Condensation on skylights

Condensation on a skylight happens when water vapor in the air turns back into liquid on a surface that is colder than the air touching it. In a Citrus County home, that usually means warm, humid air inside the house reaching the underside of the glass or the frame on a cool morning, or the outside glass dropping below the dew point after a warm, wet day.

The key factor is the dew point. Air can only hold so much moisture at a given temperature. When the surface temperature of the skylight falls below the dew point of the air next to it, droplets form. Humidity raises the dew point, so a home running high indoor moisture from cooking, showers, or a dehumidifier-free crawlspace will fog a skylight sooner than a drier house. Single-pane glass, older frames without thermal breaks, and skylights with little insulation between the glazing layers are more likely to get cold enough to trigger it.

A leak is different. A leak is liquid water getting past the flashing, curb, seals, or glazing and entering the roof structure or ceiling. It tends to show up as drips, stains, soft drywall, or water that follows a rafter. Condensation shows up as beads or film on the glass, often worst at the edges of the frame, and it usually clears as the surface warms during the day. It can still cause harm. Water running down the frame can rot wood curbs, stain the ceiling around the light, and encourage mold, so it matters even when no water is coming through the roof.

Telling the two apart is the first diagnostic step. Condensation that appears only on certain humid days, on the interior glass, without staining, points to moisture and temperature. Water that appears after rain, runs from a specific spot, or leaves a stain points to a leak. When the source is unclear, a contractor who handles skylight repair and leak work in the area can check the flashing and seals.

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