Do Skylights Lose a Lot of Heat?

It is easy to picture heat leaving through the glass itself, as if the pane were a single leaking point. In practice a rooflight loses warmth through three separate routes, and they do not lose it at the same rate.

• The centre of the glass, where the panes, the gas fill and any coating do the work.
• The frame, which is usually aluminium and, without a thermal break, conducts heat outward far faster than the glass beside it.
• The edge of the sealed unit, the narrow band where the spacer bar holds the panes apart and where heat can bridge from pane to pane if the spacer itself conducts.

A rooflight specified well treats all three. One without a thermal break in the frame, or with an old-style aluminium spacer at the edge, can lose more heat through those two paths than through the glass they surround, whatever the glass itself is rated at.

Even a good modern rooflight sits well behind an insulated wall or roof slope for heat loss, and it is worth knowing that plainly rather than expecting glazing to match masonry. In round terms, a well insulated cavity wall or roof might sit under 0.2 W/m²K, while even a well specified modern rooflight rarely gets much below about 1.0 W/m²K. That gap is physics, not a fault in the product: glass and the frame around it will always pass heat faster than a thick, continuous layer of insulation.

Our guide to reading a rooflight U-value sets out why the whole-unit figure, not just the centre-pane number, is the one that matters for a real installation, and why two suppliers can quote different figures for what looks like the same glass.

Aluminium is the frame material most rooflights are built from because it holds a large pane in a slim profile, which is exactly what keeps the frame edge out of the sightline from a bed or a sofa. That same readiness to conduct heat is a poor property in winter, when an untreated aluminium frame behaves like a small radiator running in reverse. A thermal break, a strip of insulating material set into the frame, splits the outer metal from the inner metal so heat cannot pass straight through. Our page on thermally broken frames and cold bridging covers how that split works and what happens without it, including the condensation that tends to follow a cold frame.

The spacer bar at the edge of the glass matters for the same reason. An older aluminium spacer conducts heat and cold straight across the narrow gap between the panes, which is where a lot of edge condensation starts. A warm edge spacer, built from a material that conducts far less, keeps that edge closer to room temperature. It is covered in full on warm edge spacers: keeping the glass edge clear.

Two layers you never see do most of the remaining work: a low-E coating fired onto one face of the glass, and a gas fill sealed into the cavity between the panes in place of ordinary air. Neither shows up as a colour or a line across the sky. Together they slow the rate at which heat radiates and conducts through the unit, without dimming the view the way a tinted or heavily reflective glass can. The invisible layers: low-E coatings and gas-filled glass explains how each one works. Adding a third pane, discussed on two panes or three: glazing a rooflight for warmth and clarity, cuts heat loss further still, at the cost of some extra weight and depth in the frame.

Heat loss through a rooflight scales with area, so a larger opening loses more in absolute terms even when the glass and frame are identical to a smaller one. That does not make a bigger rooflight the wrong choice; it makes sizing a balance rather than a default toward the maximum the roof structure allows. Balancing daylight and heat loss when sizing sets out where the daylight gained from extra glass starts to flatten off while the heat loss keeps climbing in a straight line.

Part L of the Building Regulations sets a limiting U-value that a new or replacement rooflight has to meet, and it treats a rooflight differently from a vertical window because of how much sky-facing glass a roof can carry. What Part L asks of a rooflight covers the figure that applies and how it is checked. We install to current Building Regulations on every job, and handle the Building Control notification where the work is notifiable, so the paperwork around a replacement does not fall to the homeowner.

The largest single improvement available is usually not upgrading a decent modern unit further, but replacing an old single-glazed rooflight or an ageing plastic dome that was never particularly efficient to begin with. Replacing single-glazed rooflights with modern glazing covers what that swap involves, and do energy efficient skylights lower heating bills? looks at how that kind of change feeds through to actual heating use. Every quote, whatever the scope, comes with a sightline sketch, so the view gained is set out alongside the glazing specification rather than treated as a separate conversation; our sightline sketch page explains how that drawing is put together. The Energy Efficient Rooflights in Peterborough page covers the full range of glazing and frame options this reasoning draws on, and what affects the cost of a rooflight sets out how these choices tend to move the price.

Does a bigger rooflight always lose more heat?

In absolute terms, yes, because heat loss scales with the area of glass and frame. That is a reason to size a rooflight deliberately rather than a reason to avoid a larger one where the room genuinely benefits from it.

Is the frame or the glass more likely to let heat out?

It depends on the unit. An untreated aluminium frame or an old-style spacer at the glass edge can lose more heat than the glass itself, which is why both are specified alongside the glazing rather than treated as an afterthought.

Can an old rooflight be improved without full replacement?

Sometimes. Where the frame is sound, the glazed unit alone can sometimes be upgraded. Where the frame itself has no thermal break, replacement is usually the route that actually closes the gap.

Does triple glazing stop all the heat loss through a rooflight?

No. It reduces it further than double glazing, but a glazed opening will still lose heat faster than the insulated roof around it, whatever the glazing specification.