Guide

Why skylights get condensation first

Why do skylights get condensation before other windows?

Because a skylight faces straight up at the open sky, so it radiates heat away far more effectively than a vertical window, which mostly faces buildings, ground and trees. It therefore runs colder and crosses the dew point sooner. Warm air collecting at ceiling level compounds it.

The geometry

Every surface radiates infrared heat towards whatever it faces. What it gets back depends on how warm that thing is.

A vertical window faces the ground, other buildings, trees and hedges — all at roughly air temperature, all radiating back. Its net loss is modest.

A skylight faces the open sky, which on a clear night is radiatively equivalent to something extremely cold. There is nothing warm in its field of view to send energy back. So it loses far more than it receives, and its temperature falls further below the air than any vertical pane's does.

How far below air temperature glass falls on a clear, calm night.
Orientation	What it faces	Typical shortfall
Skylight / roof window	Open sky	Largest — several degrees
Vertical, exposed	Half sky, half ground	Moderate
Vertical, under an eave	Mostly the overhang	Small
Vertical, facing a wall	Another building	Near zero

Same effect puts frost on a car roof while the doors stay clear, and it is the whole reason dew forms at all.

The second reason: warm air rises

Radiation explains why the glass is cold. Convection explains why the air meeting it is the worst possible air.

Warm air is less dense, so it rises and collects at ceiling level, carrying whatever water it picked up on the way. A skylight therefore presents the coldest surface in the room to the most humid air in the room.

That is an unlucky combination and it is why roof windows show condensation on nights when nothing else in the building does.

What actually helps

Same two levers as any window — raise the surface temperature, or lower the dew point of the air touching it — but the practical measures differ:

Keep air moving at ceiling level. A stagnant warm layer against cold glass is the problem. Even slight circulation breaks it up.
Do not close the blind tight. It isolates the pane from the room's warmth and traps a pocket of air against the coldest surface available. Leaving a gap is usually better than sealing it.
Ventilate when the outdoor air is drier. Which is a question about the outdoor dew point — in winter it almost always is, on a humid summer night it may not be.
Check the reveal, not just the glass. The plasterboard around a roof window is often the part that suffers, because it stays damp longer than the pane does.

When it is a fault

Two cases where the geometry is not the explanation:

Water sealed between the panes. The unit has failed. A repair, not a ventilation problem.
Water tracking down the surround and staining it. The condensation may be normal, but the detailing is not shedding it. Worth looking at rather than living with.

Everything else — misted glass on a cold morning, water on the outside after a clear night — is a skylight behaving exactly as its position dictates.

Common questions

Why does my skylight have condensation but my windows do not?

Two reasons stacked. It has an unobstructed view of the open sky, so it radiates heat away faster than any vertical pane and runs several degrees colder. And warm, moist air rises, so the air touching it is the most humid air in the room.

Is skylight condensation a sign of a fault?

Usually not. It is what you would predict from the geometry. It becomes a concern when the water is between the panes — a failed seal — or when it is persistent enough to be soaking the surround and reveal.

Does condensation on the outside of a skylight mean anything is wrong?

No, and it is the most common place to see it, because a roof window has the clearest possible view of the sky. It means the unit is insulating well enough that the outer pane is no longer warmed from inside. It clears once the sun reaches it.

How do I stop condensation on a skylight?

Raise the glass temperature or lower the dew point of the air reaching it. In practice that means keeping air moving at ceiling level, avoiding a blind that traps a cold layer against the glass, and ventilating when the air outside is drier than the air inside.

Do skylight blinds make condensation worse?

They can. A closed blind isolates the glass from the room's warmth, so the pane runs colder, and the air trapped between blind and glass reaches its dew point sooner. The same is true of heavy curtains over ordinary windows.

Sources

NOAA / National Weather Service — Dew point vs humidity

Published 1 August 2026