Sometimes, and less often than in a vertical window. Triple glazing is worth paying for in a large fixed rooflight over a continuously heated room, on a north-facing slope, or where the ceiling is high and the warm air collects against the glass. It is usually not worth it in a small opening unit on a pitched roof, where the frame and the spacer already dominate the whole-unit figure and the third pane has little left to improve. The honest answer depends on the size of the unit and the hours the room is heated, not on the product.
What the third pane actually buys
A third pane adds a second cavity, a second gas fill and, usually, a second low-emissivity coating. On the glass alone, that takes a good double unit from roughly 1.0 W/m²K down towards 0.6, and a very good triple can go below that. On paper it looks like a 40 per cent improvement, which is why brochures lead with the centre-pane figure.
The improvement is real. The coatings are doing most of the work rather than the extra sheet of glass, which is worth knowing, because a double unit with two coatings and a krypton fill can get closer to a mediocre triple than the price difference suggests.
Where the improvement gets diluted
The number that goes in your roof is the whole-unit figure, which averages the glass, the spacer and the frame across the actual product at a stated size. Frames and spacers do not improve when you add a pane. They usually get slightly worse, because the unit gets thicker and the frame has to grow to hold it.
So a 40 per cent gain at the centre of the glass turns into something more like 20 to 25 per cent on the whole unit for a large rooflight, and considerably less than that on a small one. On a 780 by 980 pitched-roof unit, where frame and edge might be a third of the visible area, the whole-unit difference between a good double and a standard triple can be small enough to argue about.
Weight, and what it does to an opening unit
Glass weighs about 2.5kg per square metre per millimetre of thickness. Adding a 4mm pane to a 1.2 square metre unit adds roughly 12kg before the heavier frame and the deeper sealed unit are counted. On a fixed flat-roof unit that is a lifting and structural question, solved with more hands and a check on the kerb.
On an opening rooflight it is a mechanism question. Hinges, friction stays, gas struts and electric actuators are rated for a load. Some manufacturers do not offer triple glazing at all in their larger opening sizes, and where they do the operating gear is different and the unit sits deeper in the roof. If you want a rooflight you can open from a pole while standing on the landing, weight matters more than the U-value does.
Daylight surrendered to the third pane
Every pane and every coating takes a share of the daylight. A double unit with one soft coating typically transmits around 70 to 75 per cent of visible light. A triple with two coatings is commonly in the high fifties to mid sixties.
Ten points of light transmittance is not nothing in a room whose only daylight comes from the roof. It is also not the disaster it sounds, because a rooflight starts with far more light available than a window does. Where we would think hard is a north-facing loft room with one modest unit, which is exactly the orientation where triple glazing otherwise makes most sense. That tension is a real one and worth resolving on the drawing rather than after installation.
The summer side of the decision
The second coating that improves the winter figure also lowers the solar factor. On a north slope that costs nothing, because there is very little direct sun to lose. On a south or west slope it means less free heat in February as well as less unwanted heat in July, and the July problem is better solved with external shading than with glass, as set out on the solar control glass page.
There is one summer point in triple glazing’s favour that rarely gets mentioned. The extra cavity slows heat travelling inwards as well as outwards, which flattens the peak a little on a hot afternoon. It is a modest effect and no substitute for shade.
Rooms where we would specify it
A large fixed flat-roof unit over a kitchen or a lounge that is heated most of the day. A rooflight over a stairwell or in a vaulted ceiling, where the warmest air in the house sits permanently against the glass and the surface temperature of the inner pane decides whether you get condensation. A north-facing slope with a large glazed area. A bathroom or utility room with high humidity, where a warmer inner pane is worth more than the heating saving.
The pattern in that list is surface temperature rather than energy. The inner pane of a triple unit runs two or three degrees warmer than the inner pane of a double, and in a humid room that is often the difference between a dry morning and a wet one.
Rooms where we would not
A single small opening rooflight in a loft bedroom that is heated in the evenings. A conservation unit with a slim sightline, where the frame is the limiting element and the third pane cannot show through it. Any position where the operating gear is the constraint. Any job where the same money has not yet been spent on the perimeter.
Being willing to say no here matters. We have talked people out of triple glazing on single-unit loft jobs more than once, and the room performed as promised.
Cheaper places to put the same money
Before the third pane, we would spend on a warm-edge spacer, a thermally broken frame, an argon or krypton fill, and the insulation and vapour control at the perimeter. Those four together will usually beat the upgrade from double to triple on a small unit, and they cost less.
The perimeter in particular is where the money hides. A triple-glazed unit set into a reveal with a cold timber ring around it will underperform a double-glazed unit set into a properly insulated one. Our rooflights and insulation page explains why the surround matters as much as the glass.
Comparing two quotations honestly
Ask three things of each. Which figure is quoted, centre-pane or whole-unit. At what size it was declared. What the light transmittance and the solar factor are alongside it. A quotation offering a triple unit at a good centre-pane figure with no size stated is not telling you what you are buying, and a fourth number, the weight, is worth adding if the unit opens.
We give a range by email once we know the roof build-up and the opening size, then a fixed price after a survey. We install VELUX, Fakro, Keylite and Roto units, in double or triple as the job needs, and our workmanship carries a ten-year guarantee. Fifteen years in the trade, Oxford and twenty five miles by road, installations and replacements only. Call 01865 704245.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
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