Why Skylights Drip in Winter

Water dripping from a skylight on a cold morning looks like a roof leak. Usually it isn’t. Skylight makers are blunt about it: CrystaLite’s technical note calls skylights “indicators of excess moisture in a home”, not the cause.

Why the skylight goes first

• Warm, moist air rises, so the ceiling plane holds the most humid air in the room. VELUX’s condensation brochure says this is why condensation is usually more noticeable on skylights.
• The glass faces the cold night sky and is often the coldest interior surface in the room.
• Metal frames without a thermal break chill the glass edges. Sunsquare points to this “thermal bridge” as a common failure.
• If the curb or shaft isn’t airtight, room air leaks up into the shaft and condenses on the cold underside of the unit.

Condensation or leak? Quick checks

1. Timing: condensation shows up on cold, clear nights and mornings, even when it hasn’t rained. A leak follows rain or snowmelt.
2. Location: condensation beads across the inside of the glass and the frame. A leak tends to track down one corner or stain the shaft from above.
3. Humidity: log the room’s RH for a week. If drips line up with high readings after showers or cooking, it’s moisture.

What helps

• Lower indoor humidity. Building Science Corporation’s BSD-012 recommends ≤30% RH at 21 °C in cold-climate heating months, and lower in very cold weather. Sunsquare cites a 30–40% winter range. Use the lower end in a cold snap.
• Exhaust at the source. Run bathroom and kitchen fans during and after use, especially when a skylight sits over a bathroom.
• Keep air moving. Leave the room door open and don’t close blinds tight across the shaft overnight; trapped air under the glass cools and saturates.
• Seal and insulate the shaft. A sealed curb and an insulated shaft keep humid air off the cold assembly. Many skylights have a condensation gutter with weep holes, so keep those clear.
• Upgrade the glazing when you replace it. Low-E insulated glass and a thermally broken frame keep the inner pane warmer.

Sources: CrystaLite technical note on condensation; VELUX moisture & condensation brochure; Sunsquare: Pitfalls of Skylight Condensation; Building Science Corporation BSD-012.

Originally published at https://dry-glass-notes.surge.sh/articles/skylight-condensation-winter.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.