Why does my skylight have condensation on it?

Water beading on the inside of a skylight is almost always condensation, not a leak. It forms when warm, moist indoor air meets glass that is colder than that air's dew point. The Virginia Energy Savers Handbook explains that condensation forms when the surface temperature of the glass, sash or frame is lower than the dew point of the humid air in the room.^[1] Skylights tend to show it first because they sit high in the room, where warm, humid air collects, and their glass faces the open night sky.

The handbook adds a second possibility: "Condensation on the glass or frame during cold weather may simply be caused by excessive indoor humidity, but might also be caused by cold air leakage around the glass."^[1] So the fix is usually inside the house, sometimes in the skylight itself. Start by working out which kind of water you have.

First, condensation, leak or failed seal?

Condensation appears on cold days, often in the morning, rain or shine. The glass mists over evenly, and it is worst over showers, kitchens and rooms with a humidifier running. Left alone, it can reduce the natural light, obstruct the view, and cause peeling paint, mold and rot.^[1]

A leak follows the weather. EPA's moisture guide uses it as the easy example of a water problem: "When it rains, water may drip around skylights."^[3] Drips at one corner, stains spreading on the shaft walls or ceiling, or water after snow melts all point to the roof, not the air. As a Forest Service roofing guide puts it, "Many apparent roof leaks are really flashing leaks,"^[6] and that is work for a roofer. See why a roof leaks only when it rains hard for how pros trace them.

Fog between the panes is neither. On double-glazed skylights, moisture trapped between the layers of glass means "seal failure, which can result in window fogging and a loss of any low-conductivity gas-fill."^[1] You cannot wipe it away, and lower humidity will not clear it.

Bring the indoor humidity down

For ordinary condensation, humidity is the main lever. EPA's advice is to "keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity."^[2] A humidity meter is "a small, inexpensive ($10-$50) instrument available at many hardware stores,"^[2] and it takes the guesswork out. The Virginia handbook gives a similar target: "Relative humidity should be between 30 and 60% when the house is heated or air conditioned."^[1] If the reading is above that range, the handbook says to find the source of the excessive moisture, and to check that you are not over-using a humidifier.^[1] More on targets in what humidity to keep your house at.

The everyday habits that move the needle most, from EPA's guidance:

• "Run the bathroom fan or open the window when showering."^[2] Leaving the fan on for a while after the shower helps clear the steam.
• "Use exhaust fans or open windows whenever cooking, running the dishwasher or dishwashing."^[2]
• "Vent appliances that produce moisture, such as clothes dryers, stoves, and kerosene heaters to the outside"^[2] where possible.
• "Increase ventilation or air movement by opening doors and/or windows, when practical. Use fans as needed."^[2] A ceiling fan on low can help keep room air moving past the glass.

VELUX gives skylight-specific advice in the same direction: ventilate 3 to 4 times daily, keep indoor temperatures above 68°F, and reduce humidity.^[4] When you do find water on the glass or sill, EPA's advice is to act quickly to dry the wet surface and reduce the moisture source^[2] so it does not soak into drywall or wood trim.

When the skylight itself is the problem

If humidity is in range and the skylight still drips, look at the unit and its shaft.

Old or single-layer glazing. Better glass runs warmer. The handbook points to low-e coatings that raise the temperature of the glass, insulating spacers, tighter frames and inert gas fills^[1] as the way modern windows cut condensation. It also warns that even a high-performance window can condense if indoor humidity is high,^[1] so new glass is not a substitute for humidity control. For old dome or bubble skylights, VELUX lists discoloration from sunlight exposure, mildew stains from moisture buildup, leaks, or condensation around the panes^[5] as signs it is time for a replacement.

A cold light shaft. Where a skylight sits above a flat ceiling, a shaft runs through the attic to the room. If you have an unheated attic above the ceiling, the light well needs insulated walls.^[1] A shaft built without insulation, or with insulation that has slumped, runs cold, and moisture can collect on its walls as well as on the glass. Checking it means going into the attic, so it is a reasonable thing to ask a pro to look at.

Air leaking around the frame. The cold air leakage the handbook mentions shows up as a draft at the frame edges or frost along one side of the glass. Sealing a skylight frame in a roof opening is not a homeowner job, because it ties into the flashing and the roof.

When to call a licensed pro

Call a roofer or skylight installer if water appears when it rains or snow melts, if stains are spreading on the ceiling or shaft, if there is fog between the panes, or if the frame or drywall is soft, stained or moldy. Those mean flashing, a failed glass seal or a damaged shaft, and every repair involves work on the roof. If the skylight is old, compare repair with replacement; see how long a skylight lasts.

Work valued at $1,000 or more needs a valid Virginia contractor's licence in the category the contractor is working in.^[7] Check a contractor using DPOR's License Lookup or by calling (804) 367-8511,^[8] or see how to check a contractor's licence. Browse more exterior answers, or find licensed help for skylight repair.

When you are ready, describe your skylight problem once and licensed local contractors send quotes or offer an estimate visit.