01Cold spots
Why some skylights leave a room chilly
A cold skylight is nearly always an old one or a badly built one, and the chill reaches you in four separate ways.
Stand under a 1980s plastic dome on a flat roof extension in February and you feel it straight away. The air near the glass is cooler, there is a slight drift of cold down onto your head, and the room never quite gets as cosy as the rest of the house. So do skylights make a room cold? That one does, and you are right to blame it. But it is the build of that particular rooflight that is doing it, not the idea of a window in the roof.
Heat leaves a room through a rooflight in four main ways. The first is straight through the glass, which is what a U-value measures (heat lost per square metre for every degree of temperature gap across the pane; lower is warmer). The second is around the edge, where an aluminium spacer bar or an uninsulated frame acts as a cold bridge. The third is air leaking past worn seals or a gappy kerb, which you feel as a draught. The fourth is the one people rarely put a name to: your body losing warmth to a cold surface above it. A pane at 8°C over the sofa makes you feel cooler even when the thermostat says 21°C.
There is also a downdraught. Air touching cold glass cools, gets heavier and drops. Over a large, poorly insulated pane that becomes a steady slide of chilly air into the room.
| Glazing | Typical U-value (W/m²K) | How it feels underneath |
|---|---|---|
| Single glazed or twin-skin dome | about 5.0 or higher | Cold to stand under, prone to dripping |
| Early double glazing, air filled | about 2.8 to 3.0 | Noticeable chill on a frosty day |
| Modern double, low-e, argon | about 1.0 to 1.4 | Little difference from the ceiling around it |
| Modern triple glazed | below 1.0 | Warmest option, heavier unit |
02Specification
What a warm rooflight is actually made of
The glass gets the attention, yet the frame, the kerb and the joint into the roof decide whether the whole opening stays warm.
A modern sealed unit is a big step up from anything fitted thirty or forty years ago. Low-e glass carries a thin metal oxide coating that reflects heat back into the room, and the cavity between panes is filled with argon, a heavy gas that slows heat crossing the gap.
The edges matter as much. A warm-edge spacer is the bar that holds the panes apart, made from a plastic or composite instead of aluminium, so the perimeter of the glass stays warmer and far less prone to condensation. Warm-edge glazing is our standard choice. On flat roofs the rooflight sits on a kerb or upstand, the raised collar that lifts it above the roof surface, and that collar should be insulated and joined to the roof insulation without a gap. If the roof insulation stops short of the kerb, you get a chilly band around the opening however good the glass is. Our page on rooflight kerbs and upstands goes through the options.
Last comes the lining: the plasterboard reveal inside, and a vapour control layer behind it so warm, damp room air cannot reach the cold timbers. Every one of those parts is chosen for heat as well as light on our jobs, and the reasoning is set out in full on our warm specification page.
The parts that keep the heat in
- Sealed unit
- Low-e coated, argon filled, double or triple
- Spacer bar
- Warm-edge, never plain aluminium
- Frame
- Timber, PVC or thermally broken aluminium
- Kerb or upstand
- Insulated, tied into the roof insulation
- Inside lining
- Insulated reveal with a vapour control layer
03Your roof
How to tell if yours is the problem
A few simple checks on a cold morning will show whether a skylight is costing you warmth.
You do not need any kit for this. Pick a cold, still day with the heating on and go and look. Put the back of your hand near the glass, then near the plasterboard beside it, and compare. Look at the corners and edges of the pane.
If what you find points to the glass alone, a misted or tired sealed unit, the fix may be a new unit in the old frame. If the frame, the kerb or the lining is the cold part, a full replacement is the sensible answer. Taking out the entire rooflight and fitting a new one is a controlled fitting under Part L of the Building Regulations. The new unit has to meet the current limit of 2.2 W/m²K, and Building Control is notified of the work. We handle that notification for you. Our guide to replacing an old rooflight walks through what happens on the day.
- Glass that feels icy compared with the ceiling next to it
- Water beads or a dark line along the bottom edge of the pane
- Mist trapped between the panes, which means the seal has gone
- A draught you can feel, or a tissue that flutters at the frame
- Black mould on the reveal or the plaster around the opening
- A dome that has gone cloudy, brittle or yellow
04Size
More glass, more care
Even the best rooflight loses more heat than the insulated roof it replaces, so the size should suit the room rather than the brochure.
Here is the straight part of the answer. A well insulated roof might sit at around 0.15 to 0.18 W/m²K. A very good double glazed rooflight is several times that. So every square metre of glass you add will let out more heat than the roof did, and anyone who tells you otherwise is selling. The point is that a modern unit loses so little that you will not feel it as a cold spot, and the daylight it brings in usually means fewer lights on and a room that warms up in winter sun.
That is why we size each window for the room it serves. A landing in a new-town semi may need one modest roof window. A long kitchen-diner under a flat roof can carry a lantern, but the lantern then deserves the full warm build. Blinds help too: a fitted thermal blind adds a layer of still air on winter nights, as covered on our blinds for warmth page. If you want to compare the heat loss of glass in the roof and in a wall, see do rooflights lose more heat than windows.
Not sure where your own roof stands? Our free survey and written quote, from Radiant Skylights, will show whether the glass, the frame or the kerb is the weak spot, before any money is spent.
Is a chilly skylight something you just put up with?
Will a new skylight put my heating bill up?
A modern, properly insulated rooflight loses only a small amount of heat, and swapping an old dome or single glazed unit for one can make the room warmer than it was. Adding a brand new opening does add a little heat loss, which the extra daylight partly offsets. Our energy efficient rooflights page covers the choices.
Is triple glazing needed to stop a cold spot?
Not usually. Good low-e double glazing with argon and a warm-edge spacer is enough for most rooms. Triple glazing helps on big lanterns, north-facing roofs or rooms you want especially snug. The numbers are set out in rooflight U-values explained.
Why does my rooflight drip in winter?
Warm, damp air from the room meets glass or a frame colder than its dew point and turns back to water. It is a sign of heat escaping, most often through old glazing, an aluminium spacer or an uninsulated kerb. A warmer build cuts it right back.
Are roof lanterns colder than flat skylights?
A lantern has more glass and more frame, so it has more to get right, but a well specified one is comfortable all year. We look at this in detail in are roof lanterns cold in winter.
