What is the most energy efficient rooflight?
A fixed triple glazed unit with a timber core, a warm edge spacer, krypton in the cavities and a properly insulated kerb underneath it. On paper that specification reaches around 0.6 to 0.8 W/m²K for the whole unit, which is roughly half the loss of the double glazed rooflight most people are choosing between.
The more useful answer is that the ranking changes once you include what the rooflight is for. Efficiency is not only heat kept in during February, it is heat kept out in July, daylight that replaces electric lighting, and useful winter gain on a south facing plane. A specification that wins on one of those can lose on the others, which is why the full picture sits on energy efficient rooflights.
Fixed units beat opening units, without exception
An opening rooflight has a moving sash, a gasket line and a hardware set. Every one of those is a route for heat and for air, and the sash frame adds a second frame section within the same overall size, which reduces the glass area and increases the perimeter.
In like for like products the fixed version typically declares 0.2 to 0.4 W/m²K better than the opening version, and its air permeability is in a different class. Where a room genuinely needs an opening for purge ventilation or for escape, that duty wins and the small thermal penalty is accepted. Where an opening was specified because it seemed useful, a pair of fixed lights with one opener between them performs better than two openers.
Ranking the types, best to worst
| Type | Typical whole unit U-value | Why it sits there |
|---|---|---|
| Fixed triple, timber core, in a pitched slope | 0.6 to 0.8 | Steep plane, no sash, insulated frame |
| Fixed triple, flat roof unit on a kerb | 0.8 to 1.0 | Near horizontal cavity convects harder |
| Opening triple roof window | 0.9 to 1.1 | Sash frame and gasket line |
| Fixed double, warm edge, argon | 1.1 to 1.3 | Good current standard |
| Roof lantern, multiple facets | 1.3 to 1.6 | Long perimeter, many bars, a ridge |
| Old twin wall polycarbonate dome | 2.8 to 3.5 | Thin skins, no coating, poor seal |
The spread from top to bottom is a factor of five, and the largest single step in that table is the last one. Changing an old dome for anything modern is the biggest thermal improvement available on a domestic roof, and it is covered on skylight replacement.
Small openings, where the perimeter starts to dominate
Thermal performance scales with area, and edge losses scale with perimeter. A 550 by 780mm roof window, the size that fits between trusses on the estates at Great Notley and South Woodham Ferrers, has about 0.43 square metres of area against 2.66 metres of perimeter.
A 1200 by 2400mm flat rooflight has 2.88 square metres against 7.2 metres of perimeter, which is a much better ratio. So the same glass and the same frame section give a worse whole unit figure in the small opening, and the small opening is where the warm edge spacer earns most of its keep. Choosing the best available specification matters more, not less, on a modest unit.
Gas fills, and why krypton appears in slim units
Argon is the standard fill and it performs best in a 16 to 18mm cavity. Below about 12mm the two panes start to conduct through the gas rather than being separated by it, and above about 20mm convection currents set up inside the cavity and carry heat across.
Krypton is denser and slower moving, and its optimum cavity is around 10 to 12mm. That is why triple glazed units built to a slim overall thickness, which is what a roof window rebate can physically accept, are filled with krypton rather than argon. It costs more and it is the reason a slim triple unit is not simply a double with a pane added. A unit lying near horizontal also convects more freely than the same unit stood upright, which is why the flat roof rows in the table above sit a step behind the pitched ones.
Coatings doing two jobs at once
A soft coat low emissivity layer on the room side of the cavity reflects long wave heat back into the room and is the single largest improvement in the glass. It is present in essentially every current unit and it is why a modern double glazed rooflight reaches figures that triple glazing needed twenty years ago.
The complication is that the most efficient winter specification is not automatically the most efficient annual one. Adding solar control rejects summer gain and also removes some free winter heat and some daylight. Getting that balance right is a question about the roof plane rather than about the product, and it is dealt with on what a g-value is.
The very best available, and what it costs you
At the top end sit units built to a passive standard, with quadruple glazing or a triple with two coatings, an insulated composite frame and a designed insulated kerb, reaching 0.5 W/m²K or better for the installed assembly.
The costs are real. Light transmittance falls into the sixties or below. The units are heavy, which changes how they are lifted onto a roof and sometimes what the structure has to carry. The kerb build-up gets deeper, which raises the unit above the roof line. And the money spent buying the last 0.2 W/m²K would usually save more energy if it were spent on the loft insulation or the airtightness of the same house. That is not a reason to avoid it, but it is a reason to sequence it.
Efficiency measured across a whole year
A rooflight is the only element of a house that gives energy back. Over a heating season a south facing rooflight in Essex collects useful solar gain on every sunny day, and the daylight it provides displaces electric lighting in a room that would otherwise have a light on at three in the afternoon in December.
Netted across a year, a well specified rooflight on a favourable plane can be close to thermally neutral. The same unit on a north facing plane is a straightforward loss, which is the case where paying for the best U-value on the market makes the most sense. Orientation changes the answer more than the product range does.
Three Essex roofs, three different best answers
A north facing rear slope on a Victorian terrace in Chelmsford: a fixed triple glazed roof window with clear glass and the best U-value the budget reaches, because there is no gain to manage and pure loss to reduce.
A flat roof over a south facing kitchen extension at Beaulieu Park: a double glazed unit with a selective solar control coating and a well insulated kerb, because rejecting August heat is worth more over a year than the third pane would be. And a large lantern on a Blackwater village weatherboard house: an aluminium system with a deep thermal break and a warm edge spacer throughout, because the perimeter length is what decides that job. Sunspire specifies the make-up, its plane and the spacer type on the fixed price after survey, so those choices are contractual rather than assumed.
Tell us about your roof
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