Winter answer
Does a rooflight make a room colder?
A glazed panel gives up warmth more quickly than the insulated roof around it, so a rooflight does cost something in winter. How much you notice depends on the age of the unit, the way it is fitted and where you sit under it.
The answer
A rooflight can make a room feel colder, because glass lets heat out faster than an insulated roof of the same size. With a modern sealed unit and a well-insulated kerb, the loss is small enough that most rooms stay comfortable. The coldest rooflights are old, single glazed or misted units, and badly sealed or uninsulated surrounds.
Why does the air under a rooflight feel cooler?
Two things: the glass loses heat quickly, and your body gives off heat towards a cold surface above you. That chill is felt as radiant cooling, even when the air temperature is fine.
Standing beneath a pane of cold glass feels different from standing beside a warm wall, even when a thermometer reads the same. Your skin trades heat with every surface in view. A cold ceiling pane draws more from you than a plastered one, so the spot feels chilly without any draught at all.
Air that sinks
Air touching a cold pane cools, grows heavier and slides down. In a tall room, that cooler air can spill off the glass and settle at head height as a gentle, steady downdraught. It is easy to mistake for a leak, but it is simply the room's air passing across a cold surface.
Glass against roof
An insulated roof build-up is thick, with layers designed to slow heat down. A rooflight replaces a strip of that with a thin sealed unit. Even a very good one has a higher U-value than the roof around it, so the patch it fills is always the coolest part of the ceiling. How much cooler is what the next sections look at, and our guide to rooflight U-values and Part L explains the numbers.
Does the age of the unit matter?
A great deal. Single glazing, early sealed units and any unit that has lost its gas or its seal let heat out far faster than a current unit with coatings, a warm edge and a gas-filled gap.
If your present rooflight feels icy, its age is a likelier cause than the idea of a rooflight itself. Glazing has changed a lot, and an old unit tells you very little about what a new one does.
What changed
A single pane has no sealed gap, no coating and nothing to slow heat down except the thin glass. Early double units had a gap, but no heat-reflecting coating, and metal spacers ran cold around the rim. Current units have a low-e coating, a gas fill, a warm edge spacer and a frame with a thermal break. Each of those features closes part of the gap.
When a unit goes cloudy
If a sealed unit's seal has failed, moist air gets between the panes and the gas escapes. The glass mists and the insulation drops with it. The unit has stopped doing the work it was built for, and it is a good reason to think about a replacement rooflight rather than living with the chill.
The limit a new unit must meet
A rooflight fitted in an existing dwelling in England has a limiting U-value of 2.2 W/m²K. A roof window has a limit of 1.4 W/m²K, or WER Band B. A unit that meets those has a far lower loss than a single pane. Our page on energy efficient rooflights shows the options we fit.
Can the way it is fitted make it colder?
Yes. Gaps in the insulation round the frame, a thin or bare kerb and an unsealed lining let warmth bypass the glass altogether, so a good unit in a poor surround can still feel cold.
People look at the glass, yet the surround often decides whether the room stays warm. Heat tends to leak around a rooflight rather than through it.
The kerb and the lining
On a flat roof, a rooflight sits on an upstand that raises it above the surface. Manufacturers set minimum heights, commonly around 150mm above the finished roof. That upstand needs insulation on the inside and outside faces. If it is built from bare timber or thin board, the heat finds a route round the frame. Inside, the lining between ceiling and glass should be sealed, with the insulation continuous behind it.
Air leaks
Warm room air that reaches the cold edge of a frame will condense there. Gaps in the seal between the lining and the ceiling let that air through. A careful detail keeps the room's air inside the room, which also helps the glass stay dry.
Work done once, in one visit
Because these details sit in the structure, they are best settled when the opening is cut. We survey the roof first, set out the build-up in the fixed written quote, and carry the work out to current Building Regulations. The notification to Building Control is dealt with by us where the work is notifiable.
Signs of a rooflight that is losing warmth
- Misting or water trapped between the panes of the glass
- A cold spot on the ceiling edge, or damp patches round the lining
- A steady downdraught under the glass on a still day
- Condensation that runs from the frame onto the sill
- A single pane, or a unit with no coating on the glass
What keeps a room comfortable with a rooflight?
A current sealed unit, an insulated kerb, a sealed lining, sensible placement away from where you sit, and blinds that close off the glass on cold evenings.
A rooflight does not have to cost you the comfort of the room. The choices that keep warmth in are mostly settled before the unit goes in.
Choose the unit for the room
For a kitchen or loft bedroom, a unit with a coating, a gas fill and a warm edge spacer is the starting point. Larger glass over a room you sit in all day may be worth a third pane, which our guide on double or triple glazed rooflights weighs up.
Place it with thought
Positioning a rooflight over the dining table puts a cold surface above where people sit. Set it towards the centre of the room, or over a worktop or walkway, and the chill is less noticeable. In a loft room, a unit near the ridge draws warm air up and away more than one low on the slope.
Cover it when it is dark
A pleated or cellular blind pulls a layer of still air across the glass. It cuts the radiant chill and holds back some of the loss from the unit overnight, the way curtains do at a window.
More on winter warmth
Will a bigger rooflight lose more heat?
More glass means more loss, because glass has a higher U-value than the roof around it. Size the opening for the daylight the room needs. A well-made unit keeps the penalty modest, and the survey looks at what size suits the room.
Does a flat roof rooflight lose more than a roof window?
They lose heat in the same way, but each has its own Part L limit, 2.2 W/m²K for a rooflight and 1.4 W/m²K for a roof window. Compare the overall figure for the finished unit, not the type of product.
Is condensation a sign that the room is too cold?
Condensation means warm, moist air has met a cold surface. It points to humidity, ventilation or a cold edge in the frame. A warm edge spacer and a sealed lining reduce it, and a bathroom or kitchen needs extraction as well.
Do I need permission to swap an old rooflight for a better one?
On a house, Class C permitted development covers a rooflight that projects no more than 150mm beyond the roof plane. A flat needs planning permission. The rooflight planning guide covers both.