Skylight condensation and moisture: why glass gets wet inside

Few things alarm homeowners more than water beading or dripping from a skylight on a cold morning. The immediate worry is a leak, and sometimes that worry is justified. But condensation, which is moisture from indoor air settling on a cold surface, accounts for many such cases, and it points to a different set of causes. This page explains how condensation forms, why skylights show it first, what indoor humidity targets published sources suggest, and how condensation differs from water entering through the roof. It is general education, not a diagnosis of any particular skylight. For the roof-side story, see flashing, curbs and why skylights leak.

How condensation forms

The Efficient Windows Collaborative glossary defines condensation as the deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as cold window glass or a frame exposed to humid indoor air. The dew point is the temperature at which water vapor in air will condense at a given state of humidity and pressure. ENERGY STAR states it the same way: water condenses on interior window surfaces when the surface temperature is below the dew point of the humid indoor air.

So two things have to line up: a cold surface and moist air. Raise the humidity of a room and the dew point rises, which means warmer surfaces start to collect water. Lower the humidity and the dew point falls, and the same glass stays dry. That is why condensation tends to appear on the coldest days, when glass is coldest, and in the rooms that make the most moisture, such as kitchens, bathrooms and laundry areas.

Why skylights show it first

The Efficient Windows Collaborative says skylights are usually the first place condensation appears, and that it signals excess moisture in indoor air. A skylight is a glazed opening in the roof, so its inner surface is exposed to outdoor cold while the room below holds whatever moisture the household produces. The sources do not rank the reasons, but the dew point rule above explains the pattern: where a surface is coldest, it crosses the dew point first. The shaft below the glass can be a cold surface of its own if it is poorly insulated.

Frames and edges matter too. The glossary describes edge effects as the two-dimensional heat transfer at the edge of glazing caused by the thermal properties of spacers and sealants, and describes warm-edge technology as low-conductance spacers that reduce heat transfer near the edge. Cold edges and frames are often where moisture shows up first, which is one reason the framing material and design affect results. The glossary also describes a thermal break as a low-conductance element placed between higher-conductance parts, often used in aluminum frames.

Indoor humidity: what the sources suggest

Building Science Corporation, in its moisture control digest for new homes, says elevated interior moisture can lead to condensation on window surfaces and to surface mold and mildew, and gives target ranges for relative humidity (the amount of moisture in the air relative to what it could hold at that temperature). For cold climates, it says relative humidity should be kept at 30 percent or lower at 70 degrees Fahrenheit during the heating months, and lower still in very cold climates in the coldest month. For mixed climates, it says 40 percent or lower at 70 degrees in heating months and 60 percent or lower at 75 degrees in cooling months. In hot-humid climates, it gives 60 percent or lower at 75 degrees during cooling periods.

The aim behind those numbers, per the digest, is to prevent 70 percent relative humidity from occurring next to surfaces, which is the level it connects to mold, mildew and other biological growth. The digest is written for new construction and for whole-house design. Individual households have different ventilation, occupancy and cooking habits, so these are reference values, not rules for your home. The digest also notes that cold surfaces from thermal bridges, including uninsulated window headers, can create high surface humidity and mold locations.

What reduces condensation

The guidance divides into two groups: warm the surface, and lower the moisture.

On the warm-the-surface side, ENERGY STAR recommends making sure the space between the window frame and the rough opening is insulated during installation, and choosing window treatments that allow air to flow over the glass. The Efficient Windows Collaborative skylight overview lists insulating the skylight well, providing adequate air movement and using high-insulating glazing with a well-designed frame. Higher-performing glazing helps because the inner surface stays warmer. ENERGY STAR notes that certified windows are more resistant to condensation but can still suffer from it in cold weather. NFRC's label includes a condensation resistance rating, where a higher number means a product resists condensation better, as described in reading NFRC and ENERGY STAR labels. The glossary adds that its condensation resistance factor is based on an AAMA standard and that the higher the factor, the less likely condensation is to occur.

On the lower-the-moisture side, ENERGY STAR recommends managing indoor humidity: vent clothes dryers directly outside, make sure kitchen and bath fans vent directly outside, and run fans during showers or cooking and for 20 minutes afterward. Building Science Corporation likewise points to source control, including direct venting of dryers, bath and kitchen exhaust, together with controlled ventilation that dilutes indoor moisture. For skylights that open, the Efficient Windows Collaborative notes that operable units let rising hot air leave from ceiling level; see fixed, vented and electric skylights. The shaft, attic and insulation around the opening are covered in skylights, attics, light shafts and insulation.

Condensation outside and between panes

Water can also form in places other than the room-side glass. ENERGY STAR explains that exterior condensation can form in warm weather: on a hot, humid day, cool air inside a house can lower the temperature of the window's outside surface below the dew point. That is a normal effect of physics and does not by itself indicate a defect.

Moisture between the panes is different. The glossary uses the term fogging for a deposit of contamination left on the inside surface of a sealed insulating glass unit, due to temperature extremes or failed seals. Factory units are sealed airtight, often with a desiccant inside the air space to absorb moisture, so haze trapped between panes points to the sealed unit rather than to room humidity. A manufacturer's warranty or a qualified professional would be the place to assess it.

Condensation or a leak?

The Efficient Windows Collaborative states that many apparent skylight leaks are actually condensation on cold surfaces. Appearance alone can mislead, though, and genuine leaks do occur at flashing, curbs, seals and cracks. Water that returns with every rain, stains that spread, or drips on a dry, mild day all deserve attention from a roofing professional rather than a guess from a web page. This page cannot say which one you have. If you suspect a leak, skylight installation, leaks and lifespan explains the broader picture, and choosing a roofing or window contractor, and what to ask covers how to find help. Roof work carries fall risk, and local codes, product instructions and a qualified professional should guide any repair.

The short version

Condensation needs a cold surface and humid air, so it is controlled by warming the glass, lowering indoor moisture, or both. Skylights often show it first, which is why published sources treat it as a signal about indoor humidity. Telling condensation from a leak is a job for a qualified professional, and local codes and product instructions govern any repair.