Skylight Condensation in Winter, or a Real Roof Leak?

Condensation or a leak: the fast answer

Condensation shows up when it is cold. A leak shows up when there is water on the roof.

Condensation forms when humid air touches a surface colder than its dew point, the temperature at which the air can no longer hold its moisture. In winter, skylight glass and frames are among the coldest surfaces in the house, and warm, moist air rises straight to them.

A leak needs water outside, from rain, melting snow or ice backing up above the skylight. If the drips follow the weather rather than the temperature, the flashing, seals, glass or surrounding roof is letting water in.

Signs of condensation vs signs of a leak

It is probably condensation if:

• Water appears during cold snaps with no rain or snowmelt
• The room side of the glass is fogged, beaded or wet
• Other windows in the house are sweating too
• Drips get worse after showers, cooking or drying laundry indoors
• You run a humidifier
• You wipe the glass dry and it is wet again hours later with no change in weather

It is probably a leak if:

• Water appears during or after rain, or during a thaw
• The drywall in the skylight shaft is stained, soft or bubbling
• A brown water stain on the ceiling is growing away from the glass
• The drip comes from the top of the shaft or a corner of the frame rather than the glass
• It started after a windstorm or hail
• Shingles around the skylight are lifted, cracked or missing

One clue is easy to miss. If the glass is clear and dry but the shaft below it is wet, water is getting in from above rather than condensing on the glass.

How to test it over a few cold days

Log when it drips. Note the date, the outdoor temperature, and whether it was raining or snow was melting. A few entries usually reveal the pattern.

Dry the glass and watch. On a cold, dry day, wipe the glass and frame completely dry. If they are wet again within hours with no precipitation, the water is condensing on the glass.

Look between the panes. Most modern skylights use insulated glass, two panes with a sealed space between them. Fog trapped between the panes that you cannot wipe off from either side means the seal has failed.

Check the attic. Damp insulation, dark staining on the roof deck or frost in the attic means moist indoor air is escaping upward too. That points to a whole-house humidity and air-sealing problem that often shows at the skylight first.

Why skylights sweat when walls do not

Skylights sweat because they are cold, high and surrounded by the warmest, most humid air in the room. Warm air rises and carries its moisture to the top of the room, where only glass and a frame separate it from the winter night.

The shaft makes it worse. A deep light well that passes through the attic is often poorly insulated and poorly sealed, so its drywall runs cold, and cold surfaces plus humid air means more condensation.

The usual moisture sources are the same every winter. Long showers without the bath fan, cooking without the range hood, drying laundry indoors, humidifiers set high and a damp basement all add water to the air.

How to reduce skylight condensation

Lower the humidity, keep air moving across the glass and warm up a cold shaft. Those steps solve most cases.

Remove moisture at the source. Run the bath fan during showers and until the mirror clears, use a range hood that vents outside, and avoid drying laundry indoors during the coldest weeks.

Turn the humidifier down. Indoor humidity that is comfortable in fall can be too high when it is bitterly cold outside. Lower the setting a little at a time and watch the glass.

Keep air moving. A ceiling fan on its winter setting, or simply leaving the door open, helps clear moist air from a bathroom or stairwell skylight.

Avoid trapping air against the glass. A closed shade in a deep shaft creates a pocket of still, cold air that sweats. If you use a shade, leave a gap so room air can reach the glass.

Insulate and seal a cold shaft. A light well that passes through the attic needs insulation on all sides and airtight drywall, or it will keep sweating no matter how low the humidity goes.

Tried the humidity fixes and still getting drips? Call (608) 909-9109 for a free on-site inspection of the skylight, roof and attic.

When it really is a leak

Treat it as a leak when water shows up with the weather, and have it checked before the next thaw. These are the usual sources.

Flashing. Skylights are flashed in pieces, with a sill flashing at the bottom, step flashing up the sides and head flashing at the top. Corners, nail holes and old flashing reused during a reroof are common failure points, and a skylight sealed with caulk instead of proper flashing tends to leak.

Seals and gaskets. The gasket between the glass and the frame hardens and shrinks with age. Once it cracks, water can run into the frame and down the shaft, as explained in our guide to skylight leaks at the flashing, seals or glass.

Cracked glass. Hail, a falling branch or temperature stress can crack a pane, and older plastic bubble skylights can become brittle and craze.

Ice dams. Snow melting over a warm attic can refreeze lower on the roof and back water up behind the head flashing. A ridge of ice above the skylight is the sign, covered in our guide to ice dams in Wisconsin.

The roof around the skylight. Sometimes the skylight is fine and nearby shingles, underlayment or a valley are failing. Water travels, so the drip is not always directly below the problem.

What to do if water is coming in now

Catch the water, protect the room and document what you see. Put a bucket under the drip, turn off any humidifier, and take dated photos of the glass, the shaft, the stains and the roof from the ground.

Leave the outside alone rather than smearing caulk or roof cement over the skylight. Sealant often traps water behind it instead of letting the flashing shed water downhill, and it has to be scraped off before a proper repair.

If the drywall is soft, swollen or smells musty, or the insulation behind it is wet, that material should be replaced rather than painted over once the water is stopped. Firm drywall with light surface spotting from condensation can usually be cleaned and repainted after the humidity is under control.

Does insurance cover a skylight leak?

The cause matters more than the damage. Sudden events such as hail cracking the glass, wind tearing the flashing loose or a branch coming through are the kind of losses homeowners policies are written for, and the resulting interior damage is often covered too.

Failed seals, worn gaskets, rotted frames and poor past workmanship are usually treated as wear and excluded. Condensation is an indoor humidity issue and is generally not covered, and ice dam leaks vary by policy, so read yours before you assume.

If a storm was involved, document it before filing. We meet your adjuster on site to point out hail damage, lifted shingles and damaged flashing as part of our storm damage work.

Repair, replace or close it up

Repair the flashing when the skylight itself is sound, and replace the unit when the glass or frame has failed. A skylight with fog between the panes needs new glass or a new unit, because no humidity change will clear it.

A flashing repair is usually the least expensive option, since it involves a flashing kit and a small area of shingles. Replacing the unit adds the skylight itself and interior finishing in the shaft. Our guide to skylight replacement cost explains what drives the price.

A flashing repair on a sound roof does not require a new roof. If the shingles are already near the end of their life, though, replacing the skylight during a reroof avoids opening the same area twice.

Removing a skylight you no longer want is also an option. The opening is framed, decked, roofed over and insulated, and the ceiling is finished flat, which leaves one less penetration in the roof.

Can skylight repairs be done in winter?

Some can. Inspection, temporary protection and water diversion can happen in cold weather, and an active leak should not wait.

Permanent shingle and flashing work goes better once the roof is clear of ice and warm enough for shingles to seal. A common winter plan is to stop the water now and finish the permanent repair in the first stretch of mild weather.

Getting your skylight checked

If your skylight has been dripping and you cannot tell why, have it looked at before the spring rains. Buckshot Exteriors is family-owned and has worked on roofs since 1979, with offices in Madison, WI and Centerville, IA.

Our free on-site inspection checks the flashing, seals and glass, the shaft and the surrounding roof, and the attic if there is access. If it is condensation, we will tell you so. Our skylights page covers repair and replacement, and our residential roofing page covers the roof around them.

Call (608) 909-9109 to schedule a free inspection. If water is coming in right now, our 24-hour storm line is the same number.