Triple glazed rooflight installation adds a third pane and a second cavity, which brings the whole-unit U-value down to roughly 0.9 to 1.1 for clear glass, and to around 0.7 to 0.9 for slim units with a solar control coating and krypton fill. In exchange it adds about 40 per cent to the glass weight, needs a deeper frame, and costs several hundred pounds more per unit. It is worth it on large units, on north-facing roofs where heat loss dominates, and in low-energy builds where the rest of the fabric is already good. It is often not worth it on small units, on south-facing roofs that overheat, or where the extra weight forces structural work that costs more than the glazing gain saved.
Triple is sold hard and it is right less often than the marketing suggests. The physics is real. The question is always whether it is real on your roof.
What the third pane actually does
A double-glazed unit has two panes and one gas-filled cavity. A triple unit has three panes and two cavities, and with low-emissivity coatings on the right surfaces it roughly halves the heat loss again compared with a good double unit. Two things follow, and only one of them is on the datasheet:
- Lower U-value. Around 0.9 to 1.1 whole unit for clear triple, against 1.3 to 1.5 for clear double and 1.2 to 1.4 for solar control double. Slim triple with krypton reaches 0.7 to 0.9.
- Warmer inner glass. This is the one people actually feel. The internal surface of triple glazing runs warmer than double, so the cold downdraught you get standing under a large rooflight in January is noticeably reduced. Customers who are glad they bought triple usually name the comfort, not the bill.
The weight and structure question
A third pane is a third sheet of glass. On the units we fit, triple adds roughly 40 per cent to the glass weight over the equivalent double unit. On a new opening designed from scratch, the structural detail simply accounts for it and it is a non-issue. On a retrofit into an existing roof, it can be the deciding factor.
The problem case is a retrofit opening where the double-glazed weight was carried comfortably by doubled trimmers, and the triple weight pushes the detail into needing a ring beam or a steel. That structural upgrade can cost more than the glazing upgrade saved. So triple is not just a glass decision, it is a structural one, and it has to be checked at survey against the actual roof rather than assumed. Where the opening is over 1.5m or the roof is an unknown quantity, a structural engineer’s calculation is the cheapest insurance on the job.
The frame has to be deep enough. Three panes and two cavities need a thicker sealed unit, which needs a deeper frame rebate to hold it. Not every rooflight system takes a triple unit, and forcing a triple unit into a frame designed for double is how you end up with squeezed cavities and lost performance. We specify a system rated for the triple build-up, not a double frame carrying a fat unit.
Frame depth, cavity width and krypton
There is a design tension in a triple unit. The overall thickness has to stay sensible so the frame and the roof detail still work, but you now have to fit three panes and two cavities into that thickness. So the cavities get squeezed, often to 10 or 12mm each rather than the 16mm a double unit enjoys.
That matters for the gas fill. Argon performs best at around a 16mm cavity and falls away as the gap narrows. Krypton conducts heat less than argon and keeps working in narrow cavities, which is exactly why krypton turns up in slim triple glazing and almost nowhere else in a domestic rooflight. In a triple unit with squeezed cavities, krypton earns its place. In a standard double unit with a 16mm gap, it is money spent on a datasheet. We go into that trade-off on argon or krypton filled rooflight glazing.
| Build-up | U-value (W/m²K) | g-value | Light transmission | Relative weight |
|---|---|---|---|---|
| Double, solar control, argon, warm-edge | 1.2 to 1.4 | 0.28 to 0.38 | 60 to 68% | Baseline |
| Triple, clear, argon, warm-edge | 0.9 to 1.1 | 0.50 to 0.58 | 65 to 70% | About 40% heavier |
| Triple, solar control, krypton, warm-edge | 0.7 to 0.9 | 0.28 to 0.36 | 52 to 60% | About 40% heavier |
Figures for a 1.2 x 1.2m reference size in a thermally broken frame. Smaller units perform worse, larger units better. Your survey confirms the declared figure for the exact size you are buying.
When triple genuinely earns its place
The honest test we apply at survey. Triple is worth it when:
- The unit is large relative to the room, so the glazed element dominates that room’s heat loss. A 3m² rooflight over a small snug is a different case from the same unit over a big kitchen diner.
- The rest of the fabric is already good. Triple in an extension with deep roof insulation and good airtightness improves a system that is already working. The same unit over a 1970s flat roof with 50mm of insulation is the strongest link in a weak chain.
- The roof is north-facing and heat loss dominates with little useful solar gain to protect.
- Comfort at the glass matters, because the warmer inner surface removes the cold downdraught. This is often the real reason.
- You are chasing a target in a SAP calculation and need the rooflight to carry more of the load.
And it is usually not worth it when the unit is small and the frame dominates, when the structure is marginal and the extra weight forces a beam, when the roof is south-facing and the room already overheats, or when the budget is finite and the upstand is not yet properly insulated. We weigh it in full on double or triple glazed rooflights, what the numbers say and answer the direct question on is triple glazing worth it for a rooflight.
The Passivhaus and low-energy case
At the demanding end, triple glazing is not a choice, it is a requirement. Certified low-U-value rooflights for Passivhaus and low-energy builds are triple-glazed, thermally isolated at the kerb, and taped airtight, because the fabric standard leaves no room for a weak rooflight. That is a distinct discipline, and it is covered on low U-value rooflights for Passivhaus and low-energy builds.
How we install it
The installation is the same careful sequence as any energy-led job, with the weight built into the structure from the start: survey and structural check, a frame system rated for the triple build-up, an insulated upstand lapped continuously into the roof insulation, an airtight and vapour-controlled perimeter, and the unit set square on the corrected junction. On coastal work at Felixstowe or Harwich the fixings are 316L stainless and the upstand goes to 200mm. The weathering-in is complete and the roof watertight before the heavy unit is ever lifted on, so the roof is never open waiting on a crane.
Does a triple glazed rooflight always need extra structure?
Not always. On a new opening designed from scratch, the structural detail accounts for the weight and it is a non-issue. On a retrofit into an existing roof, the extra 40 per cent of glass weight sometimes pushes the detail from doubled trimmers into needing a ring beam, and that upgrade can cost more than the glazing gain saved. It depends on the span, the existing structure and the unit size, which is why we check it at survey rather than assume it.
Is triple glazing worth it on a small bathroom rooflight?
Usually not. On a small unit like a 600 x 900mm bathroom rooflight the frame dominates the whole-unit figure, so the third pane changes very little of the overall performance while adding cost and weight. The money is better spent on a good thermally broken double unit with a warm-edge spacer and a properly insulated upstand. Triple earns its place on large units, not small ones.
Get triple priced against a good double unit
We quote triple and high-performance double side by side, with the whole-unit U-value, the g-value and the weight implication written on each, so you can see what the third pane buys you on your roof rather than in a brochure. One fixed price, a 10-year workmanship guarantee, and installers with 25 years on rooflights.
Request a fixed quote, read the pricing on how much a triple glazed rooflight costs, or start from the hub at energy efficient skylight installation.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.