Guide · Energy Efficient Rooflights
Thermal bridging at the rooflight perimeter
A thermal bridge at a rooflight perimeter is a route by which heat crosses the insulation line through material that conducts faster than the insulation it interrupts. Around a rooflight there are usually three of them working at once: the kerb or the trimmer timber, the frame sitting on it, and the junction where the internal lining meets the opening. Added together they frequently lose as much heat as the glass they surround, and none of it appears on the quote for the unit.
This page is about that two or three hundred millimetres of construction. What the bridge is made of, how it is measured, why it hurts a small unit more than a large one, and what a properly resolved detail looks like in a warm deck, a cold deck and a trimmed pitched slope. The wider specification question, glass and frames and gas fills, sits on the energy efficient rooflights hub.
What crosses the insulation line, and what does not
An insulated roof is a continuous layer with a very low conductivity. Rigid polyisocyanurate sits around 0.022 W/mK. Mineral wool between rafters is nearer 0.035. Anything that punches through that layer and touches both the warm side and the cold side is a bridge, and the only question is how badly it conducts.
Softwood runs at roughly 0.13 W/mK, which is about six times the conductivity of the board it replaces. Structural steel is near 50. Untreated aluminium is around 160, which is several thousand times the insulation. A single continuous metal component from the inner face to the outer skin does more damage than a whole course of timber, which is why the material of the upstand deserves as much attention as its height.
Psi values, and heat measured by the metre
Junctions are not measured in the same units as surfaces. A wall or a roof has a U-value in watts per square metre per kelvin. A junction has a linear thermal transmittance, the psi value, in watts per metre per kelvin, because the loss follows a line rather than an area.
A well resolved rooflight perimeter can sit around 0.05 W/mK. A poorly resolved one, with uninsulated timber and no thought given to continuity, will pass 0.3. Take a 1200 by 1200 unit: that is 4.8 metres of perimeter. At 0.30 W/mK the junction alone loses about 1.4 W/K, while the glazed area of 1.44 square metres at a whole unit figure of 1.4 loses about 2.0 W/K. The edge is doing forty per cent of the damage of the thing it holds up.
Why a small rooflight suffers most
Perimeter grows in proportion to length. Area grows with the square of it. So the smaller the unit, the more perimeter it carries for every square metre of glass, and the more the junction dominates the total.
| Unit size | Glazed area | Perimeter | Metres of edge per m² of glass |
|---|---|---|---|
| 600 x 900 | 0.54 m² | 3.0 m | 5.6 |
| 1000 x 1000 | 1.00 m² | 4.0 m | 4.0 |
| 1500 x 1500 | 2.25 m² | 6.0 m | 2.7 |
| 2500 x 1200 | 3.00 m² | 7.4 m | 2.5 |
This is the argument against scattering several small units across a roof when one or two larger ones would give the same daylight. Three 600 by 900 rooflights and one 1500 by 1500 deliver similar glazed area, but the three small ones carry nine metres of junction against six.
Timber kerbs, and the section that conducts
The common site built upstand is a box of 47 by 100 or 47 by 150 softwood, decked and dressed. The timber is doing a structural job and it is doing it well. The trouble is that the same section runs from the warm deck up to the top of the kerb where the frame lands, so the heat has an unbroken path through solid wood.
Two things fix it and both are cheap. Insulate the outer face of the kerb, taking the roof insulation up and over so the timber sits inside the envelope rather than through it. Or build the upstand as an insulated cassette, with rigid board bonded between an inner and outer skin. The choice usually falls out of who is building the roof, which is why it is a question for the survey rather than the day itself.
Metal upstands, and the fixings that short the break
Proprietary metal kerbs arrive with insulation already built in and a thermal break in the profile, which is a genuinely good starting point. What undermines them is the fixing schedule. A stainless screw driven from the outer skin through the insulation into the inner one restores a metal path across the barrier, and enough of them around the perimeter puts most of the bridge back.
Good profiles keep fasteners on one side of the break, or run them through an isolating polymer sleeve. On site the thing to look at is the fixing pattern before the covering goes on, because afterwards it is invisible and permanent. The same logic applies to brackets, restraint straps and anything used to hold the kerb square during setting out.
Warm deck roofs, where the geometry is on your side
In a warm deck flat roof the insulation sits above the structural deck and below the waterproofing, so the whole roof build-up above the joists is warm and dry. That makes the rooflight detail relatively simple: carry the deck insulation up the outside of the kerb, lap the waterproofing over it, and land the frame on an insulated head.
The detail worth insisting on is that the insulation on the kerb meets the insulation on the deck with no gap at the corner. A twenty millimetre void at the base of an upstand, repeated all the way round, is a surprisingly effective heat exhaust, and it is exactly where offcuts get forced in rather than cut to fit. It takes ten minutes longer to cut them properly.
Cold deck roofs, and the harder version of the problem
A cold deck puts the insulation at ceiling level with a ventilated void above it. That means the kerb, the deck and the frame all sit outside the insulated envelope, and the insulation line has to turn upwards at the opening and travel through the void to meet the frame.
Done well, that is a lined and insulated shaft from the ceiling plane up to the underside of the unit, sealed at both ends, with the ventilated void continuing past it on all four sides. Done badly, it is a plasterboard tube with a draught in it, and the ceiling around the opening runs cold enough to mark. Cold deck construction is common in Essex on garage conversions and on older rear additions, and it is the build-up where the perimeter detail moves from important to decisive.
Inverted and green roof build-ups
An inverted roof carries the insulation above the waterproofing, held down by ballast, paving or a growing medium. The kerb then rises through a thick, heavy layer and the insulation has to be taken up the upstand and terminated above the finished surface, not at deck level.
Green roofs add depth. A substrate of 80 to 150 millimetres means the visible height of the kerb above the growing medium and the actual height above the waterproofing are different numbers, and both matter: one for weathering, the other for where the insulation stops. Where the two get confused the insulation is terminated at the membrane, leaving the top half of the kerb cold and buried in damp substrate.
Pitched slopes, and the trimmer around the opening
A roof window in a rafter slope has no kerb, but it has a trimmed opening: doubled rafters at the sides, trimmers above and below. That frame of solid timber replaces insulation on all four edges, and the doubled rafters are a hundred millimetres of continuous wood where there used to be quilt.
The insulation between the rafters therefore has to be brought right up to the frame and then continued around the reveal, using a thin high performance board where the depth is not available. The gap that habitually gets left is between the top of the insulation and the underside of the window frame, on the head side, where access is worst. It is a strip fifty millimetres wide and it will show on the ceiling as a dark line within two winters.
Every gap left at the head of an opening gets found by the first cold night with a clear sky.
Where the bridge shows itself on the ceiling
A bridge announces itself before it does any harm, if you know what to look for. The first sign is dust marking: airborne particles settle preferentially on cooler surfaces, so a cold line around the opening picks up a grey shadow on the plaster over a couple of years. Painting it out works for another two years, then it returns in the same place.
The second sign is localised condensation. Even misting on the glass is a room humidity matter and is covered on the hub and its condensation page. Water at one corner of the reveal, or on the lining rather than the glass, is a thermal bridge or an air path at that exact point, and it is worth photographing with a date on it.
Temperature factor, and the number that predicts mould
Designers use a surface temperature factor, written fRsi, to judge whether a junction will stay dry. It compares the internal surface temperature at the worst point of the junction with the difference between inside and outside air temperature, and it is dimensionless.
For dwellings the accepted threshold is 0.75. Below that, the coldest point of the junction sits close enough to the dew point that mould growth becomes likely in a normally occupied room, regardless of how well the occupants ventilate. A bare timber kerb through an insulated deck routinely calculates below 0.75 at the internal corner. An insulated kerb with a continuous line comfortably clears it. The figure is the reason the detail is worth drawing rather than describing.
Modelling a junction rather than guessing
Psi values and temperature factors come out of two dimensional heat flow modelling of the actual junction, run against the standard method. It is not exotic and it is not expensive, and on a project with a large lantern or an unusual build-up it turns an argument into a number.
For a routine domestic rooflight nobody models anything, and the practical substitute is to use a detail that has been modelled by somebody else. Manufacturers of insulated kerbs publish junction details with declared psi values, and following the published detail exactly, including the insulation thickness and the fixing arrangement, inherits the performance. Departing from it on site to suit a joist position does not.
Finding an existing bridge without opening anything up
Thermal imaging on a cold morning is the direct method, and it is unambiguous. Choose a day where the outside temperature is at least ten degrees below the inside, shoot before the sun has been on the roof, and image from inside. A resolved perimeter reads as a faint outline. An unresolved one reads as a bright band a hundred millimetres wide running all the way round.
Without a camera, the back of a hand is a surprisingly good instrument. Run it across the ceiling from a metre away towards the reveal on a cold January morning. Room temperature to the edge, then a cool zone, means the insulation stops short of where the lining does.
What the loss is worth across a heating season
Putting a figure on it keeps the argument honest. Take the 1200 by 1200 unit again and assume the junction is improved from 0.30 to 0.06 W/mK by insulating the kerb properly. That saves about 1.15 W/K across 4.8 metres of perimeter.
Over a heating season in Essex, with something in the order of 2,000 kelvin days of demand, that works out at roughly 55 kilowatt hours of heat a year for one rooflight. It is not a dramatic number in isolation. It is, though, a permanent one, it costs almost nothing to secure during the install, and the comfort and condensation benefits at the junction are worth considerably more than the heat.
Essex build-ups, and what changes house to house
The county gives three recurring cases and each shifts the emphasis. A Victorian rear addition in Chelmsford or Colchester usually has a cold deck flat roof over the back room, so the kerb detail is the difficult one and the insulated shaft matters more than the glass. An interwar semi with a hipped concrete tile roof takes a trimmed roof window, and the risk sits at the head of the opening where the quilt gets pushed rather than cut.
A trussed roof estate house at Great Notley or Beaulieu Park has a tight, well insulated envelope, which means the perimeter is a larger share of a smaller total, and any bridge is felt immediately as a cold line. Weatherboarded and timber framed stock around Stock and the Blackwater villages brings shallow build-ups where high performance thin boards earn their cost.
Getting the detail agreed before the opening is cut
The perimeter is decided by whoever forms the opening, which on a new extension is the roofer or the builder rather than the glazier. That makes it a sequencing question. Sunspire sets out the kerb construction, the insulation continuity and the responsibility for each part in writing at survey stage, so the detail is agreed while it is still a line on paper.
Three things belong on that paper: what the upstand is made of and whether it is insulated, where the roof insulation terminates against it, and who is lining and sealing the reveal. Settled early, all three are trivial. Settled after the covering is dressed and the unit is set, one of them is going to be a compromise, and it will be the one you cannot see. The full order checklist is on the energy efficient rooflights page, and indicative figures are on the costs page.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →Condensation, ventilation and rooflights
Read the guide →Triple glazing in a rooflight and when it pays
Read the guide →Warm-edge spacers and where condensation starts
Read the guide →Low-emissivity coatings and argon fills
Read the guide →
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