Condensation Inside a Skylight: Causes and How to Stop Drips

Water dripping from a skylight on a cold winter morning, when it has not rained in a week, is one of the most confusing problems a homeowner sees. Nine times out of ten the cause is condensation inside skylight glass and shafts, not a roof leak. Warm, humid indoor air rises, hits the cold glass and frame, and turns to water that runs down the shaft walls or drips onto the floor. The fix starts with confirming the source, then lowering humidity, improving ventilation and adding insulation where needed.

The quick answer: if water appears in cold, dry weather, beads evenly across the glass and frame, and gets worse after showers or cooking, it is condensation. If water shows up during or right after rain or snowmelt, stains run from one corner and the glass stays dry, suspect a leak. For condensation, reduce indoor humidity to about 30 to 40 percent in winter, run bathroom and kitchen exhaust fans, insulate the skylight shaft and consider upgrading old single-pane or failing glazing.

Why Condensation Inside Skylight Glass Happens

A skylight is a window pointed at the coldest part of the sky. On winter nights it loses heat faster than a wall window, and the glass surface can drop well under room temperature. Warm air rises, so the air at the top of a skylight shaft is the warmest and often the most humid air in the room. When that air touches glass cooler than its dew point, water forms.

Several factors make it worse:

• High indoor humidity from showers, cooking, laundry, aquariums, houseplants or a new home still drying out
• Bathroom and kitchen skylights directly over moisture sources
• Older single-pane or failing double-pane glass with poor insulating value
• Uninsulated shafts running through a cold attic, where the shaft walls themselves sweat
• Air leaks around the frame or shaft that let warm air reach cold surfaces
• Tight homes with little fresh air exchange

Condensation or Roof Leak? How to Tell

A simple test helps. On a dry day, wipe the glass and frame completely dry. Tape a small square of plastic wrap tightly to the interior glass. If water forms on the room side of the plastic, it is condensation. If water appears under the plastic or behind the frame, it is coming from the glass or roof side. You can also measure indoor humidity with a hygrometer. Readings over 50 percent in winter strongly suggest condensation.

Water trapped between double panes is a different problem: a failed seal. That fog will not wipe off and means the glazing needs replacement.

Safety for Skylight Work

Most inspections happen from a ladder. Use a sturdy step or extension ladder on firm, level flooring, keep three points of contact and have a helper hold it. Do not lean out to the side. Skylights over stairwells or in high vaulted ceilings may need scaffolding or a pro. Never walk on a skylight from the roof, and leave any roof-side inspection on two-story homes or steep roofs to a roofer.

Moisture-damaged drywall in shafts often grows mold. Wear gloves and an N95 mask when cleaning small spots after fixing the moisture source. Mold covering more than about 10 square feet should be handled by a remediation professional.

Fixes, From Cheapest to Most Involved

1. Lower Indoor Humidity

In winter, aim for roughly 30 to 40 percent relative humidity, lower during deep cold snaps. Turn down or shut off a whole-house humidifier. Use a dehumidifier in damp areas, vent the dryer outdoors and cover pots while cooking.

2. Use Exhaust Fans

Run bathroom fans during showers and for 20 to 30 minutes afterward. Use the range hood when cooking, ideally one ducted outside. Check that fans actually exhaust to the exterior, not into the attic.

3. Move Air Across the Glass

A ceiling fan on low, or leaving the shaft open rather than covered by a blind that traps humid air, helps keep the glass slightly warmer and drier. Opening venting skylights briefly on mild days flushes moist air.

4. Insulate and Air-Seal the Shaft

From the attic, check whether the shaft walls are insulated. Bare drywall exposed to a cold attic sweats on the inside. Add insulation batts or rigid foam board to the attic side of the shaft and seal gaps where the shaft meets the ceiling with caulk or foam sealant rated for the use. Working in the attic means stepping only on joists and wearing a mask, gloves and eye protection around insulation.

5. Add a Condensation Channel or Absorbent Strip

Some skylight frames include condensation gutters that collect drips and let them evaporate. Clean these channels so they work. Absorbent strips are a temporary aid only.

6. Upgrade the Glazing

Old single-pane or failed insulated glass loses heat fast. Modern units with low-E double or triple-pane glazing and warm-edge spacers run much warmer on the interior surface. If the frame is old and leaking air too, full skylight replacement may be the better choice.

Tools and Materials

• Hygrometer
• Sturdy ladder and a helper
• Plastic wrap and painter’s tape for the test
• Dehumidifier
• Insulation batts or rigid foam, foam sealant and caulk
• Gloves, eye protection and an N95 mask
• Soft cloths for cleaning the frame

Troubleshooting

• Drips only after showers. Bathroom fan is undersized or not running long enough.
• Frost on the interior frame. Very cold glass plus high humidity; lower humidity and improve insulation.
• Peeling paint or stains on the shaft. Long-term condensation; dry, prime with a stain-blocking primer and repaint after fixing moisture.
• Water after rainstorms. Likely flashing or seal failure; call a roofer or skylight specialist.
• Fog between panes. Failed seal; replace the glazing unit.

Room-by-Room Considerations

Where the skylight sits changes the approach. Bathroom skylights face the most moisture, so condensation inside skylight frames there usually calls for a stronger exhaust fan, longer run times and possibly a humidity-sensing fan switch that runs automatically. Kitchen skylights pick up steam from cooking; a ducted range hood that vents outdoors makes the biggest difference.

Bedroom skylights can fog overnight simply from occupants breathing in a closed room. Cracking a door, running a ceiling fan on low or using a small dehumidifier helps. Living areas with vaulted ceilings trap warm, moist air at the peak, so ceiling fans set to push air downward in winter can mix that air back toward the floor and reduce condensation at the glass.

Tubular skylights (the small round tubes) can also collect condensation in the tube when warm air leaks into a cold attic section. Sealing the ceiling ring and insulating the tube in the attic usually solves it.

Protecting Floors and Finishes Under the Skylight

Drips from a skylight often land on hardwood, carpet or furniture. Until the fix is complete, set a towel or tray in the drip zone and wipe the frame each morning. Check wood floors for gray stains or cupping, and dry carpet pads promptly so they do not develop odors. Repainting a stained shaft is simple once the moisture is under control.

Mistakes That Make It Worse

Some quick fixes backfire. Covering the shaft opening with a tight blind or panel traps humid air against the cold glass and often increases condensation inside skylight frames. Running a humidifier at high settings through winter adds far more moisture than the glass can tolerate. Caulking over weep holes or condensation channels in the frame blocks the path that lets small amounts of water drain or evaporate. And painting over water stains without fixing humidity only hides the problem until next winter. Address humidity, airflow and insulation together, and the drips usually stop within a few weeks.

Cost Ranges

A hygrometer is inexpensive, and a dehumidifier runs from modest to mid-priced. DIY shaft insulation often costs under a hundred dollars in materials. A new bathroom exhaust fan installed by a pro typically runs a few hundred dollars. Replacing skylight glazing or the whole unit can range from several hundred to a few thousand dollars, depending on size, type, roof pitch and whether flashing must be redone.

When to Call a Pro

Call a roofer or skylight specialist if water appears during rain, if flashing looks damaged or if the skylight is on a steep or high roof. Hire a contractor for glazing replacement, full skylight replacement or attic work you cannot safely access. An electrician should install or rewire exhaust fans. For widespread mold, rotted framing or soaked insulation, bring in a remediation or water damage pro.