Thermal transmittance
The U-factor is a measure of the total heat transfer through a fenestration system expressed in W/(m²•C) [Btu/(sf•h•F)]. The lower the U-factor, the less heat will be transferred through the fenestration system.

Solar heat gain coefficient
The ratio of solar heat gain entering the space through the glazing product to the incident solar radiation, SHGC includes directly transmitted solar heat and absorbed solar radiation that is then re-radiated, conducted, or convected into the space. The lower the SHGC, the less solar heat is admitted.

Visible transmittance
A measure of the fraction of visible light a fenestration system allows into the building, VT is dimensionless and is expressed as a decimal less than 1.0. The lower the value, the less visible light is transmitted into the building.

Air leakage resistance
The flow of air through a fenestration product is measured at a specified pressure differential across the product. Leakage is expressed in L/s•m² [cfm/sf].

Condensation resistance factor
The CRF rating number is obtained under standard test conditions as prescribed in AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors, and Glazed Wall Sections. It is essentially the ratio of the difference between an average inside surface temperature and the outside air temperature, and the difference between the inside air temperature and the outside air temperature. CRF allows for comparison of the relative performance of fenestration systems based on the point at which an objectionable amount of condensation occurs The CRF is dimensionless and expressed as a number between 1 and 100—the higher the figure, the higher the resistance to condensation.