Choosing the glass is the decision that people expect to be hard, and it turns out to be easier than it looks. A double glazed skylight and a triple glazed rooflight are both good products. They differ in warmth, weight, summer behaviour and look, and once you know what each does, the right one for your room tends to be plain.
This guide walks through the differences in order: what the extra pane adds, what it takes away, how the rules treat each, and which rooms suit which. It sits inside our wider energy efficient rooflights hub, and it pairs well with the guide to rooflight U-values, which explains the number that sits behind this whole comparison.
We say skylight and rooflight interchangeably. The first is what homeowners search for, the second is what the trade writes on drawings. The glass is the same.
What a double glazed unit is
A double glazed rooflight is two panes of glass held a few millimetres apart by a spacer bar around the edge, with the gap sealed shut. That sealed gap is the whole idea. Still air or gas in a narrow space carries heat badly, so the gap slows the escape of warmth from the room below.
A modern unit improves on the plain sandwich in two ways. A thin coating on one of the inner faces reflects heat back into the room. And the gap is often filled with a gas that conducts heat more slowly than air does. Together these turn two ordinary panes into a very respectable insulator.
On a flat roof the inner pane is toughened or laminated for safety, because it is the pane you see from inside and the one that would be hit if something fell. The outer pane faces the weather and takes the sun, the rain and the cold.
What the third pane adds
Go to three panes and you gain a second sealed gap as well. Each extra gap is another layer of slow-moving air or gas between the room and the sky. The result is a lower whole-unit U-value than a comparable double unit, so less heat escapes on a cold night.
There is a second benefit that is easy to overlook. The inside face of a triple unit stays warmer in winter. A warmer inner pane is less likely to mist with condensation, which matters in a kitchen where kettles and pans put moisture into the air, and in a bathroom.
Noise drops too. Rain on glass is audible from inside, and an extra pane and gap soften the drumming. A third pane also makes the unit calmer to live under on a blustery night.
What the third pane costs you in other ways
Nothing in building is free of trade-offs, and the third pane has several. We would rather you heard them from us.
- Weight. Three panes weigh more than two. On a large flat rooflight or a roof lantern the extra weight puts more load on the frame and the upstand, which can mean a stronger frame section.
- Depth. The unit is thicker, so the glazing bar and edge detail are chunkier. On a slim, minimal design the difference is visible.
- Light. Each pane absorbs and reflects a little daylight. A triple unit lets through a little less than a double, though the sky is bright enough that the difference rarely changes how a room feels.
- Cost of choice. A third pane is a more involved product to make, and it is priced that way. We discuss the choice at the survey and put the figure in a fixed written quote.
None of these rules triple glazing out. They are reasons to choose it with open eyes rather than as a reflex.
Both can meet Part L
For an existing dwelling, Approved Document L sets the limits in Table 4.2. Roof windows must reach 1.4 W/m²K or WER band B. Rooflights must reach 2.2 W/m²K. A properly specified double glazed unit can meet the standard for the product it belongs to, and so can a triple glazed one.
That is worth saying plainly, because it is sometimes suggested that triple glazing is needed to comply. It is not. Compliance depends on the whole-unit figure for the actual product and size, which includes the spacer and the frame, not on the pane count alone. A well-made double unit in a good frame can beat a poorly made triple one.
Whichever glass you choose, it will be specified to clear the limit that applies to your opening.
What the rules cap
- 1.4 W/m²K
- for roof windows, or WER band B, when work is done to an older home
- 2.2 W/m²K
- for rooflights over flat roofs in an older home
Summer is where the choice gets interesting
Winter is easy to reason about. Summer is subtler, and it is where many buyers are surprised. The roof is the surface the sun hits hardest, so a rooflight faces the full strength of it. More insulation does not by itself keep a room cool, because the heat that matters in July comes in as sunlight through the glass, not as conduction through the frame.
That is why solar control glass exists. A coating on the outer pane reflects part of the sun's heat while letting daylight through. It can be added to a double unit or a triple one, and it matters more to summer comfort than the pane count does. If your room faces south or sits under open sky with no shade, ask about it. The options, including opening units and blinds, are covered in our answer on what a rooflight does to the temperature of a room.
The trade-off to know is that a strong solar coating cuts a little daylight and slightly tints the view. That is the price of a cooler room, and for a sunny kitchen extension many people consider it worth paying.
Which suits which room
There is no universal winner, so here is how the choice tends to fall room by room, based on what each space asks of the glass.
A kitchen extension with a roof lantern asks for a lot: even light over the table, no condensation from cooking and comfortable temperatures in all seasons. Triple glazing and a sensible solar coating answer that well, and the frame is built to carry it. See our roof lanterns page for how those are made.
A landing or hallway with a small flat rooflight has simpler needs. It is a passing space rather than one you sit in, so a good double glazed unit is often entirely sufficient, and it keeps the frame slim.
A loft bedroom lit by a roof window has its own tighter limit, and a well-made double unit can meet it. A bedroom may favour the quiet and the warm inner pane that triple brings. For a bathroom the warm pane helps against misting.
A walk-on rooflight or a conservation-style slim unit pulls the other way. Heavy walk-on glass is already laminated in several layers for strength, and a slim conservation rooflight is chosen for the narrowness of its frame. Both are specialist choices, covered on the walk-on rooflights and conservation rooflights pages.
One more consideration is replacement. If you are swapping an old rooflight, the existing opening and kerb set limits on what will fit. A tired, single glazed or early double unit can usually be replaced with a modern one in the same opening, and the new glass is a clear step up in warmth. We check the condition of the kerb and the roof around it at the survey, because a better pane on a tired upstand wastes the gain.
Maintenance deserves a thought as well. Glass that you can reach is glass you will keep clear, and glass you cannot reach needs a plan. Think about how you will clean and reach the glass. A pitched roof window is within arm's reach from inside, while a lantern on a flat roof is cleaned from outside. A heavier triple unit changes nothing about that, but a self-cleaning coating on the outer face keeps the view clear between visits, and it is another option worth raising.
Double against triple at a glance
The table below sets the two side by side so you can see where each one leads. It describes the character of each build, not exact figures, because the numbers depend on the make, the frame and the size.
| Quality | Double | Triple |
|---|---|---|
| Heat kept in | Good | Better |
| Inner pane in winter | Cool | Warmer |
| Weight on frame | Lighter | Heavier |
| Frame look | Slimmer | Chunkier |
| Daylight let through | A touch more | A touch less |
| Rain noise | Audible | Softer |
Frames, spacers and the parts people forget
Glass is only part of the story. A great pane in a poor frame disappoints, so look at the rest of the unit alongside the pane count.
The spacer bar around the edge can be metal or a less conductive warm-edge type. The warm-edge type reduces the cold bridge at the perimeter, and it improves the whole-unit figure regardless of how many panes sit inside it.
The frame matters just as much. Aluminium suits large flat rooflights and lanterns because it is strong and slim, and a thermal break inside it stops cold running through to the interior. uPVC and timber insulate well on their own and suit other designs. The materials are a separate decision from the glass, and the two should be chosen together.
Finally, the kerb underneath. On a flat roof the glass sits on an insulated upstand, and poor insulation there lets heat out round the edge of even a fine unit. We build the upstand properly and finish the inside neatly, so the plasterwork and the lining are part of the job rather than left behind.
How we help you decide
Behind all of this sit a few plain questions that we ask at every survey, and that you can ask yourself now. The choice comes down to them. How much do you use the room? Does the roof get strong sun? How slim do you want the frame? How quiet should the room be in the rain? Answer those and the pane count usually settles itself.
The free survey is where this becomes concrete. We measure the opening, look at how the roof is built and talk through the options with samples and the room in front of us. You then receive a fixed written quote that does not change on the day, and you also get our 10-year workmanship guarantee in writing. Fitting a typical rooflight itself takes a day or two.
If you want to understand the figure behind these comparisons, the guide to rooflight U-values explains it, and the answer on whether rooflights lose heat speaks to the worry behind it. For the honest list of drawbacks, read the downsides of skylights. Planning questions live in our skylight planning permission guide.
A message on WhatsApp (07880 284133) starts it, as does a note to info@vertexskylightsmiltonkeynes.co.uk.
Glass questions, answered
Is a triple glazed rooflight worth it?
It is worth it where the room is used all day, where condensation is a worry or where quiet matters. In a passing space such as a landing, a good double unit often does the job well.
Does triple glazing keep a room cooler in summer?
Not on its own. Summer heat arrives as sunlight, so solar control coating does more than a third pane. It can be added to either build.
Do I need triple glazing to meet Building Regulations?
No. Part L sets limits on the whole-unit figure, 1.4 W/m²K for roof windows or WER band B, and 2.2 W/m²K for rooflights. A properly specified double glazed unit can meet the one that applies.
