A rooflight quote can come back with a choice in it: double glazing or triple. Both are sealed glass units, and both can be made to suit the opening. The difference is one more pane and one more gap, and what that buys depends on the room under the glass, the roof carrying it and the figure the guidance sets as the minimum. This guide walks through each of those, so the choice can rest on facts about your roof and not on a feeling that more glass must be better.
What a third pane actually adds
A double glazed unit holds two panes of glass with a sealed gap between them. A triple glazed unit holds three, with two gaps. The extra layer can slow the heat passing out through the glass.
Makers list their own variants and sizes, and what is offered depends on the product. VELUX, for one, makes its flat and curved glass rooflights with either two panes or three, and gives energy efficiency and sound reduction as the reasons. That is why a rooflight quote needs to name the glazing, and why two quotes for the same opening in the roof can describe different units.
A third pane is an addition to the unit, and a double glazed unit can still reach the guidance figure, depending on the unit as a whole. A well made double glazed rooflight can do the job in plenty of rooms. The room under the glass and the roof carrying it show when it does and when the third layer is worth having.
The figure the Welsh guidance sets
In Wales the reference is Approved Document L (Wales), and it is guidance, so its figures are written as what a product should reach. For a new or replacement rooflight in an existing home, the U-value should be no worse than 2.2 W/m²K. A roof window, which sits in the plane of the roof and not on a kerb or upstand, has a tighter figure: 1.4 W/m²K, or a Window Energy Rating of Band B. Both figures sit in Table 10.1 of the document, and the same limits appear again in Table 4.1.
The document describes a rooflight as a glazed unit set out of plane with the roof surface, on a kerb or upstand, and notes that people also call it a skylight. If you are unsure which of the two you have or want, the way it sits relative to the roof slope settles it. The number itself gets fuller treatment in our guide to good U-values for rooflights.
For this choice, 2.2 is a floor and not a target. A unit that reaches it satisfies the guidance. Whether you want to go well beyond it is a question about comfort and use, and the next sections deal with that. The Welsh edition due on 4 March 2027 leaves the window, roof window and rooflight values unchanged. The wider picture on Part L and rooflights in Wales sits on its own page.
Reading two quotes side by side
When two quotes give two numbers, check what each number covers. Under the Welsh guidance, a rooflight is rated across the whole developed surface area of the finished unit, and that surface is often larger than the opening cut in the roof.
A kerb supplied in one piece with the rooflight is counted inside the 2.2 figure. A number quoted for the glass alone cannot therefore be set against a number for the finished assembly. Ask each supplier to give the figure on the same footing, and be wary of any comparison built from centre-pane values.
Look at the other lines of the quote too. Is the glazing named in full? Is the kerb part of the price or separate? Ours is a fixed written quote, so you can hold it beside another. At the survey we look at the whole assembly, from the upstand to the glass, because that is what the roof has to live with.
Rooms where the third layer earns its place
A loft bedroom in January is the clearest case. The room is warm, the glass above the bed can be the coldest surface in it, and the bed is where you spend the hours when the heating is lowest. Cold glass in that position is felt as well as measured.
The same reasoning applies to a kitchen extension used for supper on winter evenings. Once the light goes, the rooflight stops being a source of daylight and becomes a large dark pane that mirrors the room, so its surface temperature is what you notice. Triple glazing can help there, and so can good double glazing in a well made frame.
Sound is the other thing a third pane can change. VELUX pairs sound reduction with its double and triple options on flat and curved glass rooflights, which may matter on a roof under a flight path or above a busy road. If rain on the glass is the concern, rain noise reduction is a separate glazing option on some roof windows, and not every variant carries it.
Where the room is a hallway, a landing or a bathroom used for short stretches, the extra layer buys less. There the questions are more about the kerb, the fit and keeping the glass clear. A sun tunnel or a smaller rooflight over a landing is judged on different grounds from a wide unit over a bed.
Weight, frame and the roof underneath
A third pane of glass can add weight. The Welsh Government's page on roof lights lists the unit's weight among the reasons a new rooflight generally needs approval: the roof has to take the load, or be strengthened to do so. Its other reasons are that the structure of the roof usually needs altering to form the opening, that the unit has to be energy efficient, and that its fire performance needs thought close to a boundary. On a lightweight timber roof, or one with a wide opening, the weight of the finished unit is part of what the survey weighs up.
The frame matters as much as the glass. The rating covers the whole unit, as the previous section explained, and the frame has to suit the glazing chosen. The upstand or kerb beneath has to be sound and suit the unit. If an existing upstand is out of true, it is better found at survey than on the day.
For rooflights set on a kerb in a flat roof, makers publish their own installation advice for each product. Rainclear, for example, advises that the kerb under its flat glass rooflights falls at least 5 degrees, to limit pooling. Advice of that kind belongs to the named product and is no rule for every unit, which is another reason the glazing choice is settled alongside the roof and not apart from it.
Questions to put to any supplier
Whatever you decide, a handful of questions keep the comparison honest. Ask which glazing is being quoted and whether it is two panes or three. Ask whether the figure given is for the glass or for the finished unit with its frame, and whether a kerb is part of it. Ask what the unit is made to fit, and whether the opening or the upstand needs work first.
It is fair to ask where the rooflight sits in the guidance as well. A unit on a kerb is a rooflight and a unit in the plane of the roof is a roof window, and the two have different figures in Approved Document L (Wales). A supplier who quotes the wrong figure for the wrong type has not compared like with like.
Finally, ask what the choice means in the room. A loft bedroom, a kitchen extension and a landing each use the glass at different hours. We would rather you left a survey knowing why a unit suits your roof than knowing only that it costs more or less than another.
Choices that sit beside the pane count
Pane count is one decision among several that shape how a rooflight lives with a room. A quote can also describe what the glass is made of and what is done to its surface, and these can change daily use more than the number of layers.
VELUX's glazing options include a dirt-repellent coating for easier cleaning, a laminated inner pane for safety and a quieter finish in rain. The laminated pane is there, in the maker's words, so that glass does not drop into the room if it breaks by accident. That is worth weighing above a bed or a kitchen worktop, whatever the pane count. What is offered shifts from one window to the next, so the options open to you depend on the product.
Self-cleaning glass exists as a rooflight option too. Pilkington names roof glazing among the applications for its Activ range, a coating that works with daylight and rain to break down and rinse off dirt. Whether it suits your roof is something we talk through at the survey.
Blinds are the other half of the evening picture. A blackout blind in a loft bedroom does what no pane count can, and it is best chosen alongside the rooflight.
How the survey settles the choice
Before any price is given, we visit for a free survey. We look at the roof, the room, the frame or kerb the glass will sit on, and what the space is used for from breakfast to bedtime. Then we recommend a unit and send a fixed written quote.
Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. A brand new rooflight in Wales will generally need approval under the Building Regulations, and exchanging a roof window for a like one is building work too. A glass-only change in a frame that stays falls outside building work as the energy rules count it. The overview of Welsh rules for skylights sets out each part.
If cost is the thing holding you back, the pages on energy efficient rooflight cost and how much a skylight costs explain what moves a quote up or down. The wider service page for energy efficient rooflights shows the full range we install.
A third pane is worth having where cold glass is felt, as in a loft bedroom, and worth less on a landing.