Measuring Performance


The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. This section on U-factor describes what a U-factor is and it’s importance in the heat loss through a window assembly.

Solar Heat Gain Coefficient (SHGC)

The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. This section on Solar Heat Gain Coefficient describes what a SHGC is and it’s importance in the amount of heat gain through a window assembly.

Visible Transmittance

The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. This section on Visible Transmittance describes what VT is and it’s importance in the amount of daylight through a window assembly.

Air Leakage

Heat loss and gain occur by infiltration through cracks in the window assembly and an air leakage rating (AL). This section on Air Leakage describes what AL is and it’s importance in the window assembly.

Condensation Resistance

How well a window resists the formation of condensation on the inside surface is expressed as Condensation Resistance. This section on Condensation Resistance describes what CR is and it’s importance in the window assembly.

National Fenestration Rating Council

The National Fenestration Rating Council (NFRC) has developed a window energy rating system based on whole product performance. This section on the National Fenestration Rating Council describes what the NFRC is, what they rate and certify, and what is displayed on the NFRC label.


The Department of Energy (DOE) and the Environmental Protection Agency (EPA) have developed an ENERGY STAR designation for products meeting certain energy performance criteria. This section on ENERGY STAR shows the ENERGY STAR performance zones and requirements.