Usually not, on a domestic rooflight. A third pane improves the U-value by a real but diminishing amount, and it adds weight, thickness and cost, all of which matter more overhead than in a wall. A well specified double glazed unit, properly installed, generally outperforms a triple glazed one installed carelessly.
What the third pane buys
A genuine improvement in the centre pane figure, and less than doubling it.
Going from single to double glazing is a transformation. Going from double to triple is an improvement of a different order, because the second cavity adds less than the first did.
It is a diminishing return rather than a linear one, and that is the heart of the decision.
Weight
The first practical objection, and on a roof it is not trivial.
A third pane adds roughly half again to the glass weight, which the frame, the fixings and the structure below all have to carry.
On a modern trussed roof that is manageable. On an old roof with slender rafters, or a light garage deck, it is a genuine consideration.
Thickness
The second, and it works against the appearance most people want.
A triple glazed unit is deeper, so the frame around it is deeper, which on a conservation or flush profile is exactly the wrong direction.
On a slim framed unit it can be incompatible with the profile altogether.
Light and solar gain
The third cost, and it is easy to overlook.
Every additional pane and coating removes some visible light and some useful winter solar gain.
On a rooflight, which exists to deliver light, that is a real subtraction rather than an academic one.
The frame is the weak point anyway
The argument that decides it on most domestic units.
On a small opening the frame is a large proportion of the whole unit, and improving the glass while leaving the frame does less than the centre pane figures suggest.
Which is why a slim, well broken frame with good double glazing frequently beats a bulkier triple glazed unit on the whole unit number.
Where triple glazing does earn its place
Three situations, and they are specific. A large glazed area where the total loss is no longer trivial. A room being built to a high fabric standard where the rooflight would otherwise be the weakest element. And a very exposed position where surface temperature and comfort matter.
Acoustic performance
A reason people cite, and it is not the right product for it.
Three equal panes do not perform as well acoustically as two unequal ones, because the mass and the cavity matter more than the count.
An asymmetric double glazed unit, or a laminated pane, outperforms a symmetric triple for noise.
Condensation on the inside
Where triple glazing does help, and it is worth knowing.
A warmer inner surface means room side condensation forms less readily, which in a humid kitchen or bathroom under an exposed roof is a real comfort benefit.
The wider remedies are on how do I stop condensation on a rooflight.
Condensation on the outside
The unexpected consequence, and it surprises people.
A very well insulated unit keeps so little heat on its outer pane that dew can form on the outside on a clear morning, obscuring the view until the sun clears it.
It is a sign of a unit performing well rather than a fault, and it is worth knowing before it happens.
Cost
Meaningfully more for the unit, and not much more to install.
Which means on a job where the installation dominates, the premium is a small proportion. On a simple modern roof it is most of the difference.
Where the same money does more
Consistently, and this is the argument we make.
Spent on the insulation detail at the reveal, on a properly sealed vapour control layer, or on correcting the roof insulation while the roof is open, it does more than the third pane does.
Handling and installation
A practical point on a heavier unit.
Getting a triple glazed rooflight onto a roof safely is more work, and on a job with awkward access it can affect the method and the cost of the day.
It is a survey question rather than an afterthought.
Whether regulations require it
No. The limiting standards are met comfortably by good double glazing.
Nobody should be told triple glazing is needed for compliance on a domestic rooflight, because it is not.
What to ask instead
Three questions that matter more than the pane count.
What the whole unit U-value is, what the frame contributes, and how the insulation will be detailed at the reveal.
Whether the frame can take it
A product question rather than a preference, and it rules the option out on some units.
Not every rooflight frame is made in a depth that accepts a triple glazed unit, particularly in the slim and conservation profiles, so on some ranges the choice does not exist.
It is worth establishing before the decision is agonised over.
What it does to a made to size order
A practical consequence on a replacement.
A thicker heavier unit changes the frame depth and sometimes the fixing detail, so a triple glazed replacement into an existing opening is not always a straight substitution for a double glazed one.
On a like for like replacement that is one more reason double is generally the sensible answer.
What we specify on the glazing
We specify good double glazing on almost every domestic rooflight, and triple where the area is large or a room is being built to a high standard.
What the numbers actually mean is on what U-value should a rooflight have.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.