What is the most energy efficient rooflight?

The five things that decide it

Low emissivity coating. Roughly halves heat loss. Standard on any decent unit.

Argon fill. Worth 0.2 to 0.3 on the U-value for almost nothing.

Warm edge spacer. Keeps the glass perimeter warmer, which stops edge condensation and improves the whole unit figure.

Solar control, on south and west. The single most consequential choice on a kitchen rooflight in this county.

An insulated kerb. Not part of the unit and frequently the biggest loss on the whole installation.

By orientation

South or west: solar control is not optional. Overheating is the problem you will actually experience.

North: no solar control needed at all. Specify for U-value alone and spend the saving elsewhere. North light is the best light in the building.

East: morning gain. Fine in a kitchen, difficult in a bedroom without blackout.

What not to buy

Triple glazing, in most cases. It moves the U-value from about 1.2 to about 0.8 at substantial cost and weight, and it transmits less daylight. The same money on solar control glass, an insulated kerb and a sealed vapour barrier does more.

The honest summary

A well specified double glazed unit on an insulated kerb, with solar control if it faces the sun, outperforms an expensive triple glazed unit on an uninsulated one.

The whole installation, not the unit

People ask this expecting a product answer, and the honest answer is that the specification of the glass is rarely where the energy goes. Three other things matter more, and all three are invisible once the job is finished.

1. The kerb, on a flat roof

A 1200 by 1800 rooflight has six metres of perimeter. An uninsulated timber kerb punches straight through the roof insulation around all of it, presenting close to a square metre of poorly performing envelope at a U-value several times worse than the glass sitting on top.

That is not a rounding error. It is comparable to leaving a patch of the roof uninsulated, and it costs a sheet of rigid board and an hour to prevent.

2. The insulation collar, on a pitched roof

The component that closes the gap between frame and structure. Without it the reveal runs cold enough for room air to condense on it, which is both a heat loss and the commonest cause of water on a plaster reveal.

A separate order line, and the first thing dropped from a cheap quotation.

3. Airtightness

Air leakage around an opening loses more heat than conduction through a small area of glass. A properly sealed vapour control layer, continuous and taped to the frame, is what prevents it, and it also stops warm moist air reaching the cold side of the construction where it would condense inside the build up.

Then the glass

Once those three are right, the unit specification matters and the numbers are straightforward.

Whole unit U-value, not centre pane. Centre pane ignores the spacer and the frame, which is exactly where heat escapes, and it is always the better figure. Whole unit is what the building experiences and what the regulations use.

A warm edge spacer rather than aluminium. Modest improvement in the number, large improvement in the thing you notice, because the band of edge condensation on cold mornings largely disappears.

Argon fill and a low emissivity coating, which any current unit has as standard.

That combination lands around 1.2 to 1.4 whole unit, comfortably inside the 2.2 the regulations expect for a replacement.

What about triple glazing

It takes you to around 0.8, at the cost of weight, daylight transmission and a good deal of money, and it does very little about solar gain, which is the more common complaint.

On most domestic rooflights the same money spent on solar control glass and an insulated kerb does more for how the room actually feels.

The order to spend money in

First, the kerb or the insulation collar, because that is where the largest avoidable loss is and it costs a sheet of board and an hour.

Second, airtightness: a continuous vapour control layer sealed to the frame, which prevents both heat loss and moisture getting into the construction.

Third, a warm edge spacer, which should be standard and which removes the edge condensation people mistake for a failed unit.

Fourth, a good whole unit U-value, which any current double glazed unit with a low emissivity coating and argon fill will give you at 1.2 to 1.4.

Fifth, and only where the unit faces the sun, solar control glass, because a rooflight gains heat as well as losing it and summer discomfort is the more common complaint.

Triple glazing does not appear on that list, and on most domestic rooflights it should not.

The number to compare, and the trap

The whole unit U value, covering the complete assembly including frame and spacer. Not the centre pane figure.

The gap between those two numbers is wider on a rooflight than on almost any other glazed element, because rooflights are frequently small and a small unit has a great deal of perimeter relative to its glass area.

A manufacturer quoting only a centre pane figure is quoting the flattering number. Ask for the other one, at the size you are actually buying, because it changes with size.

The specification that gets you there

A warm edge spacer, which addresses the edge loss directly and is the best value item on the list.

A low emissivity coating and an inert gas fill.

A thermally broken frame, particularly on aluminium and on any lantern, without which the internal frame faces run cold and collect condensation.

Triple glazing where the unit is large enough for the centre of the pane to dominate, and not where it is small.

The part no datasheet covers

The installation, and it is worth as much as the product.

An insulation collar around the frame on a pitched roof, or a fully insulated kerb on a flat one. Insulation carried continuously around the reveal. A vapour control layer taped to the frame.

A well specified unit in a badly detailed opening performs worse than an ordinary unit in a well detailed one.

The other half of efficiency

Solar control. A rooflight that makes a room unusable in summer, so the blinds stay down and the room needs cooling, is not efficient however good its U value.

Specify for the orientation: solar control on south and west, maximum light on north.