FAQ's and Useful Resources
Much of the terminology used to determine window and patio door energy efficiency can be confusing. Below are several commonly used terms and their definitions.
U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25 and are measured in Btu/h·ft²·°F. The lower the U-factor, the better the window insulates.
Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits.
Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see.
Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf·m/ft².
Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows. Windows do not cause condensation. Condensation happens when water vapor in the air comes into contact with a surface whose temperature is lower than the dewpoint of the air the water vapor will condense on that surface. The amount of water vapor in the air is called the relative humidity. Controlling the relative humidity is the most effective way to avoid objectionable condensation. WINDOWS DO NOT CAUSE CONDENSATION, EXCESS INTERIOR HUMIDITY IS THE SOURCE. WINDOWS JUST HAPPEN TO BE A COOLER SURFACE FOR THE VAPOR TO CONDENSE ON.
Ways to control household humidity:
1. Shut off all household humidifier units.
2. Ventilate regularly. Air out the entire house for a few minutes each day. Ventilate kitchen, laundry, bathrooms and other moisture producing rooms during use to the exterior. Do not vent to another room, crawl space or attic.
3. Run exhaust fans longer and more often.
4. Open fireplace damper.
5. Verify crawl space, basement and attic ventilation is large enough and remain open year round. As you tighten up your house to be more energy efficient the original ventilation may not be enough to do the job now.
Features in windows that will help increase condensation resistance:
1. Windows made with frame material that have low thermal conductivity
2. Improved Glazing materials such as: Insulated glass, LoE coatings, triple glazed and warm edge glass spacers
Useful Links:
Energy Star
Window Maintenance (.pdf)
Troubleshooting Field Problems (.pdf)
Mulling Oversize Units (.pdf)
