Replacement Window GuideCondensation forms when moist indoor air meets a cold window surface. ENERGY STAR reports that certified windows are more resistant to condensation but even they can suffer from it in cold weather, and the NFRC rates a window's resistance on a Condensation Resistance scale of 1 to 100. This guide explains the causes and the levers you control, with every figure tied to its source.
Why condensation forms
Condensation is a moisture problem, not always a window defect. ENERGY STAR notes that certified windows are more resistant to condensation, but even they can suffer from it in cold weather, and advises managing indoor humidity to minimize the risk.
The moisture comes from everyday living, cooking, showering, and breathing, and it collects on the coldest surface in the room, which is often the glass. When indoor air holds more water than the cold surface can tolerate, the water condenses out onto the pane.
The Condensation Resistance rating
The NFRC provides an optional rating for this exact problem. The NFRC window ratings guide describes Condensation Resistance as an optional rating that measures a window's ability to prevent moisture on the interior of a window, on a scale of 1 to 100.
The same guide states that a higher rating means less condensation on and around a window. When two windows carry the same U-factor, the one with the higher Condensation Resistance number is the better choice for a humid or cold-climate home.
Warmer glass resists sweating
A better-insulated window keeps its interior surface warmer, which raises the temperature at which condensation appears. The NFRC glazing guide describes double glazing as two layers of glass separated by inert gas that significantly reduces heat transfer.
Because the inner pane of an insulated unit stays closer to room temperature, it fogs less readily than a single pane. Low-e coatings and gas fills, which the NFRC guide lists as glazing advancements, contribute to that warmer inner surface.
Air leakage and moisture
Sealing matters too. ENERGY STAR notes that proper insulation during installation helps minimize condensation risk, and the NFRC window ratings guide describes Air Leakage as an optional rating for the volume of air that can infiltrate a home through a window, where a lower number means less air leakage.
Drafts move moist air into cold gaps around a poorly sealed window, where it can condense out of sight. A tight installation and a low air leakage rating reduce both the draft and the hidden moisture it carries.
What you can do
The practical fixes follow from the causes. Lower indoor humidity, since ENERGY STAR ties condensation resistance to managing indoor humidity, and improve ventilation in kitchens and bathrooms where moisture is generated.
When replacing windows, compare the NFRC Condensation Resistance numbers directly, choosing the higher figure for problem rooms. Persistent fogging between the panes of a sealed unit is different, signaling a failed seal that ventilation cannot fix.