Skylight condensation: is it a leak or just humid air?
Water on the glass is usually humidity, not a leak. A few checks will tell you which.
Water on a skylight is more often condensation than a leak. Warm, humid indoor air meets cold glass, and the moisture drops out as droplets. That is physics, not a failure. But it can look identical to a leak, and the two need different fixes, so it is worth telling them apart.
The quick test: where is the water?
Look at where the moisture sits. Droplets on the inside face of the glass, especially on cold mornings, are condensation. A drip or stain that appears at the frame edge, on the light shaft, or on the ceiling around the skylight after rain points to a leak.
The timing helps too. Condensation shows up on cold, humid days and clears when the air warms or dries. A leak shows up after rain, even when the indoor air is dry.
Why skylights sweat more than windows
A skylight is glass sitting in a cold roof, and warm air rises straight toward it. Your kitchen, bathroom, and a houseful of people all put moisture in that air. Add a cold surface and you get droplets.
Older single-pane units and ones with worn seals sweat the most, since the inner glass stays colder. Newer insulated units resist it better but are not immune in a humid room.
Moisture between the panes is a different problem
If you see fog or droplets trapped between two layers of glass, the sealed gap has failed. You cannot wipe it off, because it is inside the unit. That is not condensation from your indoor air and it will not clear up by running a fan.
It means the glass unit needs replacing, which is a job for a skylight installer. Do not try to drill or vent it yourself.
What helps with ordinary condensation
Reduce the moisture reaching the glass. Run bathroom fans during and after showers, and use the range hood when you cook. Make sure those fans vent outdoors, not into the attic. A dehumidifier in a very humid room can help.
Wiping the glass is fine but it only treats the symptom. If water keeps forming, the room has more humidity than it should, and the fix is ventilation, not sealant.
When it really is a leak
If water collects at the frame or stains the shaft after rain, read our page on skylight leaks. Most skylight leaks come from the flashing around the unit, not the glass. Check what you can from inside, and from the ground with binoculars. Do not climb up to caulk it. Sealant smeared on flashing hides the real problem and usually makes the repair harder.
When a skylight sweats and the ceiling below is also stained, you may have both problems at once. Look at ceiling stains to read the pattern.
Frequently asked questions
Why is my skylight wet inside?
Usually because warm, humid indoor air is condensing on cold glass. It appears on the inside face on cold days and clears when the air warms or dries. A leak, by contrast, shows up after rain.
How can I tell skylight condensation from a leak?
Check where and when. Droplets across the inside of the glass on cold mornings are condensation. Drips or stains at the frame or light shaft after rain are a leak. Timing with the weather is the best clue.
What does moisture between the glass panes mean?
The sealed gap between the layers has failed and the unit lets in damp air. It will not clear by reducing indoor humidity. The glass unit needs replacing by a skylight installer.
How do I reduce skylight condensation?
Cut indoor humidity. Run bathroom fans and the range hood, make sure they vent outdoors, and consider a dehumidifier in a very damp room. Sealant will not fix condensation.
Should I climb onto the roof to check my skylight?
No. You can see most of what matters from inside and from the ground with binoculars. If the problem is on the roof, describe it to a roofer instead of climbing up.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)