Low U-Value Rooflights for Passivhaus and Low-Energy Builds
A rooflight for a Passivhaus or low-energy build has to do three things at once: reach a very low whole-unit U-value, usually 0.9 W/m²K or better and often well under that, sit on a kerb that is thermally isolated from the cold outside, and be taped airtight so it does not blow the air test. That means a certified triple-glazed unit, a preformed insulated upstand rather than a bare timber box, and a continuous airtightness and vapour control line taped and bonded around the whole opening. The unit is the easy part. The junction and the air test are where low-energy rooflight installations are won or lost.
What we do is install to the discipline these builds demand: we work to the designer’s or the certifier’s details, and we detail the opening so the air test result is not put at risk by the rooflight. The building’s Passivhaus certification is set and signed off by the designer and certifier, and our job is to make the installed rooflight match the calculation.
The three demands of a low-energy rooflight
A certified low U-value unit
Low-energy builds work to a fabric standard that leaves no room for a weak element. A rooflight at 2.0 W/m²K would be a hole in an envelope built to 0.15. So these builds use certified triple-glazed units, typically with a solar control coating and krypton fill in the slim cavities, reaching a whole-unit figure around 0.7 to 0.9 W/m²K. The figure has to be the declared whole-unit value at the installed angle, not a flattering centre-pane number, because the certifier will want the real one. We cover why those two numbers differ on specifying U-values and g-values for rooflights.
A thermally isolated kerb
On a standard job a bridged kerb costs you a cold reveal and some mould. On a Passivhaus job it also wrecks the thermal model, because the perimeter of the opening is a long line and any bridge there shows up in the calculation and on the certifier’s thermal check. So the kerb has to be thermally isolated: a preformed PIR or GRP-faced insulated upstand that takes the timber out of the thermal path almost entirely, with the insulation continuous from the roof deck up and over. We explain the failure this avoids on thermal bridging at rooflight junctions.
Airtightness that passes the test
Passivhaus and low-energy builds are pressure-tested, and the target is far tighter than a standard dwelling. A rooflight is a hole cut in the airtight layer, so it is a prime suspect for a failed test. Every joint around the opening has to be taped and bonded into the continuous airtightness line, and the vapour control layer reinstated with it. There is no room for expanding foam pretending to be a seal here. This is the same detailing we do on every job, done to a standard where it is measured rather than assumed. The general version is on air leakage around rooflights.
Sequence with the air test in mind. On a certified build the airtightness membrane at the rooflight has to be continuous with the wall and roof airtight layers, and it usually has to be in and taped before the test is due. We coordinate the rooflight airtightness detail with the main contractor’s airtight line rather than treating it as a separate island, because a perfect rooflight seal that does not join the rest of the envelope still fails the test.
Who does what on a certified build
| Element | Who is responsible | Our part |
|---|---|---|
| Unit certification | The glazing manufacturer, tested to standard | Specify and supply a unit with the declared whole-unit figure the design needs |
| Passivhaus certification of the building | The Passivhaus designer and certifier | Install to their kerb and airtightness details |
| Air test | An independent air-test engineer | Detail the opening so the rooflight does not fail it |
| Building Control | Local authority Building Control | Notify the work and deal with the Building Control inspection for you |
To be plain about it: the unit carries its own test certificate from the manufacturer, and the building’s Passivhaus certification is set by the designer and certifier. We install to their details. Notifiable work is handled through your local authority Building Control, which we take care of for you and which on a low-energy build sits alongside the certifier’s own checks. We cover that route on meeting Part L with replacement rooflights.
The install discipline that gets you through the air test
- Work to the drawings. On a certified build the kerb and airtightness details are drawn. We build them, we do not improvise them.
- Preformed insulated kerb. Thermally isolated upstand, insulation continuous with the roof, no timber left in the thermal path around the perimeter.
- Continuous airtight line. Membrane and tape bonded to the deck, carried up the kerb, terminated at the frame, and lapped into the main envelope airtight layer.
- Vapour control reinstated on the warm side, continuous with the roof layer, so the low-energy build-up does not trap moisture around the opening. The hidden failure this avoids is on interstitial condensation in the roof build-up.
- Certified unit set square on the corrected junction, fixed without cold straight-through bridges.
- Coordinate the test. The airtightness detail is finished before the air test is due, so the rooflight is signed off rather than blamed.
What it means locally
Low-energy and Passivhaus-standard builds turn up across our area on newer plots at Kesgrave, Martlesham, Sproughton and the growing edges of Ipswich, where a modern trussed roof and a deep insulated build-up leave the rooflight as the element most able to spoil the numbers. On coastal low-energy work at Felixstowe or Harwich the airtightness and thermal discipline sits on top of the marine specification, so the fixings are 316L stainless and the kerb goes to 200mm as well as being thermally isolated. The two requirements do not conflict, they just both have to be met.
What U-value does a Passivhaus rooflight need?
It depends on the individual building’s energy model rather than a single fixed number, but in practice it means a certified triple-glazed unit reaching a whole-unit figure around 0.7 to 0.9 W/m²K, and often the design will ask for the declared figure at the installed angle rather than a centre-pane value. The Passivhaus designer sets the target for your build. Our job is to supply a unit that meets it and to detail the kerb and airtightness so the installed performance matches the calculation.
Can you install rooflights for a Passivhaus or low-energy build?
Yes. We install to the details the Passivhaus designer and certifier provide, and we detail the kerb and airtightness to the discipline these builds demand. The building’s certification is set and signed off by the designer and certifier, and our job is to make the installed rooflight match the calculation. What we bring is 25 years of rooflight detailing and airtightness work applied to a standard where it is measured by an air test rather than taken on trust.
Get a low-energy rooflight detailed properly
Send us the design details or the certifier’s requirements and we will specify a certified unit to hit the target, detail the kerb and airtightness to match, and give you one fixed price with the declared whole-unit U-value on it. A 10-year workmanship guarantee, and installers with 25 years on rooflights.
Request a fixed quote, read triple glazed rooflight installation, or start from energy efficient skylight installation.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.