When shopping for replacement windows, it is important to be able to decipher their performance ratings. By understanding these ratings you will then be able to make a side by side comparison of the various replacement windows available. The National Fenestration Rating Council, NFRC for short, is the organization responsible for establishing these ratings based on overall product performance. Windows that meet their standards will receive a label with, usually, four ratings specified. The information provided below will guide you in understanding these labels and enable you to make accurate product comparisons.
The U-Factor of a window assembly indicates the rate of heat loss and tells you how well the window insulates. Typically, this value ranges from 0.25 – 1.25. The lower this number is, the better the window performs in regards to resistance to heat flow and its ability to properly insulate your home. When establishing this rating, the NFRC takes into account the whole window unit which includes the frame, glazing, and spacers. Since the center of glass U-Factor is sometimes referenced, it’s important to note that the whole window U-Factor is usually higher and is the rating to consider.
Solar Heat Gain Coefficient (SHGC)
The official definition of SHGC is: “the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1.” Windows will begin to absorb heat as the sun beats down on them. What this number tells us is how much of that heat is blocked by the replacement window. The higher the SHGC is, the more solar heat is allowed to pass through and in turn raises the internal temperature of your home. How this rating applies to you though is dependent on your local climate. Homes in a Southern climate, where the sun’s heat can become brutal during summer, should utilize replacement windows with a low SHGC rating. In cooler Northern climates, a window with a high SHGC rating can use the sun’s radiant heat to help warm a house.
Visible Transmittance (VT)
This is a purely optical property that indicates the amount of visible light permitted to pass through the window. VT is also measured on a scale of 0 to 1, where a higher value indicates more light transmitted. So in order to maximize daylight you will want to choose a replacement window with a high VT rating. Improvements in modern glazing systems utilize Low-E coatings to allow visible light to pass through the window while simultaneously rejecting solar heat gain.
Air Leakage (AL)
Cracks in the window assembly allow air to pass through resulting in heat loss or gain. Air leakage is measured, and expressed, as the equivalent cubic feet of air passing through a square foot of window area per minute. A replacement window with a high AL rating can result in a drafty window which in turn makes it less energy efficient. It’s best to choose a window with an AL rating of 0.30 or less.
Condensation Resistance (CR)
Condensation Resistance is an optional rating on the NFRC label but should be understood regardless. Condensation Resistance is measured on a scale of 0 to 100. The higher the resistance factor, the less condensation build up the window allows to form on its internal surface.
It’s important that you are able to understand these ratings to ensure you choose a product that fully suits your, and your homes, needs. You will also be better able to communicate with the installer you hire to replace your windows. Now that you can read and understand the NFRC labels you are better equipped to make an accurate comparison between different replacement windows and can be well on your way to a successful home improvement!