Triple glazing in a rooflight, and when it pays

What it costs

Money. Substantially more per unit.

Weight. A third pane is roughly fifty per cent more glass. On an opening unit that means a higher rated actuator; on a large fixed unit it can affect handling and the structure.

Light. Each pane and each coating absorbs a little. A triple glazed unit transmits noticeably less daylight than a double, which on a rooflight is working against the reason you bought it.

Where the money does more

Three things beat triple glazing for the same spend, in this order.

Solar control glass on a south or west facing unit. The difference between a room you use in July and one you avoid.

An insulated kerb. On most installations the upstand loses more heat than the glass. Insulating it is cheap and it is skipped constantly.

A sealed vapour control layer. Prevents the condensation that gets blamed on the glazing.

Do all three and the room performs better than it would with triple glazing and none of them.

When triple is right

On a very large glazed area where the absolute heat loss is significant. In a new build chasing a whole-house target where the rooflight has to pull its weight in the calculation. Or where acoustic performance is the real driver and the extra pane is being specified for sound rather than heat.

What we do

Fit it where somebody wants it, and say this first. It is not a bad product; it is usually the wrong place to spend the money on a rooflight.

The honest arithmetic

Triple glazing takes a rooflight from a whole unit U-value of around 1.2 to around 0.8. That is roughly a third less heat loss through the glazed area, and on a typical domestic rooflight the glazed area is a small proportion of the roof.

On a two square metre unit the absolute saving is real and small. Set against a substantially higher price, the payback on thermal performance alone runs to decades.

That is not an argument that triple glazing is bad. It is an argument that on most domestic rooflights it is not where the money should go first, and there are four specific costs that make the case weaker than it looks.

1. Daylight

A third pane and two additional coated surfaces reduce visible light transmission. A clear double glazed unit transmits around 75 to 80 percent; a triple glazed one noticeably less, and a triple glazed unit with a solar control coating less again.

On a rooflight that matters more than on a wall window, because you fitted the rooflight specifically to get daylight into a room that did not have enough. Reducing transmission works directly against the purpose.

In a north lit room, where the light is even and glare free and there is no unwanted gain, that reduction is pure loss.

2. Weight

A triple glazed unit is substantially heavier than a double glazed one of the same size, and weight has consequences at three points.

On an opening unit the actuator has to move it. An actuator specified for a double glazed sash and asked to move a triple glazed one works harder on every cycle, and actuator life is already the shortest of any component at ten to fifteen years.

On a manual unit the sash is heavier to operate, which on a high level window with a pole is a genuine nuisance.

On a large fixed unit or a lantern, the additional weight feeds into the structural design and into how the unit gets onto the roof, which on a big pane already means a crane or a glass robot.

3. Cost

Substantially more for the unit, and on an opening unit potentially more again for a heavier duty actuator. It is money that could go to the kerb, the glass coating and the vapour control layer, all of which do more.

4. It does nothing about the actual complaint

Ask people what is wrong with their rooflight and the answer is far more often that the room is too hot in summer than that it is cold in winter.

Triple glazing improves the U-value and barely touches the g value, which is the number governing solar gain. A triple glazed south facing rooflight without a solar control coating will still make a room uncomfortable in July.

Solar control glass, at a fraction of the cost, addresses exactly that.

The better place to spend it

The kerb. A 1200 by 1800 rooflight has six metres of perimeter, and 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. Insulating its outer face is a sheet of rigid board and an hour.

That single detail, on many installations, saves more heat than the step from double to triple glazing does.

The insulation collar, on a pitched roof. Without it the reveal runs cold enough to condense, which is both a heat loss and the commonest cause of water on a plaster reveal.

Airtightness. A continuous vapour control layer sealed to the frame. Air leakage around an opening loses more heat than conduction through a small area of glass, and it carries moisture into the construction as well.

A warm edge spacer, which should be standard rather than an upgrade, and which removes the band of edge condensation an aluminium spacer produces.

Solar control glass, where the unit faces south or west.

When triple glazing genuinely makes sense

In a very high performance building where the whole fabric is specified to that standard and the rooflight would otherwise be the weak point in an otherwise consistent envelope. In a passive standard house, a 1.2 rooflight is out of keeping with a 0.1 wall.

On a north facing unit where there is no solar gain to control, so the g value does not matter, and where the light is even enough that the transmission penalty matters least. This is the orientation where triple glazing makes most sense, which is the opposite of what people assume.

On a large fixed unit with no actuator to overload and where the structure is being designed anyway.

Where noise is a genuine problem and the additional mass helps, though an acoustic laminate interlayer does more for less weight.

How we advise on it

Fit triple glazing where somebody wants it, having set out all of the above first. What we will not do is present it as the obvious upgrade, because on most domestic rooflights it is not.

Where a customer’s real concern is a cold room, we will look at the kerb detail and the airtightness before the glazing, because that is usually where the heat is going.

Where the concern is summer heat, we will specify solar control glass and an opening vent at the highest point, and explain that a third pane would not have solved it.

Why the answer is different in a roof

In a vertical window, triple glazing is a straightforward improvement. In a rooflight it is a trade with several terms, and on a small unit it is frequently the wrong purchase.

The reason is the ratio of edge to area. A rooflight is often small, so the perimeter carries a large share of the total heat loss. Adding a third pane improves the middle of the glass, which is the part already performing best, while the spacer, the frame and the junction with the roof insulation continue to lose what they were losing.

That is why the whole unit U value and the centre pane figure diverge so sharply on small rooflights, and why a triple glazed small unit is a much smaller real improvement than the headline number suggests.

Where it does pay

Where the glass area is large enough that the centre of the pane dominates the calculation. A big flat rooflight or a generously sized roof window is a genuine candidate.

Where the rooflight is a substantial proportion of a well insulated envelope, so the weakest element is disproportionately visible in the overall performance. In a highly insulated new build or a deep retrofit, that is a real argument.

And in a room where the internal surface temperature matters for comfort as well as for energy: sitting under a large expanse of glass in January is noticeably more pleasant when the inner surface is warmer.

What it costs beyond money

Weight. A third pane adds real mass. On an opening unit the hinges are balanced for a specific weight, and the operating effort rises. On a powered unit the actuator works harder every cycle.

Thickness. The unit is deeper, and the frame has to accommodate it. On a conservation rooflight, where the whole product exists to sit low in the roof plane, that extra depth undermines the reason for buying it. Gaining a modest thermal improvement at the cost of the low profile that secured consent is a poor trade.

Light. Each additional pane and coating reduces transmission. On a north facing rooflight in a room that is already dim, that is a genuine loss.

Winter solar gain. A lower g value cuts summer overheating and also cuts the free heat arriving on a south facing slope in February.

It is rarely a retrofit

A frame designed for a double glazed unit will frequently not accept a thicker, heavier one, and the hinges are balanced for the original weight.

So the realistic upgrades within an existing frame are a better coating, a better spacer, a laminate or solar control, all of which stay within the same overall thickness.

The better spend on a small unit

A good double glazed unit with a warm edge spacer, a low emissivity coating and a gas fill, combined with an insulation collar around the frame, insulation carried continuously around the reveal, and a vapour control layer taped to the frame.

That combination costs less than triple glazing and delivers more in the actual building, because it addresses the losses that are largest rather than the ones that are easiest to put on a datasheet.

The honest summary: triple glazing is worth it on a large unit in a well insulated building, and the money is better spent on the edge and the perimeter on a small one.

Acoustics, which is a separate question

People buy triple glazing expecting a large acoustic improvement and are frequently disappointed.

Three panes of similar thickness resonate together, and a symmetrical unit is not a good acoustic performer regardless of how many layers it has.

What works for sound is asymmetry and damping: two panes of different thickness, with an acoustic laminate interlayer in one of them. That combination outperforms a symmetrical triple glazed unit for rain noise and for traffic noise, and it costs less.

On a rooflight over a bedroom, that is the specification to ask about rather than a third pane.

The condensation angle

A genuine benefit and the one least discussed.

A triple glazed unit has a warmer inner pane surface, so condensation forms at a lower temperature and in a narrower range of conditions.

In a bathroom or a kitchen with high humidity that is worth something real, though a warm edge spacer and good perimeter detailing deliver most of the same benefit for far less.

The external condensation surprise

Very well insulated units sometimes develop condensation on the outside face on clear still mornings, because the outer pane stays cold rather than being warmed from within.

It is harmless, it clears as the sun comes up, and it is a sign the unit is performing rather than failing. Worth knowing so it is not reported as a defect.