Why a roof window gets condensation on the inside, and how to stop it

Air holds moisture as invisible vapour. Warm air holds more than cool air, and when it meets a cold surface it gives some of that moisture up as droplets.

Every room makes vapour. Breathing, cooking, showers, drying clothes and even a few houseplants add to it. In a loft room the vapour rises and gathers under the roof, because warm air climbs and the slope above it has nowhere else for it to go. Roof glass is also thin compared with a wall, and it faces the open sky, which on a clear night drains heat from the pane faster than a window in a sheltered wall. That combination gives a roof window the coldest inner surface in the room, so it is the first place the moisture shows.

The temperature at which air can hold no more vapour is called the dew point. When the inside face of the glass sits below the dew point of the room air, it mists. That is why condensation is worst on still, cold mornings and eases on a mild, breezy afternoon.

A little film on the pane in the morning, gone by breakfast, is normal in a cold snap. It becomes a concern when the glass runs with water every day, or when the timber lining and plaster around the window turn grey or spotted. At that point the moisture is landing on surfaces that cannot dry out.

There are two kinds of misting that look alike from across the room. The answer to each is different, so it is worth telling them apart first.

1. On the room face of the glass. You can wipe it off with a cloth, and it returns after a cold night. This is ordinary condensation from room air, and it is what this page covers.
2. Between the two panes. You cannot reach it, and wiping changes nothing. It means the sealed unit has failed and moist air has got into the cavity. The fix is a replacement unit or window, and our hub on misted and failed rooflight replacement covers that route.

A quick test is to look at the glass in low, angled light and run a finger over the clouded area. Droplets that smear are on the room side. A haze that stays put is inside the unit.

Some condensation on the outer face of a roof window on a cold, clear dawn is also normal. It forms because the outer pane is cooler than the night air, which is a sign that the glazing is doing its job of keeping heat in. It clears as the sun comes up and needs no action.

The surest way to stop moisture on the glass is to remove it from the air before it reaches the pane. That takes air movement, and it takes it at the source.

The bathroom and the kitchen produce the bulk of the vapour in a house, so extract fans there should run during use and for a while afterwards. Part F of the Building Regulations asks for ventilation in a dwelling, including extract in wet rooms, and the level expected changes when windows are replaced or added. Closing the bathroom door while showering and opening a window afterwards stops the vapour heading up the stairs to the loft.

The roof window can help itself. Some have a ventilation flap, or a slot at the head of the frame, that lets a little air through with the window closed. Our answer on whether roof windows need trickle vents explains when one is worth having. Opening the sash for ten or fifteen minutes each morning clears the night's moisture, and because the window is at the highest point, it lets the warmest, dampest air out first.

In a loft bedroom, the situation is slightly different. Two people breathing all night in a small room release a good amount of vapour, and a closed door keeps it in. A trickle of ventilation through the night, or an electric opening rooflight on a timer or sensor, is one way to manage it without getting out of bed.

A room that is heated hard for two hours and then left to go cold invites condensation. Steady, gentle heat is better.

Each time the room cools, the glass reaches the dew point first. A constant low temperature keeps the pane warmer than the same average delivered in bursts. If the loft room has no radiator at all, a small panel or a thermostatic valve set to a low background level is enough to keep the glass above the point where it mists.

Blinds are a second cause. A blackout or pleated blind pulled tight to the pane leaves the glass behind it colder, and trapped room air reaches it through the gaps at the edge. Leave the blind open for a short time each morning. The nursery page on blackout blinds for a roof window discusses this trade-off, and blinds for roof windows compares the types.

Do not cover an air slot with a blind headrail, and keep furniture and curtains clear of the glass. Air needs to flow across the pane to keep it warm.

If you are fitting or replacing a window, the glazing build sets how cold the inner pane gets. This is one of the few points where the quiet choice and the warm choice agree.

Three things in the specification matter:

• Low-emissivity coating. A thin layer on the inner face of the cavity reflects heat back into the room, so the inner pane stays warmer.
• A warm-edge spacer. The spacer between the panes is a weak point. An insulating one keeps the edge of the glass warm, and that edge is where misting starts.
• The frame and lining. A frame that does not conduct cold inward, with insulation fitted tight to it and an airtight lining, stops the cold reaching the room surface of the reveal.

The same laminated inner pane that cuts rain noise is also a little warmer to the touch than a thin single sheet, which is why it features in our quiet spec. The finish around the opening is explained in roof window linings and the reveal. For the rest, energy efficient rooflights explains the thermal side.

Does a bathroom roof window mist more?

It has more moisture to deal with, so it will mist sooner without an extractor. With a working fan and a window that opens, it stays clear. See putting a roof window in a bathroom for the detail.

Can condensation damage the lining or the plaster?

If water runs down the glass every day, it will stain timber and lift paint on the reveal. Dry it with a cloth and deal with the cause, not the glass.

Is a dehumidifier a good idea?

It can help in a room where ventilation is hard to add, such as a loft bedroom with a closed door. It treats the air, while the glazing and the ventilation treat the cause.

Will a new window stop it completely?

A better window cuts it a great deal, because the inner pane stays warmer. It cannot cancel a humid room. Get a quote and we will look at the glazing and the room together. Our main page on roof window installation sets out the service.