Quality replacement windows are one of the best home improvement investments you can make. However, with so many replacement windows available and with most manufacturers touting their product as the best, it is easy to be overwhelmed by all the numbers and information. When it comes to finding the highest quality replacement windows, we always recommend turning to the National Fenestration Rating Council (NRFC) and the American Architectural Manufacturers Association (AAMA) for guidance. Both bodies provide accurate information to help homeowners measure and compare energy performance of not just windows, but also doors and skylights. They offer a uniform, independent rating and labelling system. So, how does the NRFC and AAMA measure window performance, and what is important for you to know about each measuring method? We have put together this quick guide to help you make an informed decision for when you next need to replace your windows.
U-FactorThe U-factor measures how well a product stops heat from escaping. U-factor ratings mostly fall between 0.30 and 1.20. The lower the U-factor, the greater a window’s resistance to heat flow, which also means the better it is at insulating your home. When it comes to performance, windows that have the lowest U-factor help keep your home warmer in winter and cooler in summer.
Solar Heat Gain Coefficient (SHGC)
The SHGC of a window measures how well it blocks heat caused by sunlight. The lower a window’s SHGC, the less solar heat it transmits. Homeowners who live in a warm climate require windows with a low SHGC so that the sun does not overheat the home. In cooler climates, like the mid-Atlantic or Northeast, homeowners often spend more money heating their homes than cooling them. Therefore, homeowners in these areas often choose a window with a higher SHGC that will allow the sun to heat their home during winter.
Visible Transmittance (VT)
A window’s VT measures how much light comes through it. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted. For example, a typical sheet of clear, annealed glass has a VT of .93, meaning 93 percent of the light that hits the glass passes through. It is important to remember that different types of glass carry different VTs. When tints are added to the glass, or coatings such as Low-E are applied, the VT can also be affected.
Air Leakage (AL)
Air leakage is indicated by an AL rating expressed as the equivalent cubic feet of air passing through a square foot of window area. Heat loss and gain occur by infiltration through cracks in the window assembly. In basic terms, the lower the AL, the less air will pass through these cracks, meaning less conditioned air will escape outside, and air from the outside will have a harder time penetrating inside.
Condensation Resistance (CR)
Condensation resistance measures the ability of a window to resist condensation on its inside surface. The higher the CR rating, the better that product is at resisting condensation formation. Though the CR rating does not predict condensation, it can provide a credible method of comparing the potential of various products for condensation development.
Design Pressure (DP) or Performance Grade
The DP of a window is based on lab pressure testing in pounds per square foot. DP requirements often vary because they are based on a window’s location in the home, the height of the home, the density of the home and wind zone designation. This DP value is particularly important for homes in a hurricane zone.
Structural Test Pressure
The structural test pressure is 150 percent of the design pressure rating. The structural rating of a window depends on the glass, the frame and the sash system. In order to get a higher DP rating, window manufacturers need to consider the thickness and possible heat-strengthening of the glass, and the use of higher-end hardware and good quality sealants in the frame and sash system.
Sound Transmission Class (STC)
STC ratings determine how much sound penetrates through a material. STC ratings are used for windows, doors, walls, and most building materials. For windows, STC ratings range from 18 to 50. The STC ratings of double-paned windows are generally between 28 to 35. If you do not consider air sealing, the difference between STC ratings comes down to glass thickness and how much air is between the glass. The more airspace and the thicker the glass, the more soundproof the window.