What is the most energy efficient rooflight?

Fixed beats opening, and it is not close

An opening unit has to have a sash, and a sash means a second frame profile, a gasket line, hardware penetrating the thermal break and a joint that has to move. Every one of those adds conducting material and air leakage at the perimeter of the glass.

The same manufacturer’s fixed version of a given rooflight will always carry the better whole-unit figure, often by a noticeable margin, and the gap widens as the unit gets smaller because the frame is a bigger share of the whole. So the first efficiency decision is not which glass but whether that opening light is genuinely needed. Where escape or ventilation requires it, it requires it. Where two rooflights are going in and only one needs to open, specify one of each rather than a matching pair.

Frame material carries more weight than the brochure suggests

Everyone reads the glass specification and skips the frame. On a rooflight that is the wrong way round, because roof units tend to be modest in area with a long perimeter, so the frame occupies a larger proportion of the whole-unit average than it would in a patio door.

Insulated composite frames, timber with a thermally broken external cladding, and glass fibre reinforced polymer sections all perform well. Plain aluminium without a proper thermal break performs badly and will drag an excellent sealed unit down with it. Slim conservation sightlines, which we fit often on Oxford stone and slate roofs, buy their appearance at some cost in the whole-unit number, and that is a legitimate trade to make knowingly.

What is inside the sealed unit

Four things inside the glazing move the number: the number of panes, the coatings, the cavity gas and the spacer bar. Panes add resistance but also weight and cost. Low-emissivity coatings reflect long-wave heat back into the room and do most of the work per pound spent. Argon in the cavity conducts less than air, krypton less still and allows a narrower cavity, which matters where a slim frame limits the unit thickness.

The spacer is the item most often left as the default. A metal spacer conducts around the whole edge of the glass and creates the cold band where condensation appears first, which is why warm-edge spacers earn their place. The coatings and gas fills are covered on their own page, and whether the third pane pays is set out on is triple glazing worth it.

One large unit against several small ones

Given a fixed total glazed area, a single large rooflight will outperform three small ones. The reason is geometry. Perimeter grows with the linear dimension while glass area grows with the square of it, so the small units carry far more frame and far more spacer for the same amount of glass, and each one brings its own thermal bridge through the roof construction.

Three openings also mean three sets of trimmers, three flashing perimeters and three chances for the insulation to stop short. If the design will tolerate it, consolidating is the cheaper and warmer option. If the rhythm of the roof demands separate units, which on a traditional Oxfordshire slate slope it often does, then the perimeter detailing has to be that much better.

Efficiency is a summer number as well as a winter one

A rooflight optimised purely for heat retention can make a house use more energy, because a room that overheats gets cooled, and mechanical cooling costs far more per unit of comfort than heating does.

The solar factor is therefore part of the efficiency question and not a separate topic. On a north slope you want the heat gain, so chase the U-value and the light transmittance and leave the solar factor high. On a south or west slope, particularly a shallow one, a low solar factor is worth more in real energy terms than the last decimal place of the U-value. Our page on specifying by orientation sets that out slope by slope.

The most efficient rooflight is often a smaller one

Even the best rooflight in the country has a whole-unit figure several times worse than the insulated roof it displaces. That is unavoidable physics and it is not an argument against rooflights, because daylight has value that a heat loss calculation does not capture.

It is an argument against oversizing. Roof glazing delivers several times the daylight of the same area in a wall, so the area needed to light a room properly is smaller than people assume. Specifying a unit a size down, and spending the difference on the glass and the perimeter detail, generally produces a room that is better lit per watt lost than the bigger opening would have been.

Where the good figure gets thrown away

We have taken out plenty of units with respectable declared performance whose openings were doing nothing at all. The insulation stopped at the trimmer face and a plasterboard box ran on from there with a void behind it. The vapour control layer was cut at the ceiling and never dressed to the frame. Both are invisible once the reveal is decorated and both undo the specification you paid for.

A mid-range unit installed with continuous insulation and a sealed vapour barrier will outperform a top-of-the-range unit installed without them, every time. That is the reason the four invisible details matter and the reason our workmanship carries a ten-year guarantee.

What we would put on a cold Oxford roof

For a north or east slope on a loft conversion, a fixed triple-glazed unit in an insulated frame with warm-edge spacers, sized to the daylight rather than to the rafter spacing, plus a separate opening unit only where escape or ventilation demands it.

For a south or west facing kitchen extension, a double-glazed unit with a selective solar control coating and a warm-edge spacer, because the third pane buys less here than the coating does and the weight is easier to carry. For a conservation slate roof under planning constraint, a slim cast-frame unit with the best glass that will fit inside the depth available, and an insulation collar specified with it rather than improvised on site.

Tell us the slope, the aspect and the roof build-up and we will give you a range by email, then a fixed price after a survey. Fifteen years in the trade, Oxford and twenty five miles by road, installations and replacements only.