Windows are important to the building envelope beyond simply keeping the rain out and bringing daylight in.  Windows fill the holes in our buildings where insulation does not exist.  Rather than being a weak point in the thermal envelope, windows, when chosen and installed correctly, can actually benefit a building’s energy performance. 

Windows can allow some of the sun’s warmth to enter the building during the heating season and block some of this warmth during the cooling season.  This change in how the window performs is a result of the window's orientation with the sun.

U-factor = thermal transmittance (includes conductivity, airflow and emissivity)

  1. the lower the better

  2. ≤ .30 qualifies for the federal tax credit

SHGC = solar heat gain coefficient (solar radiation released inward)

  1. lower is generally better, depending on orientation and shading

  2. ≤ .30 qualifies for the federal tax credit

AL = Air Leakage (cubic feet per minute, per square foot of window area)

  1. optional rating on NFRC labels

  2. the lower the better (ideally ≤ .30)

  3. casement windows usually offer the lowest AL

Durability and longevity

  1. Vinyl, aluminum and fiberglass require less maintenance than wood

  2. Fiberglass’s strength to weight ratio is greater than vinyl and wood

  3. Fiberglass’s expansion and contraction properties closely resemble that of glass

For more information: 
The Energy Efficient Windows Collaborative
Moisture Performance_Pro_Moisture.html