Air Quality

Condensation on Windows in Winter? Why Seattle Homes Get Damp and How to Fix It

Published November 3, 2026

Why windows sweat more in a Seattle winter

Warm air can hold more water vapor than cold air. When warm, humid indoor air touches a surface that's colder than its dew point, the vapor turns back into liquid — the same thing that happens to a cold glass of water on a summer day. Windows are usually the coldest surface in a room, so they show the problem first.

Western Washington makes this especially common:

• Long, damp, mild winters. Outdoor air is often humid, so cracking a window doesn't dry the house as much as it would somewhere colder and drier.
• Homes stay closed up for months. From October through April, most of the moisture you make indoors stays indoors unless something removes it.
• Older housing stock. Many homes around Seattle still have older windows, aluminum frames, or limited exhaust fans — all of which make condensation more likely.
• Tighter remodels. New windows, insulation and air sealing save energy, but they also trap moisture if ventilation wasn't upgraded at the same time.

Is window condensation a problem?

It depends on where it shows up and how much there is.

Persistent moisture can lead to peeling paint, swollen or rotting wood sills, stained drywall, musty smells, and mold that can bother anyone with allergies or asthma. The EPA generally recommends keeping indoor humidity below 60%, and ideally somewhere around 30–50%. In a Seattle winter, aiming for the lower half of that range usually keeps windows much clearer.

Common causes of high indoor humidity in winter

Bathroom and kitchen fans that don't do their job

Exhaust fans are your main defense against moisture. Many older fans are undersized, noisy enough that people avoid them, or vented into the attic instead of outdoors. Some are simply clogged with dust.

Drying clothes indoors

Hanging laundry on racks adds a surprising amount of water to the air. A clothes dryer that isn't vented outside — or a crushed or disconnected dryer vent — does the same.

Crawlspace and basement moisture

Damp crawlspaces are common in the Puget Sound region. Without a proper vapor barrier, ground moisture can rise into the living space all winter.

A heating system that isn't moving enough air

Rooms with poor airflow — a closed-off bedroom, a room at the end of a long duct run, or a space with blocked registers — stay cooler, so their windows reach the dew point sooner. A clogged filter or leaky ductwork can make this worse.

Too little fresh air

Tightly sealed homes without a dedicated ventilation system can't swap out stale, humid air. That's why newer homes often include a heat recovery ventilator (HRV) or energy recovery ventilator (ERV), which brings in fresh outdoor air while recovering much of the heat from the air going out.

A humidifier set too high

If your furnace has a whole-house humidifier, or you run a portable one, it may be adding moisture you don't need in our climate.

Unvented combustion

Gas appliances that aren't venting properly release water vapor — and potentially carbon monoxide. If condensation suddenly gets much worse near a furnace, water heater or fireplace, or you notice headaches or a stuffy feeling indoors, treat it as a safety issue. Our guide to what to do if your carbon monoxide alarm goes off covers the warning signs.

DIY checklist: how to reduce window condensation

These steps are safe for most homeowners and often make a visible difference within a week or two:

1. Get a simple hygrometer. An inexpensive digital humidity monitor tells you whether you're actually above 50%, so you're not guessing.
2. Run bathroom fans during showers and for about 20 minutes after. A timer switch makes this automatic.
3. Use the range hood when cooking — especially when boiling water — and make sure it vents outdoors if it's designed to.
4. Dry laundry in a vented dryer and check that the outside vent flap opens freely.
5. Open blinds and curtains during the day. Coverings trap cold air against the glass and keep it below the dew point.
6. Keep supply and return vents clear of furniture, rugs and drapes so warm air reaches the windows.
7. Replace your HVAC filter on schedule so air keeps moving through the system.
8. Turn down or turn off any humidifier for the winter if your readings are already above 40–45%.
9. Run a dehumidifier in a basement or problem room if readings stay high.
10. Wipe wet sills each morning and clean any small mold spots with a mild detergent solution, wearing gloves.

When to call a pro

DIY fixes handle everyday moisture. Call an HVAC professional if:

• Humidity stays above 55–60% even after you've used fans and cut down indoor drying
• Condensation or mold keeps coming back in the same rooms
• Some rooms are noticeably colder than others or barely get any airflow
• Your bathroom or kitchen fans don't seem to pull air, or you're not sure where they vent
• You've recently remodeled, added insulation or replaced windows and the problem started afterward
• You have a whole-house humidifier and aren't sure how to set or service it
• Condensation got worse suddenly near a gas appliance, or you smell exhaust

A technician can measure humidity and airflow room by room, check your ductwork and filter, inspect combustion venting, and tell you whether a ventilation upgrade like an HRV or ERV, a whole-home dehumidifier, or simple duct and airflow fixes would solve the problem. At Yeti, you'll get honest advice on what's actually needed and upfront pricing approved before any work starts. Learn more about our indoor air quality services.

Our licensed and insured technicians help homeowners across Seattle, Bellevue, Kirkland, Redmond, Renton, Shoreline, Lynnwood, Bothell, Edmonds and Everett get damp, stuffy winter air under control.

How to keep it from coming back

Once you've brought humidity down, a few habits keep it there:

• Check your hygrometer weekly through the winter, especially after cold snaps.
• Schedule seasonal HVAC maintenance so filters, blowers and ductwork keep air moving.
• When you replace windows or air-seal your home, plan ventilation at the same time.
• Keep gutters and downspouts clear so rainwater doesn't end up in the crawlspace.

Windows dripping every morning or seeing mold around the frames? Call (206) 823-0670 or request a callback. We're available Mon–Sat 7am–7pm, with 24/7 emergency service.