Condensation resistance
Why warm interior glass surfaces matter, and how vacuum glass and fiberglass frames change the picture.
Why condensation happens
Water condenses on any surface that is colder than the dew point of the air touching it. In winter the coldest surfaces in a room are usually the window frame and the edge of the glass. Condensation is a comfort problem, then a mold problem, and finally a durability problem for sills, drywall and finishes.
Condensation Resistance (CR) and CRF
NFRC publishes a Condensation Resistance rating (CR, 1–100, higher is better) derived from the simulated interior surface temperatures of the frame, edge-of-glass and center-of-glass. AAMA's Condensation Resistance Factor (CRF) is the older, test-based measure used in commercial specifications. Both reward the same thing: warm interior surfaces.
What warm surfaces need
- A frame that does not conduct. Pultruded fiberglass conducts a fraction of the heat that aluminum does, so the interior face of an F-Series frame stays close to room temperature.
- A warm edge of glass. The vacuum insulating unit in a ZERO package has no gas cavity to lose heat through, and the argon cavity's Kodispace 4SG thermoplastic spacer keeps the edge warm.
- Glass that insulates. The interior surface of a vacuum-glass unit stays within a few degrees of room temperature even at −10 °F outside, which is why condensation on GlasFëns glass is rare even at high indoor humidity.
Installation matters too
A perfectly rated window installed with an uninsulated gap to the rough opening will still be cold at the perimeter. Continuous insulation and an airtight interior seal around the frame are part of the condensation strategy. See Installation and the air-barrier interface.
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