April, 2008

Understanding the NFRC Ratings

This "apple to apple" comparison lets you rate windows and doors for energy-efficiency.

By Steve Easley

Today, many building and energy codes require NFRC-rating labels on doors and windows, which contain information designed to help builders and consumers select energy-efficient products. Building and energy codes often require this label so that the inspector can see that the window that is installed matches the window that was modeled on the plans submitted for a building permit.

NFRC stands for the National Fenestration Council—a non-profit organization that has been around since the 1990s. The NFRC is a collaborative effort of manufacturers, representatives from the Department of Energy, utilities, and other interested parties that sets standards for rating the energy performance of windows and doors. The NFRC label allows buyers to determine the energy performance of windows or glass doors. Before this label became popular, manufacturers could devise their own testing protocols to rate a window any way they chose. This led to a lot of confusion when selecting windows because it was difficult to compare products on an “apple to apple” basis. So what do the numbers on the label really mean? NFRC labels contain ratings for the total product—not just the glass. For example, a window with an aluminum frame would have a higher U-factor (lower R-value) than a wood or vinyl window with the same glass. This label is also important because it gives you specific information about the window’s winter performance, summer performance, air leakage, and the amount of light the window lets in. It even provides  information on the window’s resistance to forming condensation. When selecting windows, be sure to use the NFRC label to compare products. Since the NFRC rating is for the total window product, all the components or features are included in the rating, which makes it easier to compare one product to another.

THERE ARE FOUR VERY IMPORTANT NUMBERS ON THE NFRC LABEL. THEY ARE:

1] U-factor, a measure of how much heat escapes through the whole product.

U-factor is a measure of how conductive a window is to heat. The lower the U-factor, the better the product is at reducing heat flow into or out of a home. Ufactor is the inverse of R-value. 1/R=U and u=1/R. So a window with a U-factor of .5 would have an R-value of 2. The U-factor best relates to a window’s winter performance. I recommend a .35 or lower for anywhere in the country that has a heating load.

2] Solar Heat Gain Coefficient

This is a rating that tells you the percentage of heat or the amount of solar radiation that enters the window as heat. The heat energy from solar radiation can be as much as 250 btu’s per square foot of energy that hits the surface of a window. That means that the amount of heat energy from the sun that strikes an exposed sliding glass door can be 10,000 btu’s per hour! To offset this heat gain would require the cooling energy of a room air conditioner or almost a ton of cooling capacity. Whole house air conditioners are rated by the ton: 12,000 btu’s per hour = 1 ton of cooling capacity. The lower the SHGC, the better the window is at reducing heat gain and cooling costs. A window with an SHGC of .35 would reflect 65% of the sun’s heat and let in 35%. I recommend a .35 rating or lower for anywhere in the country that has air conditioning. (The exception here is if you have a solar-heated home that is designed to accept as much of the sun’s heat as possible.)

3] Visible Light Transmission (VLT):

This rating is the percentage or measure of how much light comes through the entire window. The higher the number, the more visible light that enters. This rating indicates how clear the glass is. Coatings or tints on glass increase energy performance but also reduce the amount of light that gets through the window. You typically do not notice an appreciable reduction in light until the VLT gets below .5. It is important to remember that this is a total window rating so grids, larger frames, and so on reduce the visible light transmission. For example, two windows can have the same glass, but if one has grids between the panes, it will have a lower VLT.

4] Condensation resistance:

This rating indicates a window’s ability to resist moisture condensation on the interior surface of the window. Condensation is annoying and can lead to mold and deterioration of the window and surrounding materials. This rating is from 1 to 100 and the higher the number, the better the window is at resisting condensation. Many energy codes require windows with U-factors of .4 or lower. SHGC’s of .35 or lower can cut heating costs up to 25% and cooling costs by up to 40%. Some products can also reduce UV rays that cause fading damage by over 80%. Any way you read it, the NFRC label takes a lot of the guesswork out of selecting windows.

Steve EASLEY is president of Steve Easley & Associates, which consults and trains on building science issues. For more info, Visit www.codecollegenetwork.com