Water beading on the room-side surface of your glass in winter is not a defective window — it's physics. Warm indoor air holds moisture; when it touches a glass surface below its dew point, that moisture condenses, exactly like a cold drink sweating in July. The glass is simply the coldest thing in the room, so it collects the evidence of your home's humidity first.
This is the opposite diagnosis from fog trapped between the panes, which you can't wipe off and which means a failed seal. If a towel clears it, you're dealing with surface condensation — read on.
Fix the Humidity First
Most winter condensation is a moisture problem wearing a window costume. Idaho homes in January should sit around 30 to 40 percent relative humidity. The usual pushers above that range: showers without the exhaust fan running (run it 20 minutes after), kitchens venting steam nowhere, humidifiers set optimistically high, firewood drying indoors, and aquariums or a jungle of houseplants. Knock the humidity down and the sweating stops — free.
When the Windows Really Are the Problem
Some glass is simply too cold to live with. Single-pane windows and old aluminum frames are the worst offenders — aluminum conducts heat so aggressively that the frame itself frosts over, and no reasonable indoor humidity keeps a single pane dry at 15 degrees outside. If your windows sweat even at 35 percent humidity, or you're scraping ice off the inside, the glass surface is running far below what modern U-factor standards deliver. Today's double-pane Low-E units keep the interior glass warm enough that ordinary household air never reaches its dew point on them — which is why condensation complaints mostly vanish after replacement.
The stakes aren't cosmetic: daily sill puddles grow mold and quietly rot sashes, stools, and the drywall beneath.
Wood Windows Inc has diagnosed sweating windows across 45 Treasure Valley winters — sometimes the fix is a bath fan, sometimes it's glass, and we'll tell you which. Free in-home measure: (208) 376-2200.
Frequently Asked Questions
What indoor humidity level prevents window condensation?
Aim for 30 to 40 percent relative humidity in winter. Above that, moisture starts condensing on the coldest surfaces in the house — which are almost always your windows. A $15 hygrometer tells you where you stand.
Is condensation on windows bad for the house?
Persistent condensation is. Water pooling on sills day after day feeds mold, stains wood, and can rot sashes and drywall below the frame. An occasional shower-steamed bathroom window is harmless; a daily puddle is not.
Will new windows eliminate interior condensation?
They raise the temperature of the interior glass surface dramatically, so the same indoor air no longer condenses on it. But if your humidity runs high — say 55 percent from an unvented crawl space — even excellent glass can sweat. Fix both sides of the equation.
Sources & Testing Data
Last reviewed 2026-07-05 by Wood Windows Inc — (208) 376-2200