Are rooflights bad for insulation?

The two numbers side by side

A roof built to current standards runs at a U-value of about 0.15 W/m2K, and a loft conversion squeezed between 150mm rafters with insulation over and under might reach 0.18. A well-specified roof window is around 1.2, and a good made-to-size flat rooflight around 1.1 to 1.3.

So the glass loses roughly eight times what the roof does, square metre for square metre. Put a different way, one square metre of rooflight loses about as much as seven square metres of roof. That ratio is the whole of the problem and it is not going to change: even a triple-glazed unit at 0.8 loses more than five times what the roof around it does.

Working it through on one loft conversion

Take a rear loft bedroom in a Gloucester semi, four metres by four, with 16 square metres of sloping ceiling and two roof windows at 1.14 by 1.18 metres, so 2.7 square metres of glass and 13.3 square metres of roof left.

The roof at 0.18 loses 2.4 watts per degree. The glass at 1.2 loses 3.2 watts per degree. So the two windows, occupying 17 per cent of the ceiling area, account for 57 per cent of the fabric loss through it. Over a Gloucestershire heating season, taking around 2,000 degree days, that comes to roughly 155 kilowatt hours through the glass. It is a real number and it is not a large one: less than a fortnight of typical heating for the whole house.

Now swap the windows for 1990s units at 2.9. The glass loses 7.8 watts per degree, the room’s fabric loss more than doubles, and the seasonal figure through the glass goes to around 375 kilowatt hours. The difference between an old unit and a current one is far larger than the difference between having a rooflight and not.

The perimeter loses more than most people expect

The declared figure covers the unit and stops at the outside face of the frame. The junction between the unit and the roof is measured separately as a linear thermal transmittance, a psi value, in watts per metre per degree.

A well-detailed perimeter with insulation taken tight to the frame runs at roughly 0.05 to 0.10 W/mK. A poor one, where insulation stops short and the timber trimmer runs cold from outside to inside, can reach 0.25 or worse. On a roof window with a 4.6 metre perimeter, that is the difference between 0.35 and 1.15 watts per degree, which at the bad end is a third of what the entire glass area loses. This is the part nobody puts on a quote and the part workmanship controls entirely. It is set out on thermal bridging at the rooflight perimeter.

Free heat coming the other way

The loss calculation only counts one direction. Over a heating season, south and west facing glass on a roof collects a useful amount of solar energy at exactly the time of year the house needs it, because the winter sun at Gloucester sits at 14.7 degrees at midday and shines straight down a shallow slope on a bright January afternoon.

On a south-facing pitch, useful winter gain through a standard low-emissivity unit can offset a meaningful share of the conduction loss through the same glass, and on a good spring or autumn day it exceeds it. North-facing glass gets very little of this, which is one of several reasons that aspect changes the specification. Daylight is the other credit: a room lit from above for eight hours in October is a room not running lights, and rooflights deliver two to three times the daylight of a wall window of the same area.

Area, and the point where more glass stops paying

Guidance for new build sets a reasonable total glazed area at around 25 per cent of the floor area of the dwelling, and rooflights count towards it. That is a sensible sanity check on a single room too.

Daylight from above follows a curve of diminishing returns. Going from nothing to one roof window in a 16 square metre loft room is transformative. Going from one to two is a clear improvement and evens out the light across the room. Going from two to four adds a modest amount of daylight and doubles the heat loss again, and it usually creates a summer problem as well, covered on will a rooflight make my room too hot. Two well-placed units almost always beat four crowded ones.

What a rooflight does that insulation cannot

The honest comparison is not rooflight against roof. It is a rooflight against the alternatives for getting daylight into the same room, because the room needs daylight and the roof was never going to provide it.

In a Stroud valley cottage cut into a bank, with a retaining wall behind and neighbours tight on both sides, there is no elevation left to put a window in. In a Gloucester terrace with a narrow rear addition, the kitchen has one small window facing a yard. In a loft conversion, the alternative is a dormer, which means a much larger area of new wall, roof and cheeks, a planning conversation, and a considerably larger construction. Set against a dormer, two roof windows are usually the lower-loss option as well as the cheaper one.

Getting the opening right in a Gloucestershire roof

The build-ups in this county vary more than most, and where the insulation goes changes with each. Interwar hipped roofs at Longlevens and Hucclecote have 100mm rafters, so a conversion needs insulation over the rafters as well as between them, and that over-layer has to be carried across the head of the opening rather than stopped at it. Trussed estates at Quedgeley and Bishops Cleeve have shallow members and a designed trimmer, so 300mm of insulation has to find somewhere to go around a frame that is only 90mm deep.

Cotswold stone slate roofs often have no underlay at all, with mortar torching between riven battens, and the ventilation path has to be re-established as part of the work. Flat rear additions across the Severn Vale are the case where the kerb decides it: a bare timber upstand is a cold ring around the glass, and insulating its outside face is the difference between a good junction and a bad one. That is on upstands, kerbs and warm roof build-ups.

What to settle before ordering

Three decisions carry nearly all the thermal outcome: the whole-unit U-value at the size being ordered, the total glass area relative to the floor, and how the perimeter is going to be insulated and sealed. Get those three right and the rooflight is a modest, quantified cost against a large gain in how the room works.

Severn records rafter depth, existing insulation type and depth, the ventilation path above it, the pitch and the aspect at survey, and the reveal detail is drawn from that rather than decided on the scaffold. Insulation runs continuous and tight to the frame, and the vapour control layer is taped to the frame rather than trimmed short of it. More on the numbers across the whole subject is on the energy efficient rooflights hub.