The short answer
Sometimes. Triple glazing lowers heat loss further than double, so it earns its place in a cold, north-facing room, a bedroom or a room you heat constantly. On a south-facing slope, or where double glazing already meets the Part L limit with room to spare, the extra weight and cost often buy little you will notice.
What the third pane adds
A triple glazed rooflight has three sheets of glass and two sealed gaps instead of one. Each gap holds a layer of gas that slows the flow of heat, so the whole unit loses less warmth to the sky. A lower u value means the glass surface on the room side stays closer to room temperature, which cuts the chill you feel beneath it and the condensation that can collect on a cold pane.
That is the real gain: comfort and a warmer surface, rather than a dramatic change to the heating bill. A rooflight is a small part of a roof, so even a big improvement in the glass shifts the total heat loss of the house only modestly.
What it costs you
Triple glazing is not free of trade-offs. Understanding them is how you decide whether it suits your roof.
- Weight. Three panes are heavier than two, which matters for a large flat-roof rooflight or a roof lantern. The frame and the structure beneath it must carry the load comfortably.
- Thickness. A deeper unit needs a deeper frame, which can change the look of a slim conservation rooflight. Some slim-profile products are only available as double glazed.
- Light. Each extra pane and coating absorbs or reflects a small share of daylight, so the room can be marginally less bright. Over a large area that adds up.
- Solar gain. A third pane also lets in less of the sun's winter warmth. On a south slope, where free warmth is useful, that cancels part of the advantage.
- Price. A triple unit is a more expensive product. We set out the difference as a line in your fixed written quote, so you can see what you are paying for.
When it makes sense
We tend to recommend a triple glazed rooflight in these situations:
- a north-facing slope, where there is little sun to offset the heat loss;
- a bedroom, nursery or study under the roof where a cold surface is noticed;
- a room that is heated through the evening and overnight;
- a large pane in an exposed spot where wind makes it colder still;
- a house where you want the most warmth from every opening, for example because the rest of the roof is already well insulated.
For a south-facing kitchen extension where summer heat is the larger worry, a well-chosen double glazed unit with solar control glass is often the better answer. Our guide to solar control glazing explains how that choice works.
Does double glazing already pass?
For an existing home the limit is set by the type of unit. A flat-roof rooflight or lantern is capped at 2.2 W/m²K. Anything in a pitched roof is classed as a roof window, so it has to achieve Window Energy Rating band B or a 1.4 W/m²K figure. A good double glazed unit with a low-emissivity coating and gas fill can meet these limits, so triple glazing is a choice for extra comfort, not a requirement of the Regulations.
Swapping a whole unit is notifiable building work, so we send the Building Control notification ourselves. If you are only changing the sealed glass in a sound frame, that is not a controlled fitting, but the frame must be able to take the thicker unit that a triple glazed pane needs. We check this at the survey.
Planning is a separate matter. Fitting a rooflight on a house is normally Class C permitted development, whatever the glazing, if it projects no more than 150 mm and meets the other conditions. The extra depth of a triple glazed unit is one more reason to confirm that projection at the survey. Our planning guide covers the exceptions.
How we decide with you
The free survey tells us which way the slope faces, how large the opening is, how deep the roof is and how the room is used. From those facts we recommend double or triple glazing, and we explain the reason in plain terms. Your quote is fixed and written down before any work begins, with the glass specification named, so you can compare it with any other quote on equal terms.
The unit is flashed and sealed on the roof, and inside the reveal is splayed if the roof permits, with plaster to finish. The work carries a 10-year workmanship guarantee. For the numbers behind the choice, our guide to reading rooflight u values against Part L shows how to read a data sheet.
A fair comparison to make
When you set a double and a triple quote side by side, ask three things. What is the whole-unit u value of each, frame included? How much daylight does each pass? And does the deeper frame change how the unit looks or sits in the roof? If the answers show a small gain for a large cost, double glazing is probably the sensible option. If the room is cold and the gain is clear, triple glazing is worth having.
