Skylights, Roof Windows And Condensation

A skylight that drips in winter and stays dry in a storm is not leaking. It is condensing, and sealing it will make it worse.

Telling the two apart

A leak follows rain. Water appears during or just after rain, it can appear in wind-driven rain from one direction only, and it does not care about the temperature.

Condensation follows cold. Water appears on cold mornings, it is worse in winter, it is worse in bathrooms and kitchens, and it can happen when it has not rained for a week.

A leak usually stains and spreads. Condensation usually beads on the glass and the frame, then runs down and drips from the same point every time.

Why condensation happens on a skylight

Warm air holds moisture. When it touches a cold surface, it cannot hold it any more, and the moisture comes out as water.

A skylight is the coldest surface in the room, because it is glass with the night sky above it. So it collects water before any wall or window does.

The moisture comes from ordinary living: showers, cooking, drying clothes indoors, and people breathing. In an apartment with a skylight over a bathroom or a stairwell, there is usually plenty of it.

Fixing condensation

Ventilation first. Get the moist air out of the room before it reaches the glass. A working exhaust fan, ducted outside rather than into the roof space, does more than anything else. The same logic that applies to [bathroom condensation and the exhaust fan](/blog/bathroom-condensation-mould-and-the-exhaust-fan) applies here.

Openable units. Many roof windows open. An openable skylight that gets used is a ventilation point rather than a condensation point.

Better glazing. Double glazed units have a warmer inner surface, so condensation forms far less readily. On a replacement this is worth specifying.

Insulation around the shaft. A skylight in a roof space usually has a shaft between the roof and the ceiling. An uninsulated shaft is a cold chimney. Insulating it warms the surfaces and cuts the condensation.

Sealing the room tighter without ventilating it makes condensation worse, not better. That is the trap.

Fixing a real leak

The flashing. Almost every skylight leak is at the flashing where the unit meets the roof. Flashings lift, corrode and get disturbed by later roof work.

The seal around the glass. On older units the gasket perishes and the glass no longer seats.

The unit itself. Acrylic domes go brittle, craze and crack under years of sun. Once crazed, they leak and they cannot be repaired.

Debris. Leaves and silt banked up on the high side of the unit hold water against the flashing until it finds a way in. On a flat or low-pitch roof this is common and easy to miss.

Like [roof and gutter leaks that are not plumbing](/blog/roof-and-gutter-leaks-that-are-not-plumbing), the water frequently appears a long way from where it entered, so the stain is not the place to look.

When replacement beats repair

An acrylic dome older than a couple of decades is usually at the end of its life. Crazed, yellowed and brittle, it will keep failing.

A modern replacement with a glass unit and a proper flashing kit costs more up front and stops the annual visit.

The trap in a strata building

The roof is common property. The ceiling inside the apartment is usually not.

Which means a skylight leak damages a lot owner's ceiling from a common property defect, and both parts need doing. Establishing [who pays for what](/blog/common-property-or-lot-owner-who-pays) before the work starts saves an argument afterwards.