 Glazing Types
When we speak of windows, we tend to use the terms single-, double- or triple-glazed. These terms simply refer to the number of panes of glass incorporated into the window unit: single glazed one pane; double-glazed—two panes; triple-glazed—three panes (Fig. 11).
All windows in Canada should be at least double-glazed. To determine the number of glazings in a particular window, hold a light next to the glass and count the reflections (Fig. 12). In a double-grazed window, for example, you'll see two main reflections, corresponding to the number of glazings. (If you look carefully at each reflection, there are actually two reflections very close together, bouncing off both surfaces of each pane.) Most window manufacturers offer several types of glazing which affect the insulation value of the window and the likelihood of condensation forming on the glass. Sometimes, transparent plastic films are used to increase energy efficiency; these are also referred to as glazings.
A variety of coatings on the glazing surfaces —plastic films, or inert gases between glazings, for example can increase the insulating value of a double-glazed window to more than equal that of a standard triple glazed window. Coatings are often used with gas fill. (See Section 7 for a more detailed discussion of special coatings and gas fills.)
Now, most windows incorporate sealed, insulated glazing (IG) units in which two or more glazing layers are sealed around the outside edge to prevent air or moisture from entering the air space, eliminating dirt and condensation between glazings. If moist air finds its way into the sealed air space, condensation may form between the glazings. This is usually caused by a faulty seal and cannot be corrected except by replacing the IG unit.
Now, most windows incorporate sealed, insulated glazing (IG) units in which two or more glazing layers are sealed around the outside edge to prevent air or moisture from entering the air space, eliminating dirt and condensation between glazings. If moist air finds its way into the sealed air space, condensation may form between the glazings. This is usually caused by a faulty seal and cannot be corrected except by replacing the IG unit.
3.4 Spacers If you look between the window panes in a conventional double-glazed window where the glass meets the frame, you will probably see a strip of material, usually made out of metal, known as a spacer. The purpose of edge spacers is to maintain a uniform separation between the panes of glass (Fig. 13).
Edge spacers have traditionally been made of hollow aluminum, containing a drying agent or desiccant designed to absorb the initial moisture present at the time of manufacture in the space between the glazings. Metal spacers conduct energy easily and are a significant source of heat loss and poor window performance. The best are insulating spacers often made from either metal-reinforced butyl rubber or insulated silicone foam. These materials do not conduct nearly as much heat (see Section 7.4).
3.5 Frames
The frame can be a major source of window heat loss. This heat loss usually results from conduction through the frame material. Heat loss can also result from air leakage losses, sometimes increased by warping in a window's frame or sashes.
The frame/sash area can account for as much as 25 per cent of the total window area in some designs as high as one-third—so frames should be at least as well insulated as the glass (Fig. 14). Several frame materials are currently in use: wood, vinyl, thermally-broken (insulated) aluminum, fibreglass and various combinations (Fig. 15).
A solid wood frame is a good choice from an energy standpoint. Colour choices are unlimited but wood requires more upkeep, such as periodic repainting inside and outside.
Clad wood frames are protected on the exterior with a covering of pre-painted aluminum or coloured vinyl.
Clad frames are more expensive than plain wood but do not require exterior painting. Claddings be well designed and assembled to prevent water from becoming trapped behind them. A few manufacturers offer a micro-vinyl coating actually coating actually a high tech vinyl paint— which is sprayed onto the wood frame. This effectively eliminates any possibility of moisture getting in between the coating and the frame.
Aluminum frames are strong and durable, and good designs are available for residential windows. However, since aluminum conducts heat rapidly, the frame and sash must have a well-designed thermal break incorporated into their construction to prevent condensation and frost from forming on the frame. The design of the thermal breaks and their location in both the sash and frame, greatly affect their effectiveness.
It is difficult to judge whether an aluminum frame is equipped with a good thermal break. The ER rating (see Section 6) is your best indicator of overall frame performance. Be particularly wary of inexpensive aluminum replacement windows. Vinyl frames provide a 100 per cent thermal break and therefore good insulation, and do not require painting. However, some manufacturers reinforce the vinyl with metal, decreasing the frame's insulation value.
Remember also that vinyl expands and contracts with temperature, opening up cracks for air leakage. Hollow sections in vinyl or fibreglass frames may be filled with foam insulation to further decrease heat loss. Again, the ER rating is your best indicator of frame performance
Heat can also be lost around the window frame, through the installation shim space, usually 25 mm (1 in.) larger than the window unit in both horizontal and vertical directions. This is the space remaining between the installed window frame and the rough opening (Fig. 16). The shim space must be insulated and sealed. See Section 11 for more details.
3.6Hardware
Window hardware includes the hinges, casement cranks, handles, latch plates, etc., of operable units (Fig. 17). The quality of hardware and hardware placement can affect the performance of the weather-stripping. Weather changes can affect the durability of the hardware design, its attachment, and window members to which it is attached. During a cold winter, opening a window may cause problems if ice builds up, making it difficult to close the window tightly.
3.7 Weather-stripping
Windows should use durable, flexible gaskets to make an airtight seal between the operating sash and the window sill and frame. An airtight seal is also needed between a door and its frame. The air-tightness of the joint between operable sashes and a window frame or between a door and frame depends on the type of weather-stripping used and the amount of pressure that can be applied on the joint.
Compression seals (Fig. 18 A) which can be squeezed tightly between the moving sash and the window sill and frame, and which are resilient over many years and not subject to cracking or other deterioration, are better than sliding seals with brush-type weather-stripping (Fig. 18 B).
You may not be able to avoid brush-type seals, especially if you are selecting horizontal sliders. On these type of windows, look for thick brush seals with a thin flexible plastic flange embedded in the brush to minimize air leakage.
How Windows Perform
Before making a decision about which windows to buy, it is useful to review how windows perform, in terms of how they allow a home to gain energy from the sun, and how they affect energy loss when the sun isn't shining
4.1 Factors Affecting Gains
There are several factors affecting the ability of windows to capture solar heat. They include 1) placement and orientation; 2) design of the window unit (and the amount of clear window opening); 3) the type of glazing used; and 4) the amount of interior and exterior shading.
Placement and Orientation
Placement and orientation of the window with respect to the sun will be the primary determining factor affecting solar gains, although some gain is possible in all directions from diffuse sky radiation. During the winter, the sun's low elevation in the sky at midday enables it to shine through south-facing windows (Fig. 19 a). These solar gains can help reduce your heating costs during the winter.
In the summer, when the sun is much higher at midday, very little sun actually strikes a south-facing window (Fig. 19 b). And what sun does reach the window is at such a low angle that it reflects off the window. Awnings or a modest eave overhang can be used to shade south windows in the summer to minimize these unwanted heat gains even further. Properly placed, these shading devices shouldn't interfere with winter solar gains.
Overheating in summer tends to occur more from unshaded west facing windows and, to a lesser extent, east windows. Well-placed deciduous trees will reduce summer overheating while permitting desirable winter solar gains.
Window Design The design and heat gain factor of a window will also have a bearing on its ability to capture solar heat. A window with a wide frame and numerous small lights separated by mullions
and muntins has less glazing area available to capture solar energy (Fig. 20 a). By contrast, a window in the same rough opening with a thin frame and one large light will have a greater proportion of glass to frame area and so will allow more sunlight into the living space (Fig. 20 b).
Glazing Choice
The number of glazing layers will also affect solar gains. For example, a triple-glazed window with ordinary glass reduces solar gain by 20 per cent compared to a single-glazed window with the same glazing area. A double-glazed unit reduces solar gain by about 10 per cent. (Fig. 21).
Glazing coatings and tints also make a difference. Clear glass transmits the most solar energy into a building. Tinted glass and glass with special insulating low-E coatings (discussed in Section 7), can reduce solar gains by up to one-third. For example, a double-glazed window with a low-E coating on one glass surface may transmit up to 20 per cent less solar heat to the interior, compared to a double-glazed window of similar area with standard glass (Fig. 22) Different types of low-E coatings vary greatly in terms of their effect on solar gains. Some that are designed for southern climates are not appropriate for use in Canada. Shading
The shading of windows, either from interior drapes and curtains, or from exterior landscape elements such as trees, will also influence the amount of solar gain. On sunny days during the winter, keep the drapes open to admit as much solar gain as possible.
Remember that the type of trees and shrubs you plant near your windows may affect the winter solar gain potential of the windows. Select deciduous trees with thin branching characteristics for southern exposures. They will provide shade in the summer but will lose their leaves in the fall and allow more sunlight through.
4.2 Factors Affecting Heat Losses
There are several processes at work which influence rates of heat loss through window components. These processes follow a basic law of nature: heat energy tends to move from warmer areas to colder areas. There is no way to get around this fundamental principle; all we can do is slow the processes down.
The principal heat transfer processes in windows are radiation, conduction and convection. In addition, air leakage is responsible for a significant portion of heat loss.
Radiation, Conduction and Convection
Absorbed by the inside pane of a double-glazed window, heat moves to the cooler outside pane and is released to the outdoors. This heat loss through windows takes place through the glazing (by radiation); across the spacer material which separates the two glazing lays at their edges and through the frame of the window (by conduction); through the movement of air in the space between the two glazings (by convection); and between the moveable or operable frame components (by air leakage) (Fig. 23).
Radiation losses through the window glass represent about two-thirds of the total heat loss in a standard window. Because ordinary glass readily emits heat to colder surfaces (i.e.., has a high emissivity), radiation losses can be reduced by lowering the emissivity of the glass (hence the term low emessivity or low-E glass)
Conduction losses in windows occur primarily through the edges and frames of the units. Advances in materials and designs that more effectively use insulating materials have dramatically reduced these losses.
Convection losses occur due to air movement between the spaces of multi-glazed windows. If the space is too small, conduction through the air is significant. If the air space is too large, the still air will begin to rise as it is heated on the warm interior side, and fall as it is cooled on the cold exterior side of the window. This convection movement of the air passes heat to the exterior. The best spacing to minimize convection losses is 12 to 16 mm between the glazings. Other gases (argon, krypton) are often used to reduce convection heat loss. Optimum spacing for these gasses can be different.
Air Leakage Air leakage is a significant contributor to energy costs during both heating and cooling seasons. Most of the air leakage of operable (i.e., openable) windows occurs between the
window's sash and frame, or the meeting rails of a sliding sash (Fig. 24). Bigger windows tend to leak less air per unit area. Air leakage can also occur in poorly constructed fixed windows between the insulated glass unit and the frame. (Remember: even in these types of windows, holes are required to effectively drain rainwater.)
Windows with the lowest leakage rates, regardless of type, tend to be fixed windows, that is, windows you can't open. Operable or openable windows come in many types, as described in Section 3. The operable windows with the least rates of air leakage are awning, casement and similar types with a closure mechanism which pulls the sash against a compression gasket, as shown in Fig. 18 (a).
Air leakage can also be a big problem if the windows are poorly or carelessly installed in the rough opening. If the space between the outside perimeter of the window frame and the rough opening isn't sealed with either caulking or foam insulation, air will leak through it. This space should be insulated and sealed before the window trim is attached.
4.3 Balancing Gains and Losses
As we have seen, there is a great deal of two-way "traffic" passing in both directions through windows. South windows often gain more solar energy during the day than they lose at night through convection, radiation and conduction losses.
North windows are usually net losers of energy, while east and west windows tend to be neutral during the heating season. However, during the summer, west windows may be net gainers of energy, posing an overheating problem for the occupants.
High-performance window technology is pointing the way to significant improvements in this balancing act between gains and losses maximizing gains when needed, while at the same time minimizing heat transmission as never before.
Condensation and Windows
5.1 What is condensation?
The occupants of a house rightly feel that condensation on the inside surfaces of windows is not good. They immediately think of obstructed visibility, reduction of the intensity of natural lighting and, above all, deterioration of interior finishes (rings, stains, peeling paint) and mould.
Superficial condensation occurs when the surface temperature of a solid (glass, sash, frame) is lower than the dew point of the humid air in its immediate vicinity. The moisture naturally present in the air in the form of vapour changes into liquid water on contact with these cold surfaces. The resulting droplets form a film of water and run down the glass when the condensation is heavy or does not evaporate fast enough. In the case of windows, condensation will often occur at the edge of the glazing because of conduction through the spacer and air convection within the glazing cavity. Such condensation can be decreased or eliminated by raising the inside surface temperature and/or decreasing the relative humidity of the indoor air.
5.2 Prevention Techniques
Reducing or eliminating condensation often means using several complementary techniques. These techniques concern the window itself, the method of installation, the interior window accessories (curtains, blinds, valances), the arrangement of heat sources (hot air registers, baseboard heating, convection heaters) and the relative humidity of the indoor air. The overall condensation resistance of a window depends on each of the above factors.
5.3 Condensation Resistance of Windows
Several techniques are used by manufacturers to increase the condensation resistance of windows. They include windows filled with a convection-limiting inert gas, low-emissivity coatings that increase the temperature of the glass, insulating spacers that reduce heat conduction, and non-conducting sashes and frames (Fig. 25).
Canadian standards make it possible to determine the condensation resistance of windows and doors. The temperature index (I), which may be found on the temporary CWDMA label (see Figure 28 on page 24), makes it easy for the consumer to compare the condensation resistance of several products. The higher the temperature index, the better the product's condensation resistance. The standard used to measure the temperature index requires a minimum index of 40. However, the higher the humidity level in the house and the lower the temperature outside, the higher the temperature index should be in order to prevent condensation problems. Any good salesperson can advise you on selecting the minimum temperature index for your house, lifestyle and locale.
5.4 Window Installation
To maintain or improve the temperature index of the window, it should be installed according to the following rules:
- position the window as close as possible to the interior finish;
- insulate the space between the window frame and the rough opening around the perimeter of the window; and
- seal the joint between the frame and the rough opening on the interior side.
Section 11 deals with window installation in more detail.
5.5 Interior Window Accessories To maintain the condensation resistance of the window, interior window accessories, such as curtains, blinds and valances, must not inhibit or impede the movement of air at the surface of the window. Any restriction of air movement will reduce the condensation resistance and thus increase "sweating" on the window. Figure 26 illustrates the various installation techniques that are recommended, acceptable or not recommended.
Arrangement of Heat Sources
Although energy-efficient windows are now available, the heat loss through them is still greater than through the adjacent walls. This is why windows with heat sources located below them are less prone to condensation. When the heating system is in operation, the air temperature will be higher in the vicinity of the window than in the centre
of the room, resulting in increased condensation resistance. Care must nevertheless be taken to ensure that the hot air from the registers does not flow directly on to the interior surface of the glass, as this could give rise to thermal stress problems in the glazed unit that could cause the glass to break.
5.7 Controlling the Relative Humidity of the Indoor Air
Replacing old windows with new, more efficient ones generally results in a significant improvement in the airtightness of the building enclosure, and thus substantial energy savings and improved comfort for the occupants.
However, when living habits and the production of humidity in the house (showers, baths, cooking activities, plants, etc.) remain unchanged, and there is a significant reduction in the rate of air exchange, the resulting relative humidity at certain times of the winter may exceed the maximum value and cause condensation on the inside surface of new windows.
Reducing the amount of humidity in the house may enable you to limit or eliminate the problems caused by humidity. Some simple but effective measures may be applied.
- If your heating system is fitted with a humidifier, or if you use portable humidifiers, disconnect them.
- Avoid hanging laundry inside to dry, and make sure that the exhaust from the clothes dryer is vented outside.
- If you have a crawl space under your house, cover the beaten earth with 0.15 mm (6 mil) polyethylene. The crawl space may have to be ventilated during the summer.
- Make sure that your basement is well drained and protected against excess moisture. Also, make sure that gutters and the slope of the land around the house drain water away from the house.
- Try not to produce too much humidity. Plants, laundry, showers and cooking without lids are major sources of water vapour.
- Avoid drying firewood in the house. A cord of wood can release more than 270 litres (60 gallons) of water.
If the signs of excessive humidity persist, you should increase the ventilation of your house. When the frequency of condensation is low (once or twice during winter), you can reduce or eliminate the problem by briefly opening two windows located on opposite walls or by turning on the kitchen or bathroom exhaust fan. If the frequency of condensation is unacceptable, you should install a controlled mechanical ventilation system (Fig. 27). Systems incorporating a heat recovery unit and a relative humidity control should be preferred.
Ratings Game
Now that you know something about window technology and how windows perform, you're probably asking yourself, "How can I verify the performance claims of manufacturers? How do I compare different window types, or the different product lines of various manufacturers? How do I decide which window type is best for my situation? Most importantly, how can I be sure I'm getting what I pay for ?"
6.1 Certification, Testing and Standards
With the rapid growth in window technology improvements, a number of organizations and industry associations have been trying to sort out performance standards and certification procedures, both for industry and the protection of consumers.
For example, the Canadian General Standards Board (CGSB) has had in place for some time an insulated glass unit (IOU) standard which sets performance limits on the durability of window edge seals (CAN/CGSB 12.8). CGSB also has a standard for sliding doors (CAN/CGSB 82.1).
The Insulating Class Manufacturers' Association of Canada (IGMAC) certification program requires manufacturers to meet quality control standards and the CGSB edge seal standard (CAN/CGSB 12.8). All IGMAC-certified products bear the following information: IGMAC logo, along with a date, the company name and the place of manufacture. This information is normally stamped into the spacer bar between the glazings or etched onto the glass if a non-metallic spacer is used.
In addition, the Canadian Window and Door Manufacturers' Association (CWDMA) has recently developed a voluntary certification program for windows and patio doors, which is the most comprehensive verification process available.
The CWDMA program will use independent auditors to verify compliance to the CSA A440 window standard (see section 6.2), the CGSB 82.1 standard for sliding doors, and an energy performance rating according to CSA A440.2 (see Section 6.3). Auditors will oversee the assembly of test samples, and ensure that the product manufactured daily is equivalent to that which was tested.
Decisions on initial certification and on-going compliance with the program, including quality control requirements, will be made by an independent Certification Council, made up of individuals not involved in the manufacture of windows and patio doors.
Testing and energy simulations are performed by the approved laboratory or simulator of the manufacturer's choice. In addition to a permanent label bearing the certification number of the specific product, a temporary label will prominently feature the Energy Rating or ER number and all the relevant A440 performance data (Fig. 28).
If a specific model complies with the standard, it may also be listed by the Canadian Construction Materials Centre (CCMC). CSA test results for various windows should be shown in the manufacturers' literature if not you may be able to obtain the data from CCMC at the National Research Council in Ottawa. Their address is listed in Section 12 of this guide. 6.2 CSA Standard
A440
The Canadian Standards Association (CSA) has established a standard (CSA-A440), called an omnibus standard because it applies to windows constructed from diverse materials (vinyl, wood and aluminum). It describes how to measure and rate a window's airtightness, watertightness, wind resistance, condensation resistance, forced entry resistance, ease of operation, and other requirements. It also sets out minimum requirements for all components and their materials, from hardware, insect screens, or weather-stripping, to finishes and adhesives. In addition, all windows must be designed to allow on-site reglazing.
The CSA-A440 rating is a minimum to start with when purchasing new windows. If a window complies with the new CWDMA certification, the manufacturer's name should be permanently marked or engraved on the glazing and a second label should be attached which shows its classifications based on airtigbtness (A1 to A3), watertigbtness (B1 to B3 or higher), and wind resistance (C1 to C3)—the larger the number, the better the performance.
Independent testing assures each model meets minimum levels of performance in the three categories. The National Building Code and most provincial building codes now require that windows used in new low-rise residential construction and renovations meet CSA-A440 requirements and have at least an At, B1 and C1 performance rating (Fig. 29).
In general, a window can only achieve high resistance to air, water and wind by incorporating tight-fitting corner joints, good seals, proper gaskets and weather-stripping. Look for a label with a minimum A1, B1, C1 rating and a high Energy Rating number (A440-2; described in Section 6.3). There are also optional parts of this standard, against which manufacturers may wish to rate their product, in such areas as condensation resistance, forced entry, etc.
6.3 The Canadian Energy Rating (ER) System
Although CSA-A440 protects the consumer and is the minimum performance standard referenced in most building codes, the bottom line for the energy conscious consumer is a window's Energy Rating, or ER number, based on the CSA-A440.2 Energy Performance Evaluation of Windows and Sliding Glass Doors standard, which applies to all windows and sliding glass doors.
A window's ER rating is a measure of its overall performance, based on three factors: 1) solar heat gains; 2) heat loss through frames, spacer and glass; and 3) air leakage heat loss. A number is established in watts per square metre, which is either positive or negative, depending on heat gain or loss during the heating season. The range is wide. Figure 30 lists the typical ER ratings for windows most commonly available.
The ER rating system is based on a formula which calculates a single ER number for a specified window size in each of seven window categories (e.g., a 600 by 1220 mm casement selected as representative of that window type). Because all window ER ratings are evaluated in the same way, this makes it easy to do comparison shopping between different manufacturer-although a consumer should be aware that the rating given on a window's label will be for that window in the standard size, and not that particular window.
In addition, data is also available from the CWDMA Certification Program. The ER number, derived from using CSA A440.2, is featured prominently on the removable label, illustrated in Fig. 28. This will allow consumers to compare the thermal performance of window and patio door products in a given price range.
Additional information is also included in the CWDMA Certification Program directory. This information is verified in two surprise audits per year.
However, there are still several things to keep in mind when comparing ER numbers of different windows. As you can see by looking at Fig. 30, most fixed windows tend to have better (higher) ER ratings than operable ones. There are two reasons for this. First, the standard size for a fixed window is nearly twice as large as most of the operable windows and thus has more glass area relative to frame area. Frames are also thinner because they do not need separate moveable sashes. This translates into more solar gains and less frame losses —hence a higher ER rating. Second, fixed windows tend to have less air leakage compared to operable ones. There is always going to be less air leakage with a window you can't open than with one you can. This translates into less heat loss and a higher ER rating for fixed windows.
It stands to reason from this discussion that, when you shop, make sure you're comparing apples to apples. Compare ER numbers within each window category— fixed, casement, sliders, etc.-because the ER ratings vary considerably for each type.
Although only intended for comparison purposes, the ER rating may be a good indicator of the effect windows will have on the annual heating costs in the home. A positive ER rating means the windows actually add more heat to the home than they lose during the heating season, decreasing the home's heating costs. An ER rating of +2 should be considered a minimum performance level for a fixed window.
A window with an ER of zero loses as much heat as it gains over the heating season. Consequently, it will have no impact on the house's annual fuel consumption.
A negative ER means a window loses more energy than it gains, making the heating system work harder. The lower the number, the more heat is lost and the harder the heating system has to work in colder weather. For example, an ER of -40 is worse than an ER of -20. An ER rating of -11 is a good minimum performance level for an operable, standard high-performance window.
However, some of the super high-performance windows coming onto the market (for instance, ones with low-profile, foam-filled fibreglass frames, insulated spacers and Low-E, gas-filled triple-glazing) may provide positive ER numbers even for operable units.
The ER rating system is a major step in giving consumers the information they need to make informed decisions about the energy performance of windows. It is a rating solely of a window's performance regardless of how, or with what materials, it was built.
Just looking at the ER numbers might not be enough information if your house differs from the average house used for the calculations (i.e., if it is heavily air-conditioned because of large internal loads, or has extreme solar gains that come with a passive solar-heated home). How do designers deal with this problem? The answer is the ERS rating system.
6.4 The ERS Rating: Adding Location and Orientation
A window's ERS rating is its ER value calculated for a specific house. It is calculated based on house type, municipal location, window orientation and window size. Although simple ER comparisons are often all that is needed for most houses, the design of an ultra-low energy home (like the Advanced House), a passive solar house, or solar spaces might warrant use of ERS for comparison. By obtaining solar heat gain, rate of heat flow and air leakage characteristics of a particular window from the manufacturer, a more accurate picture of window performance is calculated according to the CSA A440.2 standard.
The ERS values are used to make comparative estimates of the effect on a home's annual heating energy requirements of installing a particular window in a specific orientation. It allows the designer to compare two seemingly identical window-by their ER numbers— while ensuring that the right window is selected for the right location and compass orientation.
6.5 Making Ratings Work for You
Energy performance is only one of many considerations in the purchase of a window. Appearance, price and durability are also important considerations. A high-performance window that meets CSA standards and has a good ER rating will also tend to be better designed and manufactured, and will offer better resistance to condensation.
High-performance windows, described in the next section, offer other benefits too better comfort levels, less condensation problems and lower sound transmission. While these benefits are not something you can readily put in the bank, they may be important to consider as you make your decision.
High-performance Windows the New Technology
The window industry has been quick to develop alternative window technologies to address most of the performance shortcomings of conventional glazing systems. Its efforts over the past decade have been nothing short of revolutionary, and the end-result is the high-performance window, which is several times better than the windows of just a few years ago.
The list of high-performance window improvements currently available- - low-E coatings, inert gas fills and insulated frame and edge components-are indicative of the recent advances.
High-performance windows come in a wide variety of window types and applications. It can be very confusing for the uninitiated to sort through the new improvements. Understanding this new technology, as well as learning to use the new ER rating system, are important steps to making informed decisions about new window purchases.
7.1 Low-E Coatings Standard window glass easily allows the sun's energy to pass through it. However, at night, it is equally effective at emitting infrared heat energy back through it to the exterior through the process known as radiative heat loss (Fig. 31). This high emissivity characteristic of conventional glazing has led researchers to develop low emissivity (low-E) coatings.
A low-E coating is a thin, invisible metallic layer only several atoms in thickness-applied directly to glazing surfaces. In a typical double-pane application, the low-E coating is normally applied to the exterior face of the interior glazing (Fig. 31).
A low-E coating works in an ingenious way: while it is transparent to short-wave solar energy, it is opaque to long-wave infrared energy. What this means is that a low-E coating allows most of the sun's solar spectrum (including visible light) to pass through the window to the interior. But the coating reflects most heat energy (from room temperature objects) back to its source, which is a benefit both in the winter, because it keeps the heat in and in the summer, because it keeps out the heat radiated from warm objects outside.
A low-E coating on one pane in a double-glazed window can give the window an insulating value about the same as a standard triple-glazed unit, without the added weight of a third glazing
The lower weight reduces wear and tear on the window's hinges, casement cranks, etc.-making it easier to operate and giving the window longer life. It also reduces transportation costs, which means lower prices.
There is usually some loss of solar contribution due to the low-E coating (Fig. 35). But while this reduces the benefits of passive solar heat gains somewhat, it is more
than offset by the improved insulative value of the low-E window at night. An added bonus is that fewer UV rays make it through, which can mean less fading of carpets and fabric. There are now many different types of low-E coatings with different performance characteristics. Northern low-E coatings are probably your best compromise in a heating climate like Canada's. They maximize solar heat gains and reduce heat loss at night. Solar control low-E coatings might be justified on west-facing windows when no other means of solar control is possible. These reduce solar heat gain as well as visibility, and are often tinted.
In most cases, the consumer has little control over window location, especially in an existing home. However, if you're designing a new home you may wish to use the ERS rating to compare different glazing options in different orientations.
7.2 Gas Fills
The other big advance in window technology has been the introduction of intert gas fills into the space between glazing (Fig. 36). The term inert refers to a class of chemically stable, non-reactive (safe) gases. Argon and krypton are the usual choice, with argon being the most common and cheapest.
Filling the space between glazing layers with argon gas does two things: 1) it reduces conduction heat loss, because argon has a lower conductivity than air; and, 2) it reduces convection losses, because it is heavier than air and suppresses gas movement between the glazings (Fig. 36). Krypton gives slightly better performance than argon and permits a smaller optimal spacing between panes (about 8 mm). A narrow pane space requires less of this much-more-expensive gas, and allows multiple-pane systems with less chance of stress breakage. Since argon is more cost-effective, an increasing number of manufacturers offer it either as a standard feature or as an inexpensive upgrade.
Low-E coatings are also applied to thin sheets of transparent polyester, and suspended in the cavity between glazings (Fig. 37) This combines a high solar transmission with a low emissivity. Some films are designed to combine low emissivity with reduced solar transmission, making them ideal for southern climates or west-facing windows if solar gains are a severe problem during the summer. 7.3 Special Films
While these films are effective in certain applications, you need to be sure that both you and the window supplier or manufacturer select the right film for the right application. Researchers are working on exciting new categories of smart windows electrochromic, thermochromic and photochromic referred to as "switchable" glazing.
The most promising are electrochromic films that allow the amount of sunlight passing through windows to be controlled by means of a small current running through a transparent electrolite layer in the window. The biggest application for these films in the residential sector will be in buildings with large amounts of west glazing, where overheating in the summer is a problem.
Stay away from the pressure-sensitive after-market films which can be applied directly to existing windows. They are designed for the commercial building market. While some of these solar control films do have low-E coatings, they also have very low solar transmission factors. In other words, the energy saved in heat retention may be more than offset by the large reductions in solar gains.
7.4 Low-Conductivity Spacers Once radiation losses have been reduced Fig. 38 through low-E films, and convection and conduction losses through the glazing have been reduced by gas fills, the spacer at the perimeter of the window becomes the weak thermal link in the window unit. As discussed in Section 3.4, most spacers have traditionally been made out of hollow aluminum. Although lightweight and durable, this metal is, unfortunately, very effective at conducting heat.
From an energy efficiency point of view, the new low-conductivity spacer is a major improvement. Many different approaches and materials are appearing in the marketplace, but performance varies considerably. Generally speaking, these spacers can improve the energy performance of a low-E, gas-filled window by as much as 20 per cent (Fig. 38). Use the ER ratings to compare spacer effectiveness.
These better spacers also keep the inside glass warmer at the perimeter, which reduces thermal stresses on the glass and reduces the likelihood of condensation in cold weather.
7.5 Better Frame Materials
Window frames are another weak link in the overall window unit, as mentioned in Section 3.5. Recognizing that up to one-third of the overall window may be frame materials and that high-performance glazing is better insulated than most conventional frame materials manufacturers have moved quickly to develop more efficient alternatives.
Window frames that combine different materials and take advantage of the strengths of each are available from a growing number of manufacturers. The best energy performance in window frames has been achieved using a fiberglass frame with foam insulation in the frame cavities.
The bottom line on frames is that if you are investing in windows with low-E coatings, gas fills and low-conductivity spacers, then select a frame material which minimizes conductive heat losses.
Remember also that frames can have a significant effect on solar gains (and the ER rating). Stronger materials that allow narrow frames and sashes like thermally-broken aluminum or fiberglass, allow more glass area and solar gain. These are called low profile frames. Again, frame thermal efficiency will be reflected in a higher ER number.
7.6 Design Summary
In the majority of cases, if you are replacing all the windows in your home, you will probably select the same glazing on all sides. In this case, make your selection on the basis of ER rating numbers. In exceptional cases, such as passive solar homes or sunspaces, more detailed comparisons may be required.
Keep the following principles in mind as you decide on your window approach:
- A few large windows are better than many small ones. Larger windows reduce the proportion of frame to glazing, and maximize overall performance.
- The thermally weakest areas of a high-performance window are frames and edges; once the centre of glazing is efficient, the frame and edge losses will be proportionally higher, so look for insulated (non-metal) spacers and thermally broken, low-profile frames.
- Avoid large areas of west-facing glass. The sunsets may be beautiful, but your air conditioning bills won't be.
- Operable windows should be limited to locations where ventilation or emergency exits are required by codes.
- Reduced frame and sash areas contribute to better overall window performance.
- Avoid clip-on mullions and muntins. They create additional shading and reduce solar gains.
- For passive solar designs, the ERS rating system may be used.
- In all other cases, select windows based on ER numbers.
The Benefits of High Performance Windows
The technology of high-performance windows may be impressive, but the benefits are equally compelling. And these benefits extend beyond the more obvious ones.
8.1 Savings in Heating Costs First and foremost, high performance windows are energy-efficient. They offer immediate savings on home heating costs. Depending on the house design and the existing levels of efficiency in the rest of the building, switching to high-performance windows should yield 9 to 18 percent reductions in space heating costs. (Fig. 39).
8.2 Savings in Cooling Costs
Concerns about energy efficiency are not limited just to the heating season. In many parts of Canada, summertime heat requires space cooling. High-performance windows work equally well at keeping the heat out in the summer months. The same low-E coating that keeps infrared (heat) energy inside the home in the winter, keeps unwanted heat out in the summer. This keeps the interior cooler and cuts down on the need for air conditioning.
Studies have shown that, for most of Canada, it is still appropriate to choose your windows on the basis of the ER number. Exceptions to this rule are those few locations where air conditioning costs are high, relative to the amount spent on space heating. In these few cases, consider west-facing windows with a lower solar heat gain potential.
8.3 Increased Comfort
In houses with conventional windows, air leakage, drafts and radiative heat loss all contribute to occupant discomfort which the heating system must try usually with only limited success to overcome. But high-performance windows are better insulated and maintain a much higher surface temperature on the interior glazing. This characteristic, together with effective weather-stripping and proper installation, makes the window "feel" warmer to the occupant.
8.4 Higher Humidity Without Condensation
Improvements in comfort extend beyond the reduction in drafts and cold spots near windows. During the winter, outside air is very dry and can significantly lower the relative humidity (RH) inside. This in turn can lead to annoying static electric shocks when touching doors or light switches, as well as dry throats and other irritants.
If you try to compensate by humidifying the air, then once you get above about 40 per cent RH, there is the risk of condensation forming on the windows. This reduces visibility and can lead to deterioration of the frame components and mould growth.
High-performance windows can change all this. The higher inside glass temperatures and improved thermal performance of edge spacer and frame components allow much higher RH levels inside (Fig. 40). This can reduce if not eliminate RH-related problems associated with poor windows. Since higher humidity levels are possible with better windows, this means reduced static shocks, improved health, and healthier plants. Figure 40 shows the relative humidity at which condensation will start to form at the center of the glass of different types of glazing. However, condensation will usually form at the edge of the glass at a lower relative humidity.
8.5 Lower Sound Transmission
The heavy gas fills in high-performance windows which reduce conductive and convective heat losses also reduce sound transmission from the exterior to the interior. The greater the number of panes, the better the sound absorption. Sound attenuation is complex, depending on frequencies and other factors; some benefit may be realized with high-performance windows.
8.6 More Daylight
The advent of high-performance windows is allowing larger glazing areas to be incorporated into house designs, in both new construction and renovations, without the penalty of either high heat loss in the winter or high heat gains during the summer. This not only enhances energy efficiency and improves the view, but it also lets in more daylight which may lead to reductions in the use of electric lighting.
You save twice when you take advantage of this natural light: first, you save on electricity used for lighting; second, because a conventional light bulb uses only 15 per cent of its energy for lighting and wastes the other 85 per cent as heat, you lower the home's cooling load in summer. This is especially the case in larger buildings.
8.7 Increased Passive Solar Potential
Conventional south-facing glazing offers, at best, a break-even proposition in terms of balancing heat gains and losses. In other words, the energy that south-facing windows gain during the day through solar inputs is about equal to the energy they lose through radiation, convection, and conduction heat loss during the night.
High-performance windows are changing all this. With the right selection of low-E coatings and gas fills, it is now possible for most windows to gain more energy during the day than they lose at night.
With window frame areas kept to a minimum, it is now possible to have even larger window areas and still get net energy gains. In addition to saving on space heating costs, this can give the house a brighter, more open feeling.
8.8 Reduced Mechanical Complexity
On a cold winter night, conventional windows (RSI 0.35 or R-2) lose about ten times as much heat as an equal area of a reasonably well-insulated wall (RSI 3.52 or R-20). It is for this reason that architects and heating contractors have been forced to locate heating registers, convectors, and radiators directly under windows. This compensates for the high heat loss and air leakage in close proximity to these windows.
Lower air infiltration and the reduced conductive heat loss of high-performance windows may make the practice of perimeter heat distribution less important. It is now possible, thanks to high-performance windows, to locate heating registers on interior walls, either at the floor level, or near the ceiling. This reduces the length, diameter and complexity of heating duct layouts. Discharge outlets on an inside wall near the ceiling may provide more comfort, especially with air conditioning.
Reducing the length of duct or piping runs in the home saves on capital material costs as well as installation time, in either new homes or renovations. Shorter duct runs with smaller diameters can also mean smaller fans and less energy to run the fans. In some cases, cost savings may offset the additional costs of upgrading windows.
Doors, Patio Doors and Skylights
9.1 Doors
Doors have less impact than windows on the energy consumption of a home unless they are patio or garden doors simply because there are fewer of them. They come in a variety of materials, some of which reduce heat flow better than others. Depending on style and insulation material, for example, metal-clad doors are more efficient than solid wooden doors. No matter what the material, ill-fitting doors lose even more energy and can make the home drafty and uncomfortable.
Heat may be lost through the door and frame, between the door, frame, and sill, through glass in patio doors or doors with windows, and between the door frame and rough frame opening (Fig. 41). Heat loss through doors can be reduced through careful choice of the door, its location, and proper installation and maintenance. You can reduce heat loss simply by placing a door out of the path of prevailing winds, by locating it on the leeward side of a house, or by providing windbreaks. Another option is the use of an air-lock vestibule which traps the air between the exterior door and the interior of the house. Properly designed and installed storm doors will provide some degree of increased efficiency, as well as protection from the weather. And, with screen inserts, they'll provide summer ventilation. In southern and western exposures, care should be taken to avoid heat build-up between the doors which may cause the main door's finish to blister. In extreme cases, the main door may actually warp due to this heat build-up.
Missing or worn weather-stripping, improperly located strike plates, frames which no longer fit the door correctly, or warped doors that no longer contact the stops are the main contributors to air leakage. These problems can all be corrected by a carpenter or competent do-it-yourselfer.
A badly deteriorated door should be replaced with a new one with energy-efficient insulation. Select good quality units and install them properly.
New insulated doors are usually made of foam and wood I covered with metal (Fig. 42). Door frames are normally wood, clad with metal or vinyl. Doors that are mainly glass and are used as windows (for the view, daylight, etc.) should be compared for energy performance by their ER rating. Glass inserts and side lights should have at least double glazing with at least 12 mm (1/2 in.) of air space between glazings and be compared on the basis of the U-value (or R-value) calculated for the complete door system. Using the existing casing, the door and frame can be replaced with a factory-made, core-insulated, pre-hung unit. Installation takes less time than site-assembled systems, air seals are tighter and more durable, and these systems come with a thermally broken, adjustable sill assembly to reduce heat loss even further. A variety of materials may be used for the door face and framing, insulation, and weather-stripping.
In summary, when selecting doors for energy efficiency, look for:
- cores of materials that maintain high insulating values;
- wood, vinyl, or thermally broken metal frames;
- weather-stripping fabricated from high-performance, durable materials;
- low air leakage rates (for pre-hung door systems);
- maintenance-free framing materials; and
- a high ER rating or a minimum of double-glazing with a 12 mm or greater air space.
For details on specific doors check with the manufacturer.
9.2 Patio Doors
Sliding glass patio doors are popular in Canada, and can be energy efficient if selected on the same basis as for windows. Sliding glass doors are covered by the CSA A440.2 Energy Rating (ER) system. With an existing patio door of good quality and in reasonable condition, air leakage may be reduced by replacing the door's gaskets, weather-stripping and hardware. Doors not used in winter can be sealed shut with a removable sealant, or covered on the interior with heat-shrink plastic kits are available to fit most doors.
A door in poor condition should probably be replaced. New sliders perform better than older models. However, hinged French doors with a centre post to close against (Fig. 43) will be more airtight, although they would still not be suitable in a more severe northern climate.
The same basic guidelines on glazing and frame materials apply for doors as for windows.
9.3 Skylights
Skylights can bring added light to a home and make it more attractive. However, they lose far more heat than a standard roof and window, and can present special problems including water leakage, condensation, and summer overheating. The bottom line on skylights is that although aesthetically pleasing, from a heat loss point of view they are lime more than a hole in the roof covered with glass. In addition, because of their different solar heat gain characteristics, they are not yet covered by the CSAA440.2 Energy Rating system.
Skylights should be as resistant to heat loss as possible. This means that high, poorly insulated curbs should be avoided. The glazing should be as energy-efficient as possible. Low-E, gas-filled, insulated glazing units are a good choice. You may also want to consider light-reflective glass to reduce overheating, although this may also reduce the amount of daylight.
An exterior awning, reflective film, or even whitewash can be applied in summer to reduce overheating problems. An extra layer of glazing on the inside of a skylight may also reduce condensation and heat loss.
If a skylight is badly deteriorated but necessary for day-lighting, a replacement unit should be purchased. It should have the same features as high-performance windows, including a tightly sealing closure mechanism. On steep roofs, an operable roof window can provide many of the features of a good quality vertical window including the ability to ventilate (Fig. 44). Roof windows can also be equipped with blinds to reduce unwanted summer sun.
Deciding What You Need
10.1 Shopping Checklist
For your protection, make sure the high-performance window you buy has CWDMA Certification, so you know the window has been tested to the CSA A440 window standard, and that performance claims have been verified. As well, the temporary CWDMA certification label will prominently feature the Energy Rating (ER) number and all A440 performance levels. Take this handy checklist when you go window shopping. Ask to see the CSA ratings.
Window Shopping Checklist
Performance results
- IGMAC label present on sealed glazing unit
- CSA A440 compliance
- CSA air leakage level (minimum A3)
- CSA water penetration level (minimum B3)
- CSA wind load level (minimum C3)
- CSA condensation resistance
- CSA forced entry
Energy performance
- Certified ER rating available
- ER: -10 for operable and +2 for fixed are good levels to aim for.
Resistance to wear and tear
- Maintenance-free materials
- Joints well-sealed
- Easily operated, well-balanced mechanisms
- Strong, durable hardware
Style
- Meet aesthetic needs
- Suitable for size and orientation
- Non-operable except for code or ventilation
- requirements
Price
- Three quotes for product and installation (site estimation)
Warranties
- Installation: minimum 90 days
- Sealed glazing unit: minimum five years
- Window: minimum one year
In northern areas, consider durability and operability under extreme conditions. Opening windows in very cold conditions can cause a heavy accumulation of ice from the condensing interior air, making them difficult to close again. Operable windows in northern houses must be extremely sturdy.
Choose a replacement window-type that is compatible with the exterior architectural style of the building. Select high-performance windows that have the same "look" as the original windows. When replacing windows with divided lights, manufacturers may provide removable grid inserts that match the appearance of the original windows or mullions placed within the sealed glazing unit. Avoid the latter if possible (at least without a solid guarantee), since many materials have been found to be incompatible with glazing and other materials.
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