Do flat rooflights cause condensation?
No. A flat rooflight does not create moisture, and nothing about the unit adds water to a house. What it does is give the room the coldest surface it has ever had, positioned at the highest point of the ceiling, facing straight up at the sky. Warm damp air rises to exactly that spot, and whatever moisture the room was already carrying becomes visible there first. So a new rooflight often coincides with condensation appearing, which is not the same as causing it. The mechanism is specific to horizontal glass and worth understanding properly, because the useful fixes are not the ones people reach for. Kitchen lanterns behave differently again, and that is covered on the roof lantern installation pages.
The glass is where the water appears, not where it comes from
Air holds water vapour in proportion to its temperature. A room at 20 degrees and 55 per cent relative humidity has a dew point of about 10.7 degrees, which means any surface in that room colder than 10.7 degrees will collect liquid water on it. Nothing else has to go wrong.
Before the rooflight went in, the coldest surface in the room was probably a window reveal or an outside corner, and the moisture spread thinly enough to go unnoticed. Now there is one square metre or more of glass sitting several degrees below everything else, and the same quantity of water in the air concentrates on it. The house did not get damper. It got a better indicator.
Horizontal glass sees more sky than any other surface in the house
This is the part that is genuinely particular to a flat rooflight rather than a vertical window or a pitched roof window.
Any surface exposed to a clear night sky radiates heat upwards and receives very little back, because the effective radiant temperature of a clear sky is far below air temperature. How much heat a surface loses that way depends on how much of the sky hemisphere it can see. A vertical window sees half. A roof window at 40 degrees sees most of it. A rooflight laid at five degrees sees essentially all of it. On a still, clear night the outer pane of a flat rooflight can drop two or three degrees below the outside air temperature, which is why frost forms on a rooflight while the wall below it is clear.
Water that cannot run anywhere, and the drip onto the floor
On a lantern or a pitched roof window, condensation forming on the inner face runs downhill to the bottom edge of the pane within minutes and collects in the frame, where it is out of sight.
On a rooflight set at five degrees it does not run. It beads, the beads grow, and at a certain size surface tension gives up and they fall. That is the moment people describe as a leak: intermittent drops landing on a worktop or a dining table on a cold morning, with no rain overnight. The tell is timing. Water from outside arrives during and after rain. Condensation arrives on cold clear mornings, worst between five and eight, and has stopped by mid-morning. If it is dripping on a frosty day with no cloud in the sky, it came from inside the room.
The drained rebate, and the weep holes that empty it
A properly built flat rooflight already expects some condensation, and the frame is designed to deal with it.
Around the inside of the aluminium frame, below the glass line, there is a continuous channel. Water forming on the underside of the glass runs to the perimeter, drops into that channel, and is carried to weep holes at the low corners of the frame, where it discharges onto the outer surface of the glass and runs away down the slope. The whole arrangement depends on the unit actually having a fall. Set a unit level, or set it with the fall running towards the corner without the weeps, and that channel becomes a small tank that fills, overflows at the low edge and marks the plaster below. The relationship between glass pitch and drainage is set out on the flat roof rooflight installation page.
The spacer bar at the edge of the pane, and what warm edge changes
Look at a rooflight running with condensation and the pattern is always the same: a band of water 30 to 60mm wide following the frame all the way round, with the middle of the pane clear.
That band exists because of the spacer bar holding the two panes apart. A traditional aluminium spacer conducts heat straight from the inner pane to the outer one, so the edge of the inner pane runs several degrees colder than its centre. A warm edge spacer, in stainless steel, structural foam or a composite, cuts that conduction substantially and typically lifts the edge surface temperature by two to four degrees. On a marginal roof that is the whole difference between a clear morning and a wet one, and it costs very little when specified at order. It is worth asking what spacer a quoted unit uses, because the answer is not always warm edge.
Dew on the outside of the glass on an autumn morning
An increasingly common call, and it is the opposite problem. The homeowner sees a milky bloom across the rooflight at seven in the morning that clears by nine, and assumes the sealed unit has failed.
What is actually happening is dew forming on the top face of the outer pane, outdoors, exactly as it forms on a car roof and on a lawn. It occurs on units with good insulating glass, because so little heat now escapes through the unit that the outer pane no longer gets warmed from inside and falls below the dew point of the outside air. It is a sign the glazing is doing its job. Water sitting between the panes, where no cloth reaches it, is a different matter entirely and means a failed sealed unit, covered on the replacing a failed or misted rooflight page.
Warm deck, cold deck, and the water that never shows itself
Surface condensation announces itself. Interstitial condensation, forming inside the roof build-up, does not, and a flat roof is where it does the most damage.
A warm deck flat roof carries the insulation above the deck with a vapour control layer directly under it, so the deck timber stays warm and dry and the moisture never reaches a cold surface. At a rooflight, that vapour control layer has to be cut, taken up the face of the kerb and taped to the frame. Left loose or trimmed short, it becomes an open route for room air into the coldest part of the construction, and the water is deposited on the underside of the deck where nobody sees it for years. Kerbs are frequently built by whoever built the extension, before the rooflight is even ordered, which is precisely why the kerb detail and the vapour layer get checked at survey rather than assumed. An uninsulated timber kerb also runs cold from top to bottom and collects a band of water at the ceiling junction that reads convincingly as a leak.
Still air in a deep reveal, and a room with no radiator under the glass
A rooflight sits on a kerb 150 to 300mm tall, and the plastered reveal below it is a deep box. Formed with square vertical sides, that box holds a pocket of still air trapped directly under the coldest surface in the room, and still air against cold glass is the ideal condition for condensation.
Splaying the reveal outwards so the ceiling opening is wider than the frame does two things at once. It throws more daylight into the room, and it lets warm room air wash across the underside of the glass instead of sitting beneath it. Moving air raises the surface temperature and carries vapour away. The other half of this is heating: a modern extension with underfloor heating and no radiator has no rising warm air to wash the glass at all, where an older room with a radiator on the outside wall generates a convection current that does the work automatically.
Low ground, river mist, and what is worth changing first
Gloucester, Tewkesbury, Longford and the vale villages sit low beside a tidal river on ground that holds mist, and external humidity through a still winter night is regularly at or close to saturation. Clear calm nights with a hard radiative frost are exactly the conditions that pull a horizontal pane coldest. On those mornings a well built, well ventilated rooflight in a Severn Vale extension will still carry a border of water, dry by mid-morning, marking nothing. That is the realistic target rather than a unit that is never damp.
The order worth working through is the cheap end first.
- Establish where the water is: room face of the glass, top face outdoors, between the panes, or on the reveal and the kerb line.
- Check that extraction in the room actually moves air, and that a cooker hood ducts outside rather than recirculating.
- Open the trickle vent or the unit itself for ten minutes after cooking or showering, using the highest opening in the room.
- Check whether the reveal is square or splayed, and whether any warm air reaches the glass at all.
- Check the kerb insulation and the vapour control layer, from above where the roof allows.
- Only then look at the glazing specification and the spacer bar.
For a unit already in place, photographs taken on a bad morning are more useful than a description. Send them to info@severnskylightsgloucester.co.uk with a note of the room, the heating and what ventilation exists, and the reply will say which of the six items above is doing the damage.
Next on this subject.
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