What the third pane actually does
A double-glazed unit is two sheets of glass held apart by a spacer, with a sealed gap between them. The gap is filled with a gas that conducts heat poorly, and one of the panes carries a thin low-emissivity (low-e) coating that reflects heat back into the room. A triple-glazed unit adds a third sheet and a second gap.
The extra gap is a second barrier, and the unit loses less heat as a result. That is the whole case for triple: a lower U-value, a figure for the heat that escapes through each square metre, lower being better. If the unit is new to you, this U-value guide sets it out.
What the third pane does not do is change the rules of glass. It is still glass. A triple-glazed rooflight loses more heat than the insulated roof beside it. It just loses less than the double-glazed version would.
It also cuts a little light. Each extra pane and coating absorbs or reflects a share of what passes through. In a bright room you will not notice. In a gloomy room where the whole point of the skylight is daylight, you may.
What triple glazing costs you
The price is more than the invoice.
- Weight. A third pane is a third sheet of glass. On a big flat rooflight or a roof lantern, that is a lot of extra kilos for the frame, the kerb and the structure below to carry. It may mean a stronger frame or extra trimming in the roof.
- Depth. The unit gets thicker, and the frame must hold it. That changes how the glass sits in the profile, and on a slim-framed conservation-style unit it may not fit at all.
- Handling. Heavy panes are harder to lift onto a roof safely. This is a job for more hands and, on larger units, a crane or hoist.
- Cost. More glass, more coating, more gas fill and a heavier frame all add up. We give you the figure in a fixed written quote after the survey.
- Diminishing return. The first sealed gap does most of the work. The second adds less.
None of that rules triple out. It means the third pane needs a reason, and the reason should be written down.
When double is the right answer
For a typical home project, double glazing meets the regulation. Part L (2021 edition) sets 1.4 W/m²K for a roof window in an existing dwelling, band B on the Window Energy Rating scale being the other route, and 2.2 W/m²K for a rooflight. Current double units with low-e coating, gas fill and a warm-edge spacer are built to those limits. You can read the regulation on GOV.UK.
Double also suits many of the rooms we work in. A kitchen extension, a landing, a bathroom or a loft room that is heated normally has an average temperature difference with the outdoors that a well-specified double unit handles comfortably. The saving from stepping up to triple on a small rooflight is also a small number in absolute terms, because the glass area is small.
It is worth remembering the other half of the picture. Frame, spacer and fitting matter as much as the number of panes. A double-glazed unit with a thermal-break frame, a warm edge and a properly insulated kerb can outperform a triple-glazed unit on a poor frame. If you are comparing two quotes, compare the whole unit figure, not the pane count.
When triple earns its place
Triple makes sense when something about the job puts extra demand on the glass.
- A big area. A large rooflight or lantern is a large patch of glass, and the heat it loses scales with the area. Here the improvement from a third pane adds up.
- A cold or exposed position. A roof that takes the wind and the clear night sky pulls more heat out of the glass.
- A room that is kept warm. A sunroom, a swimming-pool room or a bedroom kept at a high temperature all winter raises the gap between inside and out.
- Surface comfort. The inner pane of a triple unit stays closer to room temperature. That helps if you sit or sleep beneath the glass, because cold glass radiates a chill onto you.
- Condensation risk. A warmer inner pane resists misting on the room side. The reasons are in the condensation answer.
- A low target. If you want to beat the regulation by a distance, triple is one route.
If you recognise your roof in two or three of those, ask for a triple option on the quote next to the double one, and compare the two figures side by side.
Things to ask for in the quote
Whichever you choose, three details tell you whether the unit is a good one.
The whole-unit U-value. Not the centre-pane figure, which is flattering, but the figure for glass and frame together. Part L is about the whole unit.
The spacer. Ask whether it is a warm-edge type. It is a cheap upgrade with a real effect around the perimeter.
The coating and the gas. The quote should name them. If it only says "energy efficient glass", ask what that means.
Also ask about solar control. A unit tuned to keep heat in during the winter admits a lot of sun in summer, and a south-facing triple unit can still overheat a room. Solar control glass is the coating that deals with that.
How we help you decide
At the free survey we check the size of the opening, the structure that has to carry the glass, the room below and the direction the skylight faces. Then we put the double and the triple options on a fixed written quote where both make sense, so the choice rests on your roof and not a general rule. Building Control notification is handled by us when the job needs it, and the fitting is guaranteed for ten years.
The wider range sits on our energy efficient rooflights page. If you already have a quote and want a second opinion on the glazing, send it by WhatsApp. When you are ready for ours, ask for a quote.