Windows should be properly chosen. High-performance windows are critical for day-lighting but also for comfort and energy savings; or for solar heat gains, in many climates.
Choose the best type of windows possible, but do not forget that windows are part of a wider puzzle – involving your climate and the design and type of home you have or want to build, and especially its insulation and air-tightness.
What are the best residential windows?
Some experts may say that the best performing windows are Canadian, or are manufactured in Germany; or that the best windows are foam-filled fiberglass-frame casement windows, with double or triple-glazing…
But this is just a very broad assessment. When applying such reasoning, we are devising expensive and very high-performance windows, for new and very energy-efficient buildings in cold climates – which is one possibility among others.
The quality of a window is a result of manufacturing details and...
- the number of glass panes;
- its insulated frames;
- its thin metallic coatings (low-e coatings);
- its glazing spacers;
- its weatherstrips;
- its inert gas fills.
There is no one-size-fits-all standard for choosing windows, and especially their glass. The most advantageous type of glass for a cold climate, won’t be the best choice for a hot or a mixed climate.
Besides, tuning the type glass for specific walls is also essential for energy savings and comfort. The best type of glass for south walls or for north walls is not the same.
What’s the best: double or triple-glazed windows?
Triple glazing is standard in some northern European countries, and mandatory in Germany.
Triple glazed windows are excellent for comfort, energy savings and to reduce noise-transmission, in cold climates.
But triple-glazing becomes less advantageous in mixed and moderate climates, and may not be advantageous at all in hot climates (besides being a lot more expensive).
Typically, high-quality double-glazing windows are a more advantageous and cost-competitive choice for moderate and hot climates.
The best Window reviews and rating
Forget about customer window reviews. They aren’t informed and are mostly focused on secondary features.
Most windows are now two and three-glass pane units. Single clear glass is a thing of the past, and is now mostly used for places like garages and basements. It’s too energy-inefficient. Clear single glass has an R-value of 1 (about U-1); clear double glazing has an R-value of 2 (about U-0.5), while clear triple glazing has an R-value of 3 (about U-0.33). Double and triple glazing can be further improved by using low-e coatings, thermal breaks and argon or other inert gas fill between the glass panes.
They do not mean much, and there is a much better way of knowing the quality of a window: its labels and their specifications:
- U-Factor (window insulation value; the capacity of the window to block the transmission of heat; how well the window prevents heat from entering or escaping through itself);
- SHGC (Solar Heat Gains Coefficient): the shading ability of the window;
- VT (Visual Transmission): how much visible light the window glass admits.
- AL (Air Leakage): the resistance of the window to air leakage.
These energy coeficients are in strict relationship with a wider range of quality features.
US WINDOW LABELS
In the US, the NFRC (National Fenestration Rating Council) rates windows on the four criteria listed above. Just look for the NFRC label on rated windows.
CANADA WINDOW RATING
In Canada, all windows are required to list their U-factor and their Solar Heat Gain Coefficient. Manufacturers must include the Energy Rating (ER) number (a coefficient combining the U-factor, the SHGC coefficient and the Air Leaking rating).
The ER ranges from 0 to 50 (the higher the better), and the Energy Star qualification depends on it, according to the climate zone (Zone A requires windows with an ER rating above 21 or a minimum U-factor of 13; zone D requires an ER rating of 34 or a minimum U-factor of 25, and so on).
The BFRC (British Fenestration Rating Council) contains bands from A to G. Only the most efficient windows carry the Energy Saving Recommended logo. For more information, see: Energy Saving Trust UK, British Fenestration Rating Council
Australia has the Window Energy Rating Scheme (WERS). Windows are rated from 0 to 5 stars for both cooling (summer) and heating (winter). The best rated windows have 5 stars...
The best Energy Star windows (North America)
Unfortunately, Energy Star requirements for windows fall short of the best.
If you are going to build an energy-efficient home you should look for lower U-factors than those recommended by the Energy Star program; similarly pay attention to the SHGC recommended by the Energy Star program.
The best rated windows & Labels and energy coefficients
When comparing the specifications of a window (its U-factor/thermal insulation, SHGC/Solar Heat Gains, and VT/Visual Transmission coefficients), make sure that you are using whole-window values. Some manufacturers use central glazing-only values - which is misleading: it suggests better thermal performance than whole-window numbers.
The best replacement windows
Many people often find themselves disappointed with their new windows, maybe because they have been poorly chosen, and haven’t the best specifications.
But in most cases the reasons are different. We should not forget that even the best-performance windows, properly selected, will not provide significant levels of comfort or energy savings in poorly insulated and leaky homes.
If your windows are working properly, do not replace them without considering the overall insulation and airtightness of your home.
The best windows for cold climates
In cold climates, consider triple or double-glazed windows with a very low U-factor and relatively high solar-gain (SHGC) coefficient.
The ideal values:
Double-glazed window specifications:
Whole-window U-value: 0.30 – 0.39 (the lower the better)
Whole-window SHGC: 0.42 - 0.55 (the higher the better);
Triple-glazed windows specifications:
Whole-window U-value: 0.19 – 0.26 (the lower the better)
Whole-window SHGC: 0.33 - 0.47 (the higher the better)
Triple glazing windows makes sense in cold climates, in energy-efficient homes. They are also great for homes facing very noisy locations.
Goals: to maximize winter solar heat gains (high SHGC) and to minimize heat loss (low U-Value).
Type of frame: low conductive foam-filled fiberglass and vinyl.
Low air infiltration: Type of window operation: casement windows
Visual Transmittance: High.
Large east- and west-facing windows may contribute to small periods of overheating in summer. The use of recessed windows and summer shading systems can be important in some cases.
The best windows for hot climates (Subtropical, hot arid, tropical)
In hot climates, homes should have double-glazed windows with a low solar-gain (SHGC) coefficient.
Double-glazed window specifications:
Whole-window U-value: 0.30 or less (the lower the better)
Whole-window SHGC: 0.28 - 0.37 (the lower the better).
Window placement, size and shading:
East- and west-facing windows are difficult to shade and can be a source of over-heating. Minimize them. They should be protected with wide porches, if possible.
Windows should mostly face the north and the south.
They should be recessed into walls or protected by thick or projecting overhangs, porches and other architectural elements.
Goal: to keep unwanted heat out of the building (low Solar Heat Gains Coefficient: SHGC and low U-value).
Type of frame: low conductive foam-filled fiberglass and vinyl frame
Low air infiltration: Type of window operation: casement windows (great for ventilation).
Good insulation (obtained with low U-value windows) is important, especially in air-conditioned buildings.
The best windows for moderate and mixed climates
In moderate and mixed climates, consider double-glazed windows with a proper solar-gain (SHGC) coefficient.
Window U-value: low.
Type of frame: low conductive foam-filled fiberglass and vinyl (low U-value).
Low air infiltration: Type of window operation: casement windows.
SHGC value: between those of windows for cold and hot climates (mentioned above); it varies according to the side of the house; high SHGC glazed should be used with caution on the sun’s facing windows (to avoid summer overheating).
Windows in the west and east side of the building should have a low SGHC (to limit solar heat gains) and be small in size (large east and west-facing windows may contribute to periods of overheating in summer).
The best Window manufacturers
The best home window glass (Low-e glass)
Modern windows have low-e coatings. In other words: the panes have thin metallic coatings on one side, able to improve their performance as thermal insulators.
Most highly insulated windows (with a low-U-factor) have also a low Solar Heat Gains coefficient — which is advantageous in hot climates but not in cold climates.
Pay also attention to the other energy specifications of the glass (and frame) of the window: the Visible light transmittance (VT) and the AL (Air leakage) coefficients.
The best window frames
Window frames dictate the strength of the window and how much maintenance it will need during its lifetime; but frames are also a critical element of the window’s insulation performance.
The overall performance of a window is dictated by the glass but also by the frame, spacers, inert gas-fills and weatherstrips.
See, for details: Aluminum, wood, fiberglass, vinyl and composite window frames
The best window spacers (Thermal breaks)
Glazing spacers are used to keep the distance between the panes and to provide the edge seal.
Be aware anyway. Spaces made with aluminum – a very conductive material – do not improve the performance of the windows.
Prefer non-aluminum spacers (often called warm-edge spacers). They provide higher insulation levels – and are a key element of high-performance windows – as much as inert gas-fills, or high-performance weatherstrips.
If you are going to order new windows, consider carefully the type of spacer. That’s important.
The best inert gas fills
Double and triple-glazed come now with argon, krypton or other gas between the panes. It’s another key element for a better thermal insulation.
Most windows are argon-filled, with a space of half an inch between the panes; increasing or decreasing such space will degrade performance. Krypton involves a thinner space (3/8 inch) between the panes, but is a more expensive gas, reserved for special applications.
The best looking windows
Windows with a low VT (Visual Transmittance) coefficient look gray, which is not agreeable. Similarly, some low-e coatings may distort light, and give windows a tinted appearance.
These type of issues can be solved by properly selecting the windows.
But be aware to features such as true divided lights. They are very common, but they leak more and have a higher U-factor (a lower insulation value) than undivided windows. In other words: what we may regard as the most beautiful may not be the best performing windows.
Five Window Coefficients
Typically, window labels include four coefficients: the U-Factor (Thermal Resistance), the Solar Heat Gains Coefficient, the VT (Visible Transmittance) and the AL (Air Leakage). But they may also include the CR (Condensation Resistance) coefficient.
We explain them below.
The U-factor is the inverse of R-value. The U-value of a window measures the capacity of the window to block the transmission of heat.
The lower the U-factor, the better (in all climates). The Energy Star requirements vary by region.
That’s a very important coefficient (the most important) in cold and mixed climates, but also in hot climates: the lower the U-factor, the more efficiently the window will block the passage of heat.
Buy windows with a very low U-factor (the lower the best, in all climates).
A U-factor of 0.35 (required by the Energy Star program for cold climates) is reasonable but not excellent.
The best double-glazed windows have a U-factor of about 0.27 (whole-window value), while high-performance triple-glazed windows have a U-factor around 0.17.
The Solar heat gain coefficient (SHGC) measuresthe capacity of the window to block the transmission of heat
The SHGC rates the fraction of solar radiation admitted through the glass and is expressed as a number between 0 and 1. The lower the number the better, in hot climates. A lower number means less solar heat…
Pay attention. It may difficult to find low U-factor glazing (with good insulation value) with a high SHGC.
Most low-e windows have a relatively low SHGC (0.27 to 0.35), which is not the ideal for sun-facing windows, intended to benefit from solar heat gains during the winter (cold climates).
Very low-e SHGC glasses may come under well-known brand names like Low-e2 and Sungate2.
The Air Leakage (AL) coefficient rates the resistance of the window to air leakage, and uses the 0-1 scale (0.06 to 1 to be more precise). The AL is measured in cubic feet per minute (CFM) and square foot of window surface area, and the lower the coefficient the better.
This coefficient varies according to the type of window operation.
Casement/awning windows, with their compression weatherstripping system, have higher AL. That’s why it may be important to select casement windows.
As the name suggests, the VT coefficient measures how much visible light the window glass admits. Prefer windows with a high VT, to get more light.
A low VT can reduce by half (or more) the light coming into a space (it will appear quite dark).
VT numbers above 0.60, allows more light, but also more glare, which is undesirable when the sun is low in the sky, that is, in the morning and late afternoon, in the East and West sides of the house. Hence, the advantage of having windows with low VT in those sides...
That’s also a good illustration of the advantage of having windows with different types of glass on the different sides of the same building.
Water condensation Resistance definition
Some windows are also rated for their resistance to water condensation, in a scale of 1 to 100 (the higher the rating the higher the resistance of the window).
Though important in cold climates, this coefficient can be in part replaced by the U-factor of the windows. A window with a high U-factor will also minimize water condensation.
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