Field Report: If you wake up to wet glass this January, it’s not just “sweating.” It is a warning that your home’s Dew Point has moved inside your walls.
- The Cause: When warm indoor air hits cold glass (below the Dew Point), it releases water. This same process occurs inside uninsulated wall cavities.
- The Risk: Water running down the glass pools on the sill, soaking into the drywall and framing below, creating invisible mold farms.
- The Fix: You must lower indoor humidity to 35-40% during freezing weather, regardless of comfort.
Across the Northern and Midwest states, homeowners are currently battling “Weeping Windows”—heavy condensation that forms overnight on glass panes. While often dismissed as a nuisance, structural engineers view this as a Category 1 Water Intrusion indicator.
The physics are simple but destructive. Your window acts as a dehumidifier. It draws moisture out of the air and deposits it as liquid water. If your windows are wet, your indoor Relative Humidity (RH) is too high for the exterior temperature.
The “Black Line” Warning
The first visual evidence of biological growth usually appears as a thin black line along the silicone caulk or glazing bead at the bottom of the window.
Run a paper towel over the black spot.
- If it smears easily like dust, it is likely just particulate matter (dirt/soot).
- If it is embedded in the silicone and requires scrubbing to remove, it is likely Cladosporium or Aureobasidium (common window molds).
The Invisible Danger: Wall Cavity Condensation
The water you see on the glass is the water you know about. The greater danger is the water you cannot see.
When a window “weeps,” the excess water drips down to the sill. If the seal between the sill and the drywall is not perfect (it rarely is), that water wicks into the drywall paper and the wood framing studs underneath.
This creates a dark, damp micro-climate inside the wall where Toxic Black Mold (Stachybotrys) thrives on the cellulose of the paper backing. By the time you see mold on the drywall surface, the colony inside the wall may be extensive.
Immediate Action Plan
Do not just wipe the windows. You must stop the source.
- Lower Humidity: In freezing temperatures, indoor humidity should not exceed 40%. Use a dehumidifier if necessary.
- Air Circulation: Open blinds/curtains during the day. Trapped air behind heavy curtains creates a cold pocket that accelerates condensation.
- Inspect Sills: Press on the drywall under the window. If it feels soft or spongy, structural rot has already begun.
Is Your Home structurally Sound?
Window condensation is just one of 10 risk factors we track. Check our full Home Risk Guide to identify the other wet zones in your property.
View Home Risk Guide