Rooflight Streaming With Condensation: What It Means
If a rooflight is streaming with condensation, act on humidity first: ventilate the room hard, run extractor fans, stop drying washing indoors, and wipe the glass so water does not run onto timber or plaster. Then check where the water is. On the room side of the glass, it is surface condensation from very humid air meeting a cold surface, and better ventilation plus a warmer unit fixes it. Between the panes, sealed inside the unit, the unit has failed and needs replacing, because there is no repair. Running water that returns every day, whatever you do, points to a failed unit or a genuinely wet room, not a passing cold snap.
Streaming is more than the usual morning mist, so it is worth acting on before it stains plaster or rots timber. Here is what to do, and how to tell whether you need a dehumidifier or a new unit.
What to do now
- Wipe the water off so it does not run onto the plaster reveal or pool on timber, where it starts staining and rot.
- Ventilate hard. Open the rooflight if it opens, and a window, to let the humid air out. Run the kitchen and bathroom extractor fans, and check they vent outside rather than recirculating.
- Cut the moisture at source. Stop drying washing on radiators or an unvented dryer, put lids on pans, and keep bathroom and kitchen doors shut while cooking or showering so the moisture does not spread to the coldest surface in the house, the rooflight.
- Run a dehumidifier in the room for a few days. If the streaming stops as the room dries, you had a humidity problem, not a failed unit.
- Keep a low background warmth on if the room has been cold and shut up, because a warmer inner glass surface condenses less.
Dehumidifier or replacement, the quick test. If a few days of hard ventilation and a dehumidifier clear it, the unit is fine and the room was too humid. If the water keeps returning, or you can see it trapped between the panes where you cannot wipe it, the unit has failed and drying the room will not fix it. That is the line between a moisture problem and a glazing problem.
How to check whether the unit has failed
| Where the water is | Can you wipe it? | What it means |
|---|---|---|
| Room side of the glass | Yes | Surface condensation, humid air on a cold surface |
| Between the two panes | No, it is sealed inside | The unit has failed, replace it |
| Running from the reveal or a corner | Wipeable but returns | Cold bridged upstand, or possibly a leak |
Water you can wipe off the inside face of the glass is surface condensation. Manage it with ventilation and, in the longer term, a warmer unit. Misting or droplets sealed between the panes, which no amount of wiping reaches, mean the edge seal has failed, the gas has gone, and moist air is trapped in the cavity. There is no repair for a failed sealed unit, it is replaced. If the water is coming from the reveal rather than the glass, it may be a cold bridged upstand condensing, covered on thermal bridging at rooflight junctions, or a genuine leak, which follows rain rather than humidity. The wider picture is on why condensation forms on rooflights.
When running water means replacement, not a dehumidifier
A dehumidifier manages moisture in the air. It cannot fix a unit whose seal has gone, and it cannot warm up a cold-bridged upstand. So replacement, not drying, is the answer when:
- The water is sealed between the panes. The unit has failed, full stop.
- Streaming returns every cold morning despite good ventilation, on an old single-glazed or early double-glazed unit whose inner surface is simply too cold. The performance gap is on how much heat old rooflights lose.
- The reveal keeps going black alongside the streaming, which means the upstand is cold-bridged and the junction needs rebuilding, not drying.
We do not offer resealing, because resealing a failed unit or a condensation problem buys a season and hides the cause. Where the unit or the junction is the problem, replacement is the honest fix.
My rooflight is streaming with water inside. What should I do first?
Wipe it so the water does not run onto plaster or timber, then ventilate hard: open the rooflight and a window, run extractor fans, and stop drying washing indoors. Run a dehumidifier for a few days. If the streaming clears as the room dries, it was a humidity problem. If the water keeps returning, or you can see it trapped between the panes where you cannot wipe it, the unit has failed and needs replacing rather than drying.
Does streaming condensation mean my rooflight needs replacing?
Not always. If it clears with hard ventilation and a dehumidifier, the room was simply too humid and the unit is fine. Replacement is the answer when the water is sealed between the panes, which means the unit has failed, or when streaming returns every cold morning on an old, cold unit despite good ventilation. If the reveal is going black alongside it, the upstand is cold-bridged and the junction needs rebuilding, which drying will never fix.
Get it diagnosed before it damages the ceiling
We tell you whether it is humidity, a failed unit or a cold bridge, and price the right fix so it does not come back. A 10-year workmanship guarantee, and installers with 25 years on rooflights.
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