Energy Efficiency Labels on Replacement Windows

Visible Transmittance – Visible transmittance, or VT, is the percentage of visible light that a window transmits. In other words, it shows the amount of daylight an energy efficient vinyl window lets in. For example, a .55 VT window lets in 55% visible light. Since light is energy and energy is heat, lower VT rating mean cooler homes in sunny environments.  

U-Value – U-Value is the measure of the rate of non-solar heat gain or loss, typically through a material or construction choice. Tests to determine a window’s U-Value are generally conducted in simulated environments with 0 degree temperatures outside, 70 degrees inside and a 15 mile per hour wind speed. The lower a U-Value a window has, the more resistant it is to heat flow and the better it is at insulating your home.  

Solar Heat Gain Coefficient – Solar Heat Gain Coefficient, or SHGC, is the amount of solar radiation that any particular window emits through both absorption and inward release. A low SHGC means that window transmits low amounts of solar heat, meaning it has good shading ability. Get low SHGC energy efficient vinyl windows if you live in particularly sunny climates, but high SHGC windows if you live in areas with lots of clouds or rain, like Washington.