Double or Triple Glazed Rooflights: What the Numbers Say
A good double-glazed rooflight sits at roughly 1.2 to 1.5 W/m²K whole unit and is the right choice for most Suffolk homes. Triple glazing reaches 0.7 to 1.1, runs a warmer inner surface that kills the cold downdraught, and cuts edge condensation, but it adds about 40 per cent to the glass weight and several hundred pounds to the price, and it can force structural work on a retrofit. Triple wins on large units, north-facing roofs, low-energy builds and where comfort at the glass matters. Double wins on small units, south-facing roofs that overheat, marginal structure, and any job where the budget would be better spent on the insulated upstand. Neither is universally right.
Triple is marketed harder than it deserves. The physics is real, but so is the cost and the weight, and the honest answer depends on the specific roof.
The numbers side by side
| Factor | Solar control double | Clear triple | Solar control triple (slim, krypton) |
|---|---|---|---|
| Whole-unit U-value (W/m²K) | 1.2 to 1.4 | 0.9 to 1.1 | 0.7 to 0.9 |
| g-value | 0.28 to 0.38 | 0.50 to 0.58 | 0.28 to 0.36 |
| Light transmission | 60 to 68% | 65 to 70% | 52 to 60% |
| Relative glass weight | Baseline | About 40% heavier | About 40% heavier |
| Edge condensation risk | Low with warm-edge | Lower | Lowest |
| Cost | Baseline | Several hundred more per unit | Most expensive |
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 exact numbers.
Factor by factor
U-value and comfort
Triple roughly halves the heat loss again over a good double unit, and it runs a warmer inner glass surface. That second point is the one people actually feel. Standing under a large rooflight in January, the cold downdraught off a double unit is real, and triple noticeably reduces it. Customers who are glad they paid for triple almost always name the comfort, not the bill. On a small rooflight in a room you pass through, that comfort gain is barely perceptible, which is half the reason triple is wasted on small units.
Weight and structure
A third pane adds roughly 40 per cent to the glass weight. On a new opening designed from scratch that is a non-issue, because the structure accounts for it. On a retrofit it can be decisive: the extra weight sometimes pushes the detail from doubled trimmers into needing a ring beam or a steel, and that structural upgrade can cost more than the glazing gain saved. This is why triple is a structural decision as well as a glass one, and why we check it at survey. The detail is on triple glazed rooflight installation.
Condensation risk
Triple runs a warmer inner surface, so surface condensation on the room-side glass is less likely, and edge condensation is reduced further. That said, a good double unit with a warm-edge spacer already handles edge condensation well, and most condensation problems we see are not the glass at all, they are a cold bridged upstand, covered on why condensation forms on rooflights. Triple helps at the margin. It does not fix a thermal bridge.
Solar gain
Here triple can actually work against you. Clear triple has a g-value around 0.50 to 0.58, higher than a solar control double, so on a south or west-facing roof clear triple lets in more summer heat than a solar control double unit would. If overheating is your problem, a solar control double beats a clear triple. You can get solar control triple, but now you are paying the most for the unit and cutting light transmission to 52 to 60 per cent. On a sunny aspect the glazing decision is often more about g-value than about the third pane.
The Suffolk cases where each one wins
Triple is worth it when: the unit is large relative to the room; the rest of the fabric is already good, as in a well insulated modern extension at Kesgrave or Martlesham; the roof is north-facing and heat loss dominates; comfort at the glass matters and you want the downdraught gone; or you are chasing a SAP target and need the element to carry more load.
Double is the right call when: the unit is small and the frame dominates, as on a 600 x 900mm bathroom rooflight; the structure is marginal and the extra weight forces a beam; the roof is south or west-facing and the room already overheats, where a solar control double beats a clear triple; or the budget is finite and the upstand is not yet properly insulated, because money into the junction beats money into the third pane.
Put simply, on a large north-facing rooflight over a well insulated room, triple earns its place. On a small rooflight, a baking south-facing one, or a job where the upstand is still a bare timber box, the money is better spent elsewhere. The direct version of the question is answered on is triple glazing worth it for a rooflight, and the cost comparison is on how much a triple glazed rooflight costs.
Is triple glazing always better than double for a rooflight?
No. Triple has a lower U-value and a warmer inner surface, but it adds weight, costs more, and on a sunny aspect clear triple can let in more summer heat than a solar control double. It is better on large units, north-facing roofs and low-energy builds. On small units, on south or west-facing roofs that overheat, or where the extra weight forces structural work, a good double unit is the right choice. The best glazing depends on the roof, not on the pane count.
Does triple glazing stop condensation on a rooflight?
It reduces the risk, because the inner glass runs warmer, but it does not fix the common cause. Most rooflight condensation we are called to is on a cold bridged upstand, not on the glass, and no glazing upgrade touches that. A good double unit with a warm-edge spacer already handles edge condensation well. If the reveal is going black, the fix is an insulated upstand, not a third pane.
Get both priced side by side
We quote double and triple against each other with the whole-unit U-value, the g-value and the weight implication on each, plus an honest view on whether the structure carries triple, so you can decide 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 what energy efficient rooflights cost, or start from 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.