DIY & Repair

How to Stop Window Condensation in Winter (Before It Rots the Sills)

Water on the inside of the glass every cold morning is not cosmetic — it is slow-motion rot. Work this ladder from free fixes to replacement-level decisions.

Why winter condensation happens

Warm indoor air holds moisture; cold glass does not negotiate. When room air touches a glass surface colder than its dew point, water condenses — and on a single-digit Great Lakes night, the inside of an old window can run cold enough to frost. The equation has only two sides: how much moisture is in your air, and how cold the glass surface gets. Every fix on this ladder pushes one side or the other.

First, read the pattern on the glass

Condensation on the room side of the glass, worst along the bottom edge, is the classic humidity-meets-cold-glass problem — keep reading. Fog or droplets between the panes of a double-pane unit is a failed seal: no amount of humidity management will clear it, and that unit needs glass replacement. Frost on the storm window with the inner window clear usually means warm moist air is leaking past the inner sash — a weatherstripping problem, not a storm window problem.

Step 1: Lower the moisture load (free to cheap)

• Run bath fans during showers and for 20 minutes after, and the range hood whenever you cook. If a fan just rattles without moving air, clean or replace it — these two rooms produce most of a house's winter moisture.
• Turn the humidifier down as it gets colder outside. Whole-house humidifiers set for comfort in November will fog the glass in January. As a rule of thumb, the colder the outdoor temperature, the lower the indoor relative humidity your windows can tolerate — on the coldest nights, older windows may demand under 30 percent.
• Check the usual suspects: a dryer venting indoors or leaking at the duct joints, firewood drying in the basement, a crawl space with bare earth, dozens of houseplants. Each quietly loads the air with water.

Step 2: Keep air moving across the glass

Still air against cold glass condenses first. Open curtains and blinds during the day so room air can wash the panes — and be aware that tight cellular shades, for all their insulating virtue, trap a cold pocket against the glass and can make condensation worse behind them. Running ceiling fans on low, opening interior doors, and keeping supply registers under windows unblocked all help. If a bedroom stays clammy with the door closed all night, crack the door.

Step 3: Raise the glass temperature

• Shrink film over the interior opening adds a still-air layer that keeps room air off the cold glass entirely. Unglamorous, nearly invisible, and genuinely effective on old single-panes from November to March.
• Storm windows — exterior storms or low-profile interior inserts — do the same job durably. A tight storm over a sound original wood window is a respected cold-climate combination, and often the right answer for a bungalow whose owner wants to keep the original sashes.
• Weatherstrip the inner sash so moist room air stops leaking into the gap and frosting the storm. The inner window should be the tight one; storms are ventilated on purpose through small weep openings — never caulk those shut.

Why you should not just ignore it

Condensation that sits on sashes and pools on sills every morning, all winter, is a water leak in slow motion. Paint peels first, then the glazing putty loosens, then the sill and lower rail go soft — and mold takes hold in the corners where water lingers. By the time wood is spongy, you are no longer buying weatherstripping; you are pricing carpentry. Wipe-downs help the symptom, but a towel on the sill every morning is a sign the assembly is losing.

When condensation means replacement

If the glass still weeps or ices at reasonable indoor humidity — after the fans, the film or storms, and the weatherstripping — the glass surface is simply running too cold, and that is a hardware problem. Modern Low-E units with argon fills and warm-edge spacers keep the interior surface warm enough that ordinary household moisture never reaches its dew point on the pane. A specialist can also tell you whether hidden rot has already spread into the framing — the measure-and-quote visit costs you nothing but an hour, and it is far cheaper to learn that in October than in January.

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