01The idea
Why every rooflight we fit is specified for heat as well as light
A skylight is a hole in the warmest part of your house, so we plan how it holds heat before we talk about how much light it lets in.
Warm air rises. In a loft room, a flat roof extension or a landing, the ceiling is where your heating ends up, and that is exactly where a rooflight goes. Get the specification wrong and you have built a very bright cold spot. Get it right and the room is as comfortable on a January evening as it is cheerful in June.
That is what we mean by a warm rooflight specification. It is not one product or one brand. It is a set of choices that run from the glass, through the edge of the sealed unit and the frame, down into the kerb or upstand, across the join with your roof insulation and into the lined reveal you see from the room. Every one of those is a place heat can slip out. Our job is to close each one.
If you have ever stood under an old skylight and felt it pulling the warmth off your head, our answer to do skylights make a room cold explains why. This page explains what we do about it.
- 2.2W/m²K, the Building Regulations limit for a new or replacement rooflight in an existing home
- 5+W/m²K, roughly where old single glazing sits
- Loweris warmer: a U-value is the rate heat passes through
02Glass
The glass: coatings, gas and how many panes
The sealed glass unit does the heavy lifting, and three details decide how well it does it.
First, the low-e coating. Low-e stands for low emissivity: an extremely thin metal oxide film on the inner pane sends warmth back towards the room rather than letting it pass out into the cold. You cannot see it, but it makes a big difference to how warm the inner glass feels.
Second, the gas in the cavity. Air conducts heat more readily than heavier gases, so the gap between the panes is filled with argon (or sometimes krypton in slimmer triple units). It slows heat crossing from the warm pane to the cold one.
Third, double or triple. A good modern double glazed rooflight with low-e glass and argon comfortably beats the Building Regulations limit. Triple glazing goes further, with a warmer inner pane and less chance of a cold downdraught, but it is heavier and dearer. One thing worth knowing: rooflight U-values are measured with the glass lying flat or at its fitted pitch, which reads higher than the same glass upright in a wall, so compare like with like. Our page on double vs triple glazed rooflights goes into the trade-off, and rooflight U-values explained unpicks the numbers.
Double glazed, low-e, argon
Our baseline. Warm enough for most loft rooms, landings and smaller flat roof openings, lighter on the frame and kinder on the budget.
Triple glazed
Worth a look for large panes over a sitting area, rooms you heat for long hours, or north-facing glass that never gets winter sun. The inner pane stays noticeably closer to room temperature.
03Edges
Warm-edge spacers and thermally broken frames
Heat takes the easiest route out, and on a poor rooflight that route is round the edge of the glass.
Every sealed unit has a spacer bar running round its edge, holding the panes apart. On older units it is aluminium, which carries heat out like a metal spoon in a hot drink. That is why old double glazing often shows a band of condensation round the bottom edge on a frosty morning. A warm-edge spacer is made from a low-conductivity material, often a polymer or stainless steel composite, so the edge of the glass stays closer to room temperature. Warm-edge glazing comes as standard on every rooflight we fit.
The frame is the next weak point. Aluminium is strong and slim, which is why lanterns and flat rooflights use it, but it conducts heat freely. A thermally broken frame splits the inside and outside sections with a strip of insulating material, so the cold outer metal never touches the warm inner metal. Timber and uPVC frames are naturally poor conductors, and timber roof windows are usually insulated around the frame too. You can read more in warm-edge spacers and thermal breaks.
Edge and frame details we specify
- Spacer bar
- Warm-edge, never plain aluminium
- Aluminium frames
- Thermally broken, inner and outer sections separated
- Timber roof windows
- Factory insulation collar around the frame
- uPVC lanterns
- Multi-chamber profiles with insulated ridge and eaves beam
04Kerb
The insulated kerb and how it meets your roof
On a flat roof the rooflight sits on a kerb or upstand, and that box is as much a part of the thermal envelope as the glass.
A kerb, or upstand, is the raised frame that lifts a flat rooflight above the roof surface so rain runs past it. Old kerbs were usually plain timber with nothing inside. Picture a ring of cold wood round the opening, and you have the reason so many flat roof extensions feel chilly under the skylight.
We use either a factory insulated upstand or a site-built timber kerb lined with rigid insulation. What matters most is the join. The insulation in the kerb has to meet the insulation in the roof build-up without a gap, so there is no strip of bare timber or blockwork connecting the cold outside to the warm ceiling. That gap is a cold bridge: a path for heat that skips around the insulation. On a warm deck flat roof the insulation sits above the joists, so the kerb insulation laps onto it. On a cold deck roof, where the insulation sits between the joists, we line the opening to carry it right up to the frame. Our guide to rooflight kerbs and upstands covers the options in more depth.
05Air
Airtight seals, inside and out
A rooflight can have perfect glass and still feel cold if air is moving around its frame.
Draughts are the part of heat loss you can actually feel. Warm, moist room air pushes up and out through any gap in the ceiling, and cold air finds its way back in. Around a rooflight the gaps hide where the frame meets the kerb, where the lining meets the frame, and where the vapour control layer in the ceiling has been cut to make the opening.
So we seal on both sides. The weather side keeps rain and wind out with the right flashing or upstand detail. The room side gets an airtight seal between the frame and the lining, and the vapour control layer is taped back to the frame rather than left hanging. That inner seal also keeps damp room air out of the cold parts of the build-up, which is one of the main reasons rooflights run with water in winter.
- Frame to kerb or roof bedded and fixed to the maker's detail
- Vapour control layer cut neatly and taped back to the frame
- Airtight seal between the inner frame and the lining
- Opening sashes checked for even compression on their gaskets
- No loose insulation left stuffed into gaps as a stand-in for a seal
06Lining
The lined reveal, condensation and cold bridges
The last few centimetres between the frame and your ceiling decide whether the reveal stays dry and warm.
The reveal is the short shaft of wall between the rooflight frame and the room's ceiling. If its sides are just plasterboard on bare timber, they will be cold, and cold surfaces in a warm room attract condensation. That is how a smart new skylight ends up with grey speckles of mould in its corners by the second winter.
We insulate the reveal sides, then line and plaster them so the finish is part of the warm layer, not a skin over a cold one. Where the frame has an insulation collar, the lining butts up to it. Where a lantern sits on an upstand, the inside face of the upstand is insulated before it is boarded. The aim is a continuous warm surface from the glass all the way to the ceiling, with nowhere for a cold bridge to sneak through.
Some condensation on the outer glass on a clear frosty morning is normal and a sign the inner pane is doing its job. Condensation on the inside, round the edges or on the reveal is the warning. Our page on whether flat rooflights get condensation explains the difference.
A warm skylight needs a continuous warm surface from the glass all the way down to the ceiling.
07At survey
What we check before we write the spec
A warm specification is only as good as what we know about your roof, so the free survey looks at the build-up as well as the space.
Before we put anything in writing, we need to know how your roof is built and insulated, which way the glass will face, how you use and heat the room, and what is already there. That tells us whether double glazing will do, whether triple is worth it, how the kerb insulation needs to meet the roof, and how the lining should go together.
All of it then appears on your quote in plain words, together with the Building Control notification that we handle where the work is notifiable. For the wider picture on efficient glazing, see our energy efficient rooflights service, our note on Building Regulations for rooflights, and what drives the price on our costs page. When you are ready, book a free survey.
- Roof type and covering, and whether it is a warm or cold deck.
- Existing insulation depth and where it sits in the build-up.
- Orientation, so we know how much winter sun the glass will catch.
- How the room is heated and used, and where people sit.
- Signs of past condensation or mould round any old unit.
- The ceiling and loft void, which shape the lined shaft.
Is a warm specification worth it?
Does a warm specification cost much more?
Warm-edge glazing and an insulated kerb are part of how we price every job, not an add-on. The bigger choices, such as triple glazing, are explained at survey so you can decide.
Can a warm spec work with an existing VELUX opening?
Yes. A replacement roof window can go into the same opening with a new insulation collar and a fresh lining, and a blind adds a little more on winter nights. See VELUX blinds for warmth.
Will a warm rooflight stop all condensation?
It stops the cold surfaces that cause most of it inside the room. Very humid rooms such as bathrooms still need good extraction or an opening rooflight to let moist air out.