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Replacement Window Ratings

Replacement window ratings are some of the most important factors that come into play when selecting the best replacement window for your home. The numbers indicate how well the window is designed and built, as well as how effectively it keeps out air and moisture.

For some consumers, these ratings are too technical and confusing (which they can be for a number of reasons). These consumers will need to rely on finding a good contractor who can suggest manufacturers and models that will provide good long term protection to the home.

For those consumers who are interested in replacement window ratings, pay particular attention to four measures; U-Factor, Air Infiltration, SHGC and Design Pressure. There are many more performance numbers in the industry, but for 95% of all residential homes, knowing these four will be more than sufficient.


U-factor (also known as U-value) is a measure of Energy efficiency plays a very important role in your window selection and the U-Factor rating that each window provides tells you what the heat loss will be. Most windows range between 0.2 and 1.3, the lower the rating the better your energy efficiency will be.

Some examples include:

Silverline 2900 Series - 0.35 (not very impressive)

Sunrise Restorations - 0.27 (impressive)

Soft-Lite Elements - 0.19 (outstanding)

Air Infiltration

Air infiltration or AI is the rating used to determine the air that leaks into the property from the outside through the window glass or the window frame. Most windows will rate their AI between 0 and 0.3, the lower the rating the less air will leak into your property.

Some examples include:

Vinylmax Hyde Park - 0.17 (not very impressive)

Great Lakes EcoSmart - 0.06 (impressive)

HiMark 800 Series - 0.01 (outstanding)

Solar Heat Gain Coefficient (SHGC)

The SHGC measures the heat that can work its way through the window glass and frame into the property. The normal ratings are between 0 and 1.0 with the lower rating being the better option if you live in a warmer climate. These ratings will help you determine how much heat will be allowed into your property on a hot summer's day. If you are using air conditioning, you don't want heat filtering through your windows and heating up the area, making your air conditioning work overtime. At the same time homes in cooler climates will want higher SHGC ratings than warmer climates.

Visible Transmittance (VT)

VT is the measurement used to determine how much light a window will allow through the glass. Average numbers are between 0 and 1, the higher the rating the more natural light will be allowed to flow into the room. Clear glass will have a high VT as it lets in lots of light, but is not very energy efficient. A low-e3 glass (very energy efficient) will typically have a lower SHGC (meaning it reflects much more heat than clear or single low-e glass) will have a lower VT. There is a balance here that consumers will have to take into consideration -- how much heat do you want to stay out of the house and how much light do you want to allow in.

Condensation Resistance (CR)

Condensation forms on any surface where the temperature is lower than the dew point of the air. During cold weather, the coldest surface in most rooms is the window. That's why you would get condensation or water droplets on the inside of the window glass. Condensation resistance numbers range anywhere from 30 to 80. The higher the CR, the better it is at resisting condensation, which can be important for homes in cold weather where condensation happens frequently in winter.

Design Pressure (DP)

Design pressure is the load, or pounds per square foot, that a window can withstand. Many windows are tested by increasing the pressure on the window until it fails. The pressure has to reach 1.5 times the design pressure for ten seconds without causing the window to undergo permanent damage. A standard DP is 30, which would mean that that window has to be able to withstand 45 pdf.

The required DP rating for a product will vary based on several factors, including geographic location, building height, and window location on the building itself. Homes on the coast, for example, may require a much higher DP (like a DP60 in some Florida coastal towns) than for homes that are located inland.

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