Why does my skylight have condensation, and should skylights have condensation?
Short answer. Skylight condensation happens when warm, humid indoor air touches glass that is colder than the air's dew point, which is common on winter mornings, in bathrooms and kitchens, and on older units. Occasional light fog that clears is normal; heavy dripping, fog between the panes, or water that shows up only when it rains points to a failed seal or a leak.
Normal vs not normal
- Normal: light fog on cold mornings that clears by midday, a bit more in a bathroom after showers.
- Too much humidity: heavy condensation that drips daily and stains the drywall in the skylight shaft.
- Failed seal: fog between the glass panes that cannot be wiped off, meaning the insulated glass unit has lost its seal.
- Leak: water that appears after rain regardless of temperature, often at a corner or the top of the shaft.
Reducing condensation
- Run bath and kitchen fans during and after use.
- Keep winter indoor humidity moderate.
- Improve air circulation near the skylight; a closed blind traps humid air against cold glass.
- Insulate the shaft walls where they pass through a cold attic.
- Consider replacing old units with modern insulated, low-e glass.
Telling condensation from a leak
Wipe the glass and frame dry and watch the timing. Water that returns after a cold, dry night is condensation. Water that returns only during or after rain, especially wind-driven rain, is a leak at the flashing or the unit. Robust Roofing's guide to skylight leaks walks through more clues. See replacing skylights with the roof.