Is triple glazing worth it in a rooflight?

The costs

Price. Substantially more per unit.

Weight. Fifty per cent more glass. On an opening unit that means a higher rated actuator; on a large unit it affects handling and sometimes structure.

Daylight. Every pane and coating absorbs some. A triple glazed rooflight transmits noticeably less light, which works against the reason you fitted it.

Three better places for the money

Solar control glass if it faces south or west. This is the difference between a room you use in July and one you avoid, and no amount of U-value improvement addresses it.

An insulated kerb. On most installations the upstand loses more heat than the glass does. It is cheap while the roof is open and impossible afterwards.

A properly sealed vapour control layer. Prevents the condensation and ceiling damp that gets blamed on glazing.

Do those three with a good double glazed unit and the room will outperform a triple glazed unit with none of them.

When triple does make sense

Very large glazed areas where absolute loss matters. New builds chasing a whole-house target. Or where acoustics are the real driver, in which case an acoustic laminate in a double unit is often the better answer anyway.

What we do

Fit it where you want it, and tell you this first. It is not a bad product. It is usually the wrong line to spend the extra on.

Usually not, and here is the arithmetic

Triple glazing takes a rooflight from a whole unit U-value of around 1.2 to around 0.8. That is a real improvement and it is a modest one, and it comes with four costs that people do not always weigh.

What you give up

Daylight. A third pane and two extra coated surfaces transmit noticeably less visible light. On a rooflight, which you fitted specifically to get daylight into a room, that works directly against the purpose.

Weight. Significantly heavier. On an opening unit that means a harder working actuator and a shorter mechanism life. On a large unit it can affect the structure and it certainly affects how it gets onto the roof.

Money. Substantially more, and the payback on the thermal difference alone is long.

Nothing on the summer side. Triple glazing does very little about solar gain, which on a south or west facing rooflight is the problem people actually complain about.

Where the same money does more

Solar control glass on any sunward facing unit. It addresses overheating, which is the more common complaint, and it costs a fraction of triple glazing.

An insulated kerb, on a flat roof. A 1200 by 1800 rooflight has six metres of perimeter, and an uninsulated timber kerb punching through the roof insulation presents close to a square metre of poorly performing envelope at a U-value several times worse than the glass. Insulating it is a sheet of board and an hour.

An insulation collar, on a pitched roof. Its absence leaves the reveal cold enough to condense, which is a heat loss and the commonest cause of water on a plaster reveal.

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

Do all four and the room will feel better than it would with triple glazing and none of them.

Where triple glazing does make sense

A very high performance building where the whole fabric is specified to that standard and the rooflight would otherwise be the weak point.

A north facing unit where there is no solar gain to control and daylight is already even and glare free, so the transmission penalty matters least.

A large fixed unit where there is no actuator to overload.

What we will do

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

The four things we would do instead

Solar control glass on any south or west facing unit, which addresses overheating, the complaint people actually have, at a fraction of the cost.

An insulated kerb on a flat roof, or an insulation collar on a pitched one. 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.

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

A continuous vapour control layer sealed to the frame, because air leakage around an opening loses more heat than conduction through a small area of glass.

Do all four and the room will feel better than it would with triple glazing and none of them, for less money.

Where it earns its place

In a room where the heat loss genuinely matters and the rooflight is a large part of the envelope: a loft bedroom with two or three windows in an otherwise well insulated roof, or a highly insulated new build where the rooflight would otherwise be the weakest element by a wide margin.

Also where noise is a factor, though the acoustic gain from three similar panes is smaller than from two panes of different thickness with an acoustic interlayer.

Where it is not worth it

On a small unit, where the perimeter dominates the heat loss. Improving the centre of a small pane while the spacer, frame and reveal detailing carry most of the loss is spending in the wrong place. A warm edge spacer and an insulation collar buy more.

On a conservation rooflight, where the extra thickness and weight push up the profile, and profile depth is the whole reason for the product. Gaining a small thermal improvement at the cost of the low profile that got the unit approved is a poor trade.

As a retrofit, because a frame designed for a double glazed unit may not accept a thicker heavier one, and the hinges are balanced for the original weight.

The costs beyond price

Weight, which affects the hinges and the operating effort, and on a large unit the structure.

Light, slightly, because a third pane and its coatings reduce transmission.

Solar gain, reduced, which is a benefit in summer and a loss in winter on a south facing slope.

The better spend, usually

A good double glazed unit with a warm edge spacer, plus an insulation collar and a properly insulated reveal. That combination beats triple glazing in a badly detailed opening, for less money.