Why is my VELUX window misting up?

So the first job is not to guess, it is to work out which of those three you are looking at, because they are three different situations with three different answers. Two of them are about the weather and the way you use the room. Only one of them is a problem with the window itself.

Reach up and wipe the glass with a dry cloth. Where the mist sits decides everything.

• It wipes off the room-side face. That is condensation, water coming out of the warm air inside your house onto the cold glass. Annoying, common on a skylight, and not a fault in the window.
• It is on the outside face and your cloth cannot reach it, but it burns off within an hour or two of daylight. That is dew on the outer pane. It looks alarming through the glass and it means nothing is wrong.
• It is trapped in the middle, between the two panes, and nothing you do from either side touches it. That is a failed sealed unit. The glazing has reached the end of its life and the pane needs swapping.

Do this before you phone anyone. The location of the water is the whole diagnosis, and it takes longer to describe than to check.

Warm air holds moisture. Cold air cannot hold as much, so when warm damp air touches a surface that is colder than its dew point, the water it can no longer carry condenses out onto that surface. A skylight is very often the coldest surface in the room and it sits at the highest point, which is exactly where warm humid air collects as it rises. So the glass mists before the walls or the ordinary windows do, and people read that as the skylight being at fault when it is really just the first place the room shows its humidity.

Where does the moisture come from? A boiling kettle, a pan without a lid, a hot shower under a bathroom rooflight, laundry drying on an airer, and simply breathing overnight in a bedroom. Loft conversions and rear kitchen extensions are the usual places this shows up, because they combine a lot of activity with a large area of glazing overhead.

The answer is ventilation, not a new window. Open the trickle flap at the top of the VELUX, which is designed to let a slow background exchange of air happen with the window shut and locked. Run the extractor fan in a kitchen or bathroom for a good while after you finish, not just during. Crack the skylight open for a few minutes when you cook or bathe. If the misting only appears on cold mornings and clears through the day, that is normal seasonal behaviour and there is nothing to put right on the glass.

Every so often the mist is on the outer pane, on the roof side, where you cannot reach it. This throws people because it looks like the window has gone wrong overnight. It has not.

On a clear, still night the outer pane radiates its heat up to the open sky and cools to below the temperature of the air around it. When it drops past the dew point of that night air, dew settles on it, the same dew you see on car roofs and lawns on the same mornings. It forms on the outside for one reason: modern double and triple glazing is good at holding heat inside the house, so very little warmth leaks out through the glass to keep the outer pane above dew point. In other words, external misting is a side effect of the glazing insulating properly. A poorly performing old unit would leak enough heat to keep its outer face warm and dry, which is nothing to envy.

It clears on its own as the sun comes up and the air moves. South-facing rooflights clear it fastest, because they catch the morning and midday sun square on. There is nothing to act on here at all.

This is the one that matters. A sealed glazing unit is two panes held apart by a spacer bar around the edge, with the gap filled with dry air or argon and a strip of desiccant inside the spacer to mop up any stray moisture. A continuous seal keeps that cavity closed. When the seal breaks down, usually after years of the glass expanding and contracting through daily heating and cooling, damp outside air works its way in. The desiccant saturates, and after that every temperature swing leaves condensation or a faint cloudy film on the inside of the cavity where you can see it but never touch it.

It does not recover. Once the cavity is breached the misting comes and goes with the weather, gets worse over the seasons, and often leaves a permanent mineral haze on the glass even on the days it looks clear. There is no cleaning it and there is no drying it out. The unit is spent, and the answer is a replacement pane, or a replacement window where the frame is tired as well.

Heat cycling is what wears the seal out, so glass that swings between hot afternoons and cool nights takes more punishment than glass that stays at an even temperature. A south-facing rooflight over the Thames Estuary bakes through the middle of the day and cools right down after dark, and that daily stretch and shrink is exactly what tests an edge seal over the years. It is one more reason orientation is worth thinking about when the glass comes out.

Match your action to the test you did at the start.

If the water wipes off the inside, work on the air in the room before anything else. Better ventilation, the trickle vent open, and the extractor running longer will usually settle it, and no glazing work is needed. If dew is forming on the outside, leave it, it will clear with the day and it is a sign the glass is doing its job.

If the misting is locked between the panes, the glazing has failed and it is time to replace it. That is a straight swap of the sealed unit or the window, and it is also the natural moment to look again at the glass specification rather than dropping in a like-for-like pane. If the window sits on a south-facing roof, the room that overheats in summer, this is your chance to move to solar-control glazing with a lower G-value, which cuts the heat and the glare coming through without making the room dark. We set out how that choice works on our energy-efficient skylight installation page, and it is the heart of our VELUX window installation work: orientation first, then the glass, then the unit.

If you would like us to look at a misted VELUX and tell you which of the three it is, tell us where the water sits and which way the roof faces. Ring 01702 898232 or request a quote, and if it does turn out to be a failed unit we will price the replacement and the glass that should go back in its place. Our skylight installation costs page gives you the range before you call.