Window condensation — at a glance
- Cause: warm, humid indoor air hitting a cold glass surface. The colder it gets outside, the lower the indoor humidity has to be to prevent it.
- Problem: water running down the glass soaks sills, frames, and the drywall below — and mold follows.
- Fix: lower indoor humidity, move air across the glass, and dry or clean affected sills promptly.
- Call a pro when mold has spread past the sill onto drywall, or the sill wood is soft. (385) 250-2825.
Why it happens in Utah homes
Utah air is dry outdoors, but winter homes trap moisture from showers, cooking, laundry, people, pets, and humidifiers. When it’s well below freezing outside, interior glass gets cold enough that even moderate indoor humidity condenses on it. Older single-pane or aluminum-frame windows — common in pre-1980 Provo and Orem homes — are the worst offenders.
A rough humidity guide
A widely used rule of thumb for indoor relative humidity in winter:
| Outdoor temperature | Indoor humidity target |
|---|---|
| Outdoor temp 20°F to 10°F | Keep indoor RH around 35% or lower |
| Outdoor temp 10°F to 0°F | Around 30% or lower |
| Below 0°F | Around 25% or lower |
A $15 hygrometer tells you where you stand. If your windows sweat at these levels, the window itself may be the weak point.
Where to look for mold
- Black or gray spotting on the window sill and in the corners of the frame
- Peeling paint or swollen wood on the sill
- Staining or bubbling on the drywall directly below the window
- Musty smell behind curtains or blinds
- Mold on the back of curtains that touch the glass
What you can do
- Run bathroom fans during showers and 20 minutes after. Use the kitchen hood when cooking.
- Turn down or off any humidifier when windows sweat.
- Open blinds and curtains during the day so room air reaches the glass.
- Wipe condensation off the glass and sill each morning during cold snaps.
- Vent the dryer outside and don’t hang laundry to dry indoors.
- Small surface mold on a hard, sealed sill can be cleaned with detergent and water and dried thoroughly. Wear gloves and an N95.
When it’s past DIY
- Mold covering more than about 10 square feet (the EPA’s general guideline for calling a professional)
- Mold on the drywall below the window or inside the wall
- Soft, rotting sill or frame wood
- Recurring mold after cleaning — usually means moisture inside the wall
- Anyone in the home with respiratory concerns. See mold health effects.
Typical Utah cost
| Scope | Typical range |
|---|---|
| Sill and frame cleaning, a few windows | $300-$900 |
| Mold removal including drywall below one or two windows | $900-$2,500 |
| Multiple rooms + wall cavity work | $2,500-$6,000+ |
Condensation mold is generally treated as a maintenance issue by insurers and usually isn’t covered.
Mold around your windows?
Call (385) 250-2825. We’ll check whether it’s surface-only or inside the wall.
Mold remediation services · Mold vs mildew · How to tell if you have mold · Winter pipe protection
Also see: Can mold grow in winter?
More guides: Bathroom exhaust fans and mold · Sweating pipes
Need help with mold in Provo? Our Provo mold remediation crew is AMRT-certified, and remediation jobs start at $500. Call (385) 250-2825.
Related new guides