Northern Zone Recommended Window Properties (mostly heating)

U-factor

Solar Heat Gain Coefficient (SHGC)

Visible Transmittance (VT)

Air Leakage (AL)

Windows: U=0.35 or less
Skylights: U=0.60 or less (20°)
Skylights: U=0.45 or less (90°)

If air conditioning loads are minimal, windows with U-factors as high as 0.40 are also energy-efficient if the Solar Heat Gain Coefficient is 0.50 or higher.

Windows: SHGC=No Requirement
Skylights: SHGC=No Requirement

To reduce heating, select the highest SHGC you can find (usually 0.30-0.60 for the U-factor ranges required in colder climates) so that winter solar gains can offset a portion of the heating energy need. If cooling is a significant concern, select windows with a SHGC less than 0.55. Select skylights with a SHGC of 0.55 or less.

Windows: VT=No Requirement
Skylights: VT=No Requirement

Select windows with a higher VT to maximize daylight and view.

Windows: AL=No Requirement
Skylights: AL=No Requirement

Select windows with an AL of 0.30 or less.

North/Central Zone Recommended Window Properties (heating & cooling)

U-factor

Solar Heat Gain Coefficient (SHGC)

Visible Transmittance (VT)

Air Leakage (AL)

Windows: U=0.40 or less
Skylights: U=0.60 or less (20°)
Skylights: U=0.45 or less (90°)

The larger your heating bill, the more important a low U-factor becomes.

Windows: SHGC=0.55 or less
Skylights: SHGC=0.40 or less

If you have significant air conditioning costs or summer overheating problems, look for SHGC values of 0.40 or less. If you have moderate air conditioning requirements, select windows with a SHGC of 0.55 or less. While windows with lower SHGC values reduce summer cooling and overheating, they also reduce free winter solar heat gain.

Windows: VT=No Requirement
Skylights: VT=No Requirement

Select windows with a higher VT to maximize daylight and view.

Windows: AL=No Requirement
Skylights: AL=No Requirement

Select windows with an AL of 0.30 or less.

South/Central Zone Recommended Window Properties (heating & cooling)

U-factor

Solar Heat Gain Coefficient (SHGC)

Visible Transmittance (VT)

Air Leakage (AL)

Windows: U=0.40 or less
Skylights: U=0.60 or less (20°)
Skylights: U=0.45 or less (90°)

A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. The larger your heating bill, the more important a low U-factor becomes.

Windows: SHGC=0.40 or less
Skylights: SHGC=0.40 or less

Select windows with a SHGC of 0.40 or less. A low SHGC is the most important window property in warm climates.

Windows: VT=No Requirement
Skylights: VT=No Requirement

Select windows with a higher VT to maximize daylight and view.

Windows: AL=No Requirement
Skylights: AL=No Requirement

Select windows with an AL of 0.30 or less.

Southern Zone Recommended Window Properties (mostly cooling)

U-factor

Solar Heat Gain Coefficient (SHGC)

Visible Transmittance (VT)

Air Leakage (AL)

Windows: U=0.65 or less
Skylights: U=0.75 or less (20°)
Skylights: U=0.75 or less (90°)

A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. Select windows with a U-factor lower than 0.65 and preferably lower than 0.60.

Windows: SHGC=0.40 or less
Skylights: SHGC=0.40 or less

Select windows with a SHGC of less than 0.40. A low SHGC is the most important window property in warm climates.

Windows: VT=No Requirement
Skylights: VT=No Requirement

Select windows with a higher VT to maximize daylight and view.

Windows: AL=No Requirement
Skylights: AL=No Requirement

Select windows with an AL of 0.30 or less.