The ƒ-factor of a product or assembly is an internal surface temperature comparison against outside air temperatures to determine the risk of surface condensation. For a well-insulated wall or roof, the ƒ-factor would be close to 1 and condensation unlikely. A poorly insulated element or cold-bridge may have an ƒ-factor of 0.5 or less.

Rooflights, by their very nature, are unable to achieve the very low U-values of the opaque parts of the roof and still allow light transmission. Typically, a rooflight with a U-value of 1.8W/m²K would have an ƒ-factor of 0.82 and a rooflight with a U-value of 0.8W/m²K would have an ƒ-factor of 0.92.

For more information on this subject, please see the Thermal Transmission section in our Zenon Technical Manual.