Two Panes or Three: Glazing a Rooflight for Warmth and Clarity

A double glazed rooflight has two panes of glass with one sealed gap between them. A triple glazed unit adds a second gap and a third pane. That sounds like a small difference, but it changes four things at once: how much heat the glass holds onto, how heavy the finished unit is, how clear the view stays at the edges, and how much the glass and frame cost to make. None of those four move together in a straight line, which is why the choice is not simply "three is always better than two".

For a rooflight the stakes are higher than for a wall window. The glass sits closer to horizontal, so it faces the sky and loses heat upward on a cold, clear night in a way a vertical pane never does. That is also the glass you are looking through from a bed, a sofa or a landing, so whatever the extra pane does to clarity and reflections matters more here than almost anywhere else in the house.

Coatings do most of the work inside each pane. A low-emissivity coating on the inner face of the glass reflects heat back into the room rather than letting it radiate out through the unit, and it is usually this coating, together with the gas fill, that separates a good double glazed rooflight from a poor one, before triple glazing even enters the conversation. Our guide to low-e coatings and gas-filled glass sets out how those two layers work together.

The figure that is usually quoted, the whole-unit U-value, includes the frame and the spacer bar as well as the glass, so a triple glazed pane fitted in a poor frame can still underperform a well-made double glazed unit. If you are comparing quotes by U-value alone, it is worth reading how that number is built up in reading a rooflight U-value before you take one figure as the whole story.

A third pane and a second gap add both thickness and weight. A triple glazed unit sits deeper in its frame, and the finished sealed unit is heavier than a double glazed pane of the same size and glass thickness. On a small rooflight over a landing, that difference is rarely worth discussing. On a large flat rooflight, or a run of roof windows in a single opening, the added weight changes what the upstand, the kerb and the roof structure need to support.

The frame matters as much as the glass here. A frame with a poor thermal break can undo a good triple glazed pane, because heat still finds a way through the metal or the reveal lining even while the glass itself performs well. Thermally broken frames and cold bridging covers how that path through the frame is closed off, and why a well-specified double glazed unit in a good frame can outperform a triple glazed unit in a poor one.

Structurally, most domestic roofs and standard rooflight kerbs are designed with the extra depth and weight of triple glazing in mind, so this is rarely a reason to rule triple glazing out on its own. It is, though, one more thing worth raising at survey stage rather than after the unit has arrived, particularly on an older roof or a wide opening. Our answer on whether triple glazed rooflights are heavier gives the short version of this.

This is where Clearview's own concern sits, because a rooflight is a window you look up through, not past. Every pane of glass has an edge, and every edge shows a faint green tint where light passes through a greater thickness of ordinary glass. With three panes instead of two, that edge tint is very slightly more visible on some units, though it is rarely something you would notice without comparing samples side by side, and low-iron glass removes almost all of it if the clarity of the sky matters to you in that particular room. The full explanation is in does triple glazing give skylight glass a green tint.

More noticeable, in practice, is reflection. Each extra internal surface in the glass build-up is another surface that can catch and bounce light, so a triple glazed unit with several coated surfaces can show a touch more internal reflection at a low sun angle than a simple double glazed pane. It is a small effect and it does not change what you see on an overcast day, but from a bed or a sofa at the golden hour it can be the difference between glass that reads as open sky and glass that reads as glass. If a coating or tint is doing work beyond warmth, our note on heat-reflective coatings and the view through the glass is worth reading alongside this one.

• A frame built to take the extra depth and weight of triple glazing is sometimes a heavier-duty specification than the same frame built for double glazing, which can carry its own cost.
• Handling and installation on a large or awkward triple glazed unit can take a little longer, particularly where two people are needed to lift and seal it rather than one.
• Over the life of the rooflight, a lower U-value reduces heat loss through that opening, which feeds into running costs, though the size of that saving depends on the size and orientation of the rooflight and cannot be stated as a fixed figure for every home.

None of this tells you whether triple glazing is worth the extra outlay on a specific roof. That depends on the room, the orientation, how the space is heated and used, and what else is being done to the roof at the same time. Our page on whether it is worth replacing a double glazed rooflight with triple works through that trade-off for an existing installation, and what affects the cost of a rooflight sets glazing choice alongside the other factors that move a quote.

Condensation on a rooflight almost always starts at the edge of the glass or on the frame, not across the middle of the pane, because the spacer bar that holds the two or three panes apart is the coldest part of the unit. A warm edge spacer, made from a low-conductivity material rather than plain aluminium, keeps that edge closer to room temperature and cuts down the misting and streaking that collects there over a winter. This matters just as much on a double glazed unit as a triple one; adding a third pane does not fix a cold-edge spacer, and a well-specified double glazed unit with a warm edge spacer can stay clearer at the edges than a triple glazed unit built with a poor one.

Where triple glazing does help is across the centre of the pane, because the warmer inner surface it produces is less likely to reach the temperature at which moisture in room air starts to condense. On a bathroom or kitchen rooflight, where humidity is higher and condensation is more likely to form, that margin can be the difference between a pane that stays clear and one that mists over most winter mornings. Warm edge spacers and the clear edge of the glass and skylight condensation: causes and cures both go into this in more detail.

Triple glazing tends to earn its keep in a handful of situations: a north-facing rooflight that gets little direct sun to offset its heat loss, a room that is heated and occupied for long hours such as a main bedroom or a home office, a build aiming at a Passivhaus or very low energy standard, or a rooflight in an exposed position with a large area of glass relative to the room it serves. Our guide to what makes a rooflight suitable for a Passivhaus sets out how far that specification goes beyond ordinary triple glazing.

A well-specified double glazed unit, with a good low-e coating, argon fill and a warm edge spacer, is often the better balance for a smaller rooflight over a landing, a hallway or a utility room, where the heat loss through one opening is modest and the extra weight and cost of a third pane buys little that the room will notice. Sizing the opening correctly in the first place has more effect on the balance between daylight gained and heat lost than the choice between double and triple glazing, which our guide on balancing daylight and heat loss when sizing covers directly.

Current Building Regulations set a maximum U-value that any new or replacement rooflight must meet, whether it is double or triple glazed, so the real question for most homes is not whether the glass will pass, since a competent double glazed specification usually clears the bar comfortably, but which specification suits the room, the roof and the outlook you are trying to keep.

A survey settles most of these questions for a specific roof faster than any general comparison can, because it puts the actual opening, orientation and room use in front of the decision rather than a set of averages. It is one of several specification choices covered in our wider guide to energy efficient rooflights in Peterborough, alongside coatings, frames and sizing.

Does a triple glazed rooflight let in less daylight than a double glazed one?

The extra pane and coatings reduce light transmittance slightly, but the difference is small on most modern units and is rarely noticeable against the difference that placement, size and frame width make to how bright a room feels.

Can a double glazed rooflight be upgraded to triple glazing later?

No. The glazed unit is sealed at manufacture, so a double glazed pane cannot be converted to triple glazing in place. Moving to triple glazing means replacing the sealed unit, and often the frame, rather than adding a pane on site.

Does triple glazing stop a rooflight overheating in summer?

Not by itself. Triple glazing is about keeping heat in during winter, while summer heat gain is governed mainly by the glass coating and area, which our guide to rooflights and overheating under Part O explains separately from glazing thickness.

Is triple glazing available on every rooflight and roof window style?

Most flat rooflights, lanterns and pitched roof windows can be specified with a triple glazed unit, though the frame, kerb and lining need to be built to take the extra depth and weight from the outset, which is why this is best settled at survey stage rather than after the opening is cut.