For the average person, window labels can be anything but straightforward. They are filled with technical terms that can be very difficult to understand, which makes getting new windows an intimidating process. Luckily, we understand window labels well enough to offer explanations of all of the complicated terms so that getting new windows will be a headache-free process.
- U-Factor: The U-factor tells you how easily heat escapes through the window. For those familiar with the term R-value (as in insulation), the U-factor is the inverse of the R-Value (R-Value = 1/U-factor). U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-value, the less heat flows through the window or the better it is an insulator. Remember: Lower is Better.
- Solar Heat Gain Coefficient: The Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight or how much heat passes through into the house. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits into the house.
- So do you want a higher or lower SHGC? Not such an easy question to answer and very house and location dependent but generally speaking you want a lower SHGC if you cool more than you heat (like in Arizona) and a higher SHGC if you heat more than you cool (like in Wisconsin). Also, it is fairly standard practice to use a high SHGC window on the south side of the house – to absorb the sunlight on the sunny side of the house – and a lower SHGC on the other sides to prevent heat loss on the cooler sides of the house.
- Visible Transmittance: Visible Transmittance (VT) measures how much light comes through a product with 0 being no light and 1 being maximum light. Some examples of VT are:
- Single-pane, clear window 0.9; 90%
- Double-pane, clear window 0.81; 81%
- Ordinary low-e 0.75; 75%
- SHGC and VT generally travel as a pair, meaning a lower SHGC generally results in a lower VT and vice versa. Don’t get too caught up with VT though, as even a window with a low VT measure looks like a normal window (though maybe a little smokey).
- Air Leakage: Air Leakage (AL) or sometimes called the “wind” rating is a measure of how drafty the window is or how much air infiltration is taking place. The lower the number the draftier the window with the most drafty, new windows rated 0.3 or above.
- For most people, the reason to take note of this rating isn’t so much for energy loss as much as comfort. A drafty window is really uncomfortable, so that is reason enough to go for a low AL window (less than 0.2). Generally speaking a traditional double-hung window will be far leakier (a higher AL) than casement or tilt & turn windows.
Note: AL is a voluntary rating and many manufacturers do not include it.
- Condensation Resistance: Condensation Resistance (CR) is another voluntary rating that measures the ability of a window to resist the formation of condensation on the interior surface of that product. You know that “sweaty” window that makes the windowsill al wet. The higher the CR rating, the better that product is at resisting condensation formation.
- CR is expressed as a number between 1 and 100 with the higher the number the better: 50 is good and 60 is excellent.
- Is this an important measure? Surprisingly it is, not so much because one should be overly concerned about interior condensation of your windows but rather because the CR rating acts as a proxy for how well the window is constructed. The better a window is built, the higher the CR will be; conversely, really cheap windows have low CR values.
- (Note: On Energystar labels there will be a map showing what portions of the country this particular window is best suited for. Clearly, hot, sunny climates need a different window from cold, gray locations and this map tells you that.)
*This information has been provided by our friends at the National Fenestration Rating Council(NFRC) . For more information on windows, including the anatomy of a window and the wide range of windows available with these ratings, please visit their site.