Double or triple glazed rooflights

A double glazed rooflight has two sheets of glass with a sealed gap between them. A triple glazed one has three sheets and two gaps. Each gap is a layer of still gas, usually argon, that slows the movement of heat from the warm side to the cold side. More gaps slow it further, so the glass loses less heat and the inner surface stays a little warmer.

That warmer inner surface matters in a rooflight more than in a wall window. Glass overhead faces the cold sky, and warm air rises to meet it, so a rooflight is often the coldest thing in a heated room. A warmer inner pane cools the air beside it less, which cuts the downdraught people feel on a winter evening and makes condensation on the glass less likely.

The coatings count for as much as the pane count. A modern double unit with a low emissivity coating on one pane and an argon fill can outperform an older triple unit without them. When you compare quotes, compare the whole specification: the coating, the gas, the spacer at the edge of the unit and the frame it sits in. The number that sums those up is the U-value, which we explain in rooflight U-values explained.

A third pane is not free, and the price on the quote is only the first part of it.

Weight

Another sheet of glass adds a good deal of weight to every square metre. On a small roof window that is easy to live with. On a large flat rooflight or a lantern it can change how the unit is handled, how many people are needed to lift it and what the kerb or frame has to carry. We cover that in is a triple glazed rooflight heavier?

Thickness

The unit is deeper, so it needs a deeper frame or glazing bars. In a sloping roof window that can mean a slightly chunkier frame around the opening. In a lantern it can widen the bars, which changes how slim the structure looks from the room.

Daylight

Each sheet of glass absorbs and reflects a little light. Three sheets pass a few percentage points less than two, and the coatings that cut heat loss tint the glass a touch. In most rooms you will not notice the difference, but in a small, dim space such as a north-facing landing, it is one more reason to ask for the light transmission figure alongside the U-value.

Solar gain

Heat from the sun passes through glass too, and a third pane lets a bit less of it in. That helps in summer and costs a little free warmth in winter. If the rooflight faces south and the room overheats, a solar control coating does far more than a third pane, which is why the next guide, solar control glass for rooflights, is worth reading alongside this one.

The room, the roof and the way you live in the house decide this, not a general rule that more glass is better.

A loft conversion bedroom with roof windows set into a sloping ceiling is where triple glazing is easiest to justify. The windows are small, so the extra weight is nothing, the room is warm at night when the glass is coldest, and the glass is close to the bed. In a 1930s semi in Heavitree or a Victorian villa in Torquay, where the loft room may be the most exposed space in the house, a triple unit takes the edge off the cold surface above you.

A kitchen extension with a lantern or a large flat rooflight is a different question. These spaces are often already well insulated at the walls and floor, so the glass is the weak point and the thermal gain from a third pane is real. But the weight and the frame depth matter more, and the structure under the glass has to be designed for it. For lanterns in particular, ask whether the third pane is offered in the frame you like and whether the bars stay as slim as the drawing suggests.

A hallway, landing or bathroom with a small fixed rooflight is the place to be most sceptical. The glass area is modest, the room is not lived in for long stretches, and the heat lost through a few tenths of a square metre is small. A good double unit is usually the sensible choice here, and the money goes further on the lining and finish.

Conservation and period work often points back to double. Slim-profile conservation rooflights are made to sit low and look like the cast iron originals, and a thicker unit can work against that. We explain the options in our conservation rooflights service page.

The table shows tendencies. A premium double unit and a basic triple unit can land closer together than the labels suggest, so use it as a prompt to ask for the actual figures rather than as a verdict.

Either way, a new rooflight or a full frame-and-glass replacement falls under Part L of the Building Regulations, the part that deals with fuel and power. The 2021 edition of Approved Document L, Volume 1, sets limiting U-values for new and replacement windows and rooflights in existing homes. Both a good double unit and a triple unit can meet them, and the document is the place to check the current limits for your type of opening.

A complete rooflight or roof window replacement is notifiable work, so Building Control must be told or a registered installer must certify it. Where notification is needed, we do it. A sealed glass unit alone, fitted to a frame that stays, is different: that is not notifiable, though the new unit should still be a sensible specification for the frame. The government's planning and building guidance is on gov.uk.

The regulations set a floor, not a target. Meeting the limit and choosing the right glass for a particular room are two different decisions, and the second is the one that affects how the room feels in January.

Two details outside the glass change how a unit performs more than the pane count alone.

The spacer is the strip that holds the panes apart around the perimeter. A metal spacer conducts heat across the edge of the unit, which leaves the glass cold at the perimeter and invites condensation along the bottom. A warm-edge spacer is made of a less conductive material and keeps the edge warmer. It is a modest change that helps both double and triple units, and we look at it in are warm edge spacers worth it in a rooflight?

The frame matters as much. A rooflight with excellent glass in a frame that conducts cold straight through will still have a chilly edge. Timber, uPVC and thermally broken aluminium all behave differently, and the way the frame is insulated and sealed to the roof decides whether the glass performance carries through to the room.

This is the part that rarely makes the quote, and it is where the room below feels the choice.

A triple unit sits in a deeper frame, and the frame's inner edge is where the lining begins. The depth of the unit changes where the lining meets the frame, so the board has to be set to the right plane, with the groove or stop bead lining up with the inside face. On a roof window this is a few millimetres. On a lantern or a kerb-mounted flat rooflight, the thicker unit and bar can change the whole proportion of the opening from below.

Insulation has to wrap the frame cleanly, because the warmest glass in the world is wasted if the lining behind the board is cold. A cold lining attracts condensation at the sill, which stains the paint and brings the problem back after it has been covered. Where the lining runs a long way, as in a flat ceiling under a pitched roof, the insulation on the loft side of the shaft matters at least as much as the glass. We cover the shaft case in how a rooflight is finished inside.

The heavier unit can also change the structure around the opening. The trimmers and the support for a large triple unit have to be sized for its weight, and in an older house with lath and plaster below, the ceiling must be supported while that work is done. Our written finish schedule, supplied with each Brightview quote, covers all of it: ceiling and roof construction, the form of the lining, the final finish and the dust control. If you are comparing glazing options, ask for the schedule for both so you can see what each does to the room.

Sometimes the frame is sound and only the glass has failed. In that case the sealed unit can be replaced on its own, and the question becomes whether the frame can take a thicker unit. Many older frames were built for a slim double unit and have no room for a third pane, so a triple replacement may mean a new frame, and a new frame makes the job notifiable.

A triple glazed skylight is also not a retrofit you can assume for a roof window made to a fixed size. The rebate depth, the opening sash and the hinges were all designed around the original unit. If the glass in an older rooflight has misted, the sensible first step is to ask what the frame was designed to carry, then choose the best unit that fits, rather than the thickest one on the price list. Our misted and failed rooflight replacement page covers how that decision is made on site.

A short sequence takes most of the guesswork out of it.

1. Decide how much time you spend in the room in winter and whether the glass is overhead or on a slope above a bed or seat.
2. Ask for the U-value, the light transmission and the solar gain figure for each option, from the same manufacturer, on the same basis.
3. Check the unit weight against the opening size, and ask how the frame or structure copes with it.
4. Ask what spacer, gas and coating each unit has, not only how many panes.
5. Look at the frame depth and the bar widths on a sample or a drawing, particularly for a lantern.
6. Check how the lining and insulation are detailed at the frame for the unit you pick.

If two options come out close, choose on the room: a loft bedroom or a living space you use in the evening leans towards triple, while a hall or bathroom rarely needs it. And if the room overheats in summer, put solar control ahead of the third pane.

The wider picture of efficient glazing, including U-values, regulations and condensation, is on our energy efficient rooflights page. Tell us about the room through the quote form, with a photo of the ceiling and the roof if you can, and we will survey it, measure the roof and the ceiling, and write up the glazing options with a finish schedule for each.