The short answer
A skylight feels cold because its inner surface is the coldest part of the room. Air touching it chills, turns heavy and slides down into the room, which you feel as a draught even when nothing is open. Warmer glazing and a warmer frame edge cure it. On an older unit that usually means a replacement, not an adjustment.
Where the chill is coming from
Glass in a roof has no insulation behind it. Its inside face takes on the temperature of the weather outside, and the room reacts to that surface in two ways.
The first is radiant. Your body gives off heat to any colder surface it faces, and a cold pane overhead is exactly that. Sit under one on a clear January evening and you lose warmth to it without any air moving at all. The room thermometer can read a comfortable figure while you still feel cool.
The second is convective. Air that touches the cold glass cools, becomes denser and falls. Warmer air rises to take its place, meets the glass, cools in turn and falls. The result is a slow loop that dumps a curtain of cool air onto whatever is beneath the opening. People often describe it as a rooflight cold draught, and they go looking for gaps in the frame. Sometimes there is one. Often there is none, and the draught is entirely the glass doing its job as a cold surface.
Both effects grow as the glass gets colder, so they grow as the unit gets older and as the night gets clearer. Our page on how heat escapes through a rooflight explains the paths in more detail.
Why older skylights are the worst offenders
A skylight from the 1980s or 1990s was built to a standard that nobody would accept today. Its inner surface runs colder than a current unit's does, and the difference can be felt.
Several features of an ageing unit add up:
- Single glazing or early double glazing. A single pane has almost no resistance to heat flow. An early sealed unit may have a narrow cavity, plain glass and a metal spacer bar.
- A failed seal. Once the edge seal lets air in, the gas fill is gone and moisture can collect in the cavity. The unit insulates worse, and you will see misting between the panes.
- A solid metal frame. Aluminium with no thermal break carries cold from the outside straight to the inner edge of the frame, where the glass meets it.
- Thin or tired upstand insulation. The kerb or surround under a flat rooflight is part of the wall of the room. If it is thin, the cold reaches the ceiling reveal.
The frame edge deserves a note of its own. The glass may be acceptable while the edge strip is where the cold lives. That ring is where condensation forms first on a cold morning and where black mould can follow. Our answer on cold bridging around a rooflight covers that path.
Checking whether it is the glass or the gaps
A draught that comes from a gap and a chill that comes from cold glass need different answers. A few minutes at home will separate them.
- On a windy day, hold a thin strip of tissue near the frame where it meets the lining. A flutter that follows gusts points to an air path.
- On a still, cold night, put the back of your hand a few centimetres from the glass, then the same distance from a nearby wall. A sharp difference is the cold surface at work, not a gap.
- Look along the edge of the glass for beads of water or a dark line. Moisture that returns daily is condensation on cold material.
- Look between the panes, from inside. Cloudiness or a droplet trail that you cannot wipe away means the sealed unit has failed.
Air leakage around an opening is a workmanship and sealing issue, and on a rooflight it is looked at when the unit is taken out. A cold pane is a specification issue, and it is solved by the glazing you choose next.
What a warmer rooflight is made of
Comfort comes from raising the inner surface temperature. Three parts of the build do that, and a replacement unit can combine all of them.
A low-E coating reflects heat back into the room rather than letting it pass through, which warms the inner pane. Gas fill in the cavity, usually argon, slows the movement of heat across the gap. A warm-edge spacer replaces the metal bar at the perimeter of the unit so the edge does not chill. Our pages on low-E coatings and warm-edge spacers and argon fill explain each, and thermally broken frames cover the frame.
The surround matters just as much. Fully insulating the upstand and lining the reveal stops the cold coming in around the glass. That is the detail in insulating around a rooflight.
A thermal blind is a sensible extra and worth a look at our page on thermal blinds for a rooflight. It does not replace a better unit. It covers the glass at night while the glass itself stays cold.
Colder rooms, and what to do about them
Will a new skylight stop condensation on the glass?
A warmer inner surface makes condensation much less likely, because moist room air meets a surface above the temperature at which it would form. A very humid room, such as a bathroom without an extractor, can still mist a cool pane, and ventilation helps.
Is the draught a sign that my rooflight is unsafe?
No. Cold air falling from glass is a comfort problem, not a structural one. If the unit is loose in its opening or the frame is visibly moving, tell us at the survey so that we can look at how it is fixed.
Can I just fit secondary glazing or film inside?
Film and add-on panels are awkward to fit on a sloped or overhead surface, and they do not address the frame edge or the upstand. A replacement sealed unit or a new rooflight treats the whole opening. See upgrading an old rooflight for the routes.
Is triple glazing the answer to a cold skylight?
Sometimes, and not always. It adds weight and cost, and a good double unit with the right coating and spacer can already bring the inner pane up to a comfortable temperature. The page on triple glazed rooflights sets out when the third pane earns its place.
If your skylight chills the room all winter, get a quote and we will note the cold spots at the survey and write the glazing build into your quiet spec. The service itself is on the energy efficient rooflights page.