Terms that explain the problem

Dew point
The temperature at which air can no longer hold its moisture. When glass is colder than the room's dew point, water forms on it.
Relative humidity
How much moisture air holds compared with the maximum at that temperature. Indoor levels that are comfortable in fall can cause condensation in a deep freeze.
Thermal bridge
A material, such as an aluminum frame, that conducts cold from outside to the indoor surface.
Window return
The drywall or wood that wraps from the wall into the window opening. Water running off the sill often stains it first.
Weep hole
A small drainage slot in the bottom of some window frames that lets water out. When blocked, water pools in the track.

Why it happens even in Colorado

Outdoor air in the Denver area is dry, but indoor air picks up moisture from showers, cooking, laundry, houseplants, and simply people breathing in a closed room overnight. When an arctic front drops outdoor temperatures well below zero, the inner surface of a window can fall below the dew point of that indoor air, and water condenses.

Older double-pane windows with aluminum frames, typical of some homes built in the 1980s, conduct cold readily and show this first. Seals in insulated glass units also fail with age, which lets the space between panes fog and makes the inner pane colder. Newer vinyl or fiberglass windows usually perform better but can still sweat in bathrooms and in bedrooms with doors closed overnight.

Whole-house humidifiers on furnaces raise the stakes. A setting that feels pleasant in November may be too high for a January cold snap.

Winter routine for problem windows

  • Wipe condensation from glass and sills each morning during cold spells
  • Open blinds and curtains so warm room air reaches the glass
  • Run bathroom fans during and after showers and the kitchen hood while cooking
  • Turn a furnace humidifier down, or off, when forecasts call for extreme cold
  • Keep furniture a few inches away from exterior walls under windows
  • Clear window tracks and weep holes of dust and debris
  • Use a small hygrometer to watch indoor humidity in the worst room

Cleaning or calling someone

  • IfSmall spots on a vinyl frame or glassThenClean with detergent and water, dry the surface, and fix the humidity pattern.
  • IfGrowth on painted wood sills that keeps returningThenClean, let dry fully, and repaint with a quality exterior-grade or moisture-resistant finish once the cause is addressed.
  • IfDrywall below the window is soft or stainedThenHave the wall checked with a moisture meter. Wet drywall usually needs to be cut out.
  • IfGrowth spreads across a large area of wallThenArrange an assessment for contained removal rather than scrubbing it yourself.

General information for Highlands Ranch, not a quote or professional advice. In an emergency, call 911.