best windows rating

The easiest way to select the best windows for your home is by looking at the information in their rating labels, namely the insulation value (U-factor) and the Solar Heat Gain Coefficient (SHGC).

Window performance labels

Top windows may cost a little more but they are more energy-efficient, and you will get lower energy bills. Do not buy low-quality uncertified windows.

Quality windows are certified by performance labels, which contain information on how much the windows can help to cool your home in the summer, or keep warm air in the winter, or resist condensation and air leaks. Window Performance Label from NFRC

The performance labels are a way to compare the windows (or doors or skylights) and to make an informed purchase…

Best Windows & Coefficient Ratings

There are two major coefficients of the performance of a window: the insulation factor (U-factor or U-value) and the Solar Heat Gain Coefficient (SHGC). But you should also consider the VT (Visible Transmission), the AL (Air Leakage) and the Condensation Resistance (CR) coefficients.

U-factor

The U-factor or U-value of a window indicates how well the window prevents the indoor heat (and cold) from escaping through its glass and sash... The U-factor is a measure of the insulation value of the windows.

The best-rated windows have a low U-value. The lower the U-value, the better the insulating value of the window.

The U-factor rating of a window falls between 0.20 and 1.20 (Energy Star/NFRC). And the lower the U-factor, the greater the resistance of the window to heat and cold flow…

In some cases a low U-factor is very important. That’s the case of cold climates, and even mixed and mild climates where it is important to keep heated air inside the home. That's also the case of some hot climates where it is important to prevent cooled air from escaping. In all these cases select a window with a very low U-factor.

This rating coefficient may not be so important in some mild climates, with very low needs of artificial heating and cooling, or in some tropical hot climates where the cooling of the house is based on natural ventilation (and not in insulation…).

Note: when considering the U-value of a window, see if this coefficient is for the whole window, and not just for the glass (or the frame). It's important to consider the whole window and not just a component of it.

SHGC

The Solar Heat Gain Coefficient (SHGC) indicates how well a window blocks the sunlight’s heat.

The SHGC of a window falls between 0 and 1. The lower the SHGC of a window the less solar heat it transmits into the house.

In other words: in climates and situations where overheating is a problem, look for a window with a very low SHGC.

In some cold climates a low SHGC isn’t important. On the contrary. But in hot climates, or in many mixed and temperate climates, a low SHGC is important to block the sunlight’s heat.

Visible Transmission

The Visible Transmittance (VT) rating indicates how much visible light is transmitted by the window’s glass. Like the SGGC, the VT is expressed as a number between 0 and 1. But in this case, the better is usually a high value: a coefficient closer to 1 very high amount of light transmitted into the house.

Windows with a high VT coefficient can be important in some climates, for natural lighting strategies. That’s the case of north facing windows in north hemisphere cold climate countries (or south facing windows, in southern hemisphere cold climate countries)…

See: Natural lighting with windows and skylights

Air Leakage Coefficient

The window’s air leakage or air infiltration coefficient indicates how much air will leak through the window (this rating is expressed in cubic feet of air passing through a square foot of window area: cfm/sq ft).

This coefficient is optional, and many window manufacturers opt by not including it in their windows. But the best-rated windows often include this coefficient, which is very important in all climates: the lower the value, the less heat loss and gain occur by air leakage through cracks associated to the sashes and elements of the window.

Note: its’ also important to choose a good window design, to get a low air leakage coefficient. The casement window design is great to get a higher air sealing… See: Hopper, awning and casement windows

Condensation Resistance

Condensation Resistance (CR) measures the ability of a window at resisting the formation of condensation. The best-rated windows have a high CR. The higher the CR coefficient, the better the window is at resisting condensation.

CR is measured as a number between 0 and 100. The best windows have a CR of 70 or more.

This coefficient – like the Air Leakage one – isn’t often very important in itself because well-built windows with good overall coefficients (U-factor, SHGC) have also good AL and CR coefficients. Anyway, the CR and the AL coefficients are a mean of getting that confirmation… .

See: Condensation and humidity in windows

USA Windows Rating

In USA, pay attention to the Energy Star performance labels and to the four climate zones associated to them. See: Best windows rating

Canada Windows Rating

Canadian Energy Star also considers four climate zones for windows and skylights, each with different U-factors and SHGC values. See: Canada Climate Zones and Canadian Energy Office

Europe & Best Windows Energy Rating

The BFRC (British Fenestration Rating Council) contains bands from A to G, where A is the most efficient rating and G the least. Only the most efficient and best windows carry the Energy Saving Recommended logo. For more information, see: Energy Saving Trust UK, British Fenestration Rating Council

Australia Window Rating

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...

For more information, see:
Australia Windows & Doors Labeling
Australian GreenHouse Office
Build.com.au