Cold bridging around a rooflight, and the damp it leaves

A cold bridge shows itself as a line. It follows the edge of the opening, and the room tells you where the heat is leaking before any instrument does.

Heat always takes the easiest route out. Through a wall or roof that is well insulated, the route is slow. Wherever a dense, conductive material crosses the insulation from inside to outside, the route speeds up, and that material becomes a bridge. Around a rooflight the usual bridges are an aluminium frame running unbroken from the inside face to the outside, a timber upstand with no insulation in it, and a gap in the roof insulation where it was cut back to fit the opening.

The effect is on the inside surface. Warm air in a room holds moisture, and when that air touches a surface cold enough, the moisture turns to water. So the telltale is a narrow band of droplets on the frame, a damp stripe on the plasterboard splay or a grey shadow of mould along the reveal, all tracing the shape of the opening. Elsewhere on the ceiling the paint stays dry, because the surface there is warmer.

The same thing happens on the glass, but for a different reason. Mist on the inside of a pane is ordinary condensation on a cold surface. A sealed unit that has failed and fogged between the panes is another matter, and our page on misted and failed rooflight replacement deals with that.

There are four places we look at a survey. Each has a cure that belongs in the specification, not one added later.

1. The frame itself. Metal carries heat well. A frame made in one piece lets the cold from outside run straight to the inner face. A thermal break is a strip of low-conductivity material, usually a polyamide or foam section, that separates the inner and outer halves of the frame. Our page on thermally broken rooflight frames shows the construction.
2. The glass edge. The spacer bar that holds the two panes apart is a line of the unit where heat travels most easily. A metal spacer makes that edge cold, and condensation collects at the corners first. A warm-edge spacer is made of a less conductive material, which is why it matters for damp as much as for warmth. See warm edge spacers and argon fill.
3. The upstand. On a flat roof, a rooflight sits on a kerb, and that kerb rises through the insulation and the waterproofing. If it is a thin shell of untreated timber, it is a bridge the full height of the roof build. A deeper, fully insulated kerb and a lining with no gaps give the frame a warm seat.
4. The joint between the two. Even a good frame on a good kerb can be let down by a gap, a void of air or a poorly filled seam where they meet. Continuous insulation, tight against the frame and finished with an airtight tape or lining, closes it.

A little condensation on a winter morning is normal. A repeat, in the same place every cold day, is a sign that the detail is wrong.

Water lying on paint and plaster softens the surface, lifts the coating and gives mould something to grow on. Black spotting along a reveal is usually mould, and it tends to return to the same strip however often you wipe it, because the surface underneath has not changed. Timber frames and linings can swell, and sealants can lose their grip where they stay wet.

The air in the room plays a part. A kitchen with a hob, a bathroom with a shower and a bedroom full of sleepers all put moisture into the air. A properly ventilated room carries it away, and a bridge that is well detailed will not collect it. Where the room is damp and the bridge is bad, the problems arrive together. We come back to ventilation in the note below, and to the wider question of heat loss in how heat escapes through a rooflight.

The cure is a set of small decisions made together, and all of them are written on the quote.

First, the frame. We specify a thermally broken one wherever the rooflight is to be used in a room that holds moisture, and on any roof where the frame will face a long cold night. Second, the glass. A warm-edge spacer goes into the sealed unit so the perimeter does not chill. Third, the kerb. It is built tall enough for the roof buildup, insulated through its full depth and lined inside with a material that keeps an unbroken surface. Fourth, the junction. Insulation in the roof is taken right up to the kerb rather than stopped short, and the lining is sealed to the frame so warm air cannot reach the cold surface behind it.

The same upstand that stops the bridge also has a part in how quiet the room is. A dense, insulated kerb gives the frame something solid to sit on, which is why the quiet spec lists it alongside the glazing. Our page on insulating around a rooflight covers the upstand and reveal in detail, and our page on the quiet spec shows how it is recorded.

The regulations give a reason beyond comfort. The energy rules for extensions and replacements, set out in Part L and rooflights, expect heat loss at junctions to be kept in check, and Building Control will look at the detail where the work is notifiable. We handle that notification.

You can gather some clues before anyone visits, and they help us arrive prepared.

Look at where the damp forms. A stripe that follows the frame points to the frame or the glass spacer. A stripe on the splay or reveal below the frame points to the kerb or the insulation. A patch in one corner only may mean a local gap. Note whether it appears only on the coldest mornings, and whether the room is one that gets steamy. Take a photo in the morning, before wiping anything, and another with the rooflight open or closed if it has a vent.

A warm hand held near the frame and then the wall will often feel the difference too, though it is not a substitute for a survey. For deciding whether the existing unit should be renewed, our page on upgrading an old rooflight explains how we tell which part is at fault. To discuss yours, get a quote, or message us on WhatsApp with the photos.

Does a cold bridge cause a rooflight to leak?

No. A cold bridge and a water entry are different faults. The bridge causes condensation on the inside face, which is water from the room air. If a rooflight lets in rain, the cause is in the waterproofing or the upstand, and the answer is a replacement detailed properly.

Will a thermal break stop all condensation?

It removes the main path for it, but not the moisture in the room. On a very cold night in a steamy room, a small amount of moisture on glass can still appear. Good extraction and a warm frame together keep it to a minimum.

Are aluminium frames always bridged?

A frame with no break is. A frame with a proper thermal break and a warm-edge unit is a far smaller bridge, and aluminium in that form is still a sound, slim choice.

Does the kerb depth matter?

Yes. A shallow kerb leaves the frame sitting close to the cold outer layers of the roof. A taller, insulated one lifts it into the warm side of the build. The energy efficient rooflights page shows the options we fit.