Are Polycarbonate Skylights Energy Efficient?

Multiwall polycarbonate is not a single sheet of plastic. It is built as two or more skins joined by internal ribs, so the panel is really a set of narrow channels, or flutes, running along its length. The insulating value comes almost entirely from the still air trapped inside those channels, not from the plastic itself, which is the same principle a sealed glass unit uses when it traps a gap of air or gas between two panes.

The number of walls and the overall thickness decide how much that trapped air can do. A thin 10mm twin-wall sheet, the kind often seen on an older flat-roof box or a budget lean-to, holds one narrow channel of air and behaves close to a single sheet of glass. A 25mm or 32mm multiwall panel, built with triple or five-wall construction, holds several channels stacked one behind the other and insulates noticeably better as a result. Our guide to reading a rooflight U-value covers how a whole-unit figure is arrived at and why it is the number that matters on a real roof, whatever the glazing material.

These are guide figures for typical constructions, not a quote for any particular product, and the real number for any panel depends on its exact build and the frame it sits in.

A thick multiwall panel is not a poor performer. Set beside an old single-glazed rooflight or a thin, ageing plastic dome, a well specified 25mm or 32mm polycarbonate panel loses heat far more slowly, and it does that at a fraction of the weight glass would add to the same opening. That matters where a roof structure was never built to carry a heavy sealed unit, or where the budget for a job has to stretch further than a glazed lantern would allow. Do skylights lose a lot of heat? sets out the same three heat-loss routes, the glass, the frame and the edge, that apply to any glazing material, polycarbonate included.

Where the improvement is most obvious is a straight swap. Replacing single-glazed rooflights with modern glazing covers that kind of upgrade, and a thicker polycarbonate panel replacing a thin one follows the same logic even where the opening stays the same size.

Glass has three things working in its favour that a polycarbonate panel does not. A low-E coating, fired onto one face of the glass, reflects heat back into the room without a visible tint. A gas fill, usually argon, sits sealed into the cavity in place of ordinary air and conducts less. And because the whole unit is sealed at the factory, that performance does not depend on air moving in and out of open-ended channels the way it can in a polycarbonate panel exposed at its edges. The invisible layers: low-E coatings and gas-filled glass explains both in full, and two panes or three: glazing a rooflight for warmth and clarity sets out what a third pane of glass adds over two.

None of that is available to add to a polycarbonate panel later. What you specify at the point of ordering is broadly what the panel will do for as long as it lasts.

Energy figures are only half of what matters on a page about polycarbonate, because most polycarbonate rooflights are built to diffuse light, not to show a clear picture of the sky through it. An opal or translucent panel scatters daylight evenly, which is useful over a kitchen island where even lighting matters more than a view, but from the bed or the sofa it reads as a bright glow rather than moving cloud or open blue. A clear-grade polycarbonate sheet exists and does show more of a real view, though it scratches more readily than glass and can dull with years of UV exposure in a way toughened glass does not.

The flutes bring a second problem specific to this material. Because the channels are open at the ends to allow drainage, dust, pollen and occasionally moisture can find their way inside over the years, and once something sits inside a flute it cannot be wiped clean the way the face of a pane can. A sealed glass unit does not have that failure mode, and an outer coating that sheds dirt and rain keeps the outward face clear for longer still, which our page on self-cleaning glass for clarity and efficiency covers. Every quote we give, whatever the glazing, comes with a sightline sketch showing what you will actually see from where you sit, stand or lie, which is where the diffusing effect of polycarbonate tends to show up most clearly against a clear glass alternative.

Energy efficiency is not only about winter heat loss. A clear polycarbonate panel with no coating lets a similar amount of solar heat through as uncoated glass, which can leave a room under it running warm on a bright afternoon. An opal or diffusing panel cuts some of that glare, but it does so by scattering the sun evenly across the whole panel rather than by managing the heat passing through it, so a room can still warm up while the actual view of sky disappears into a flat white glow. Solar control coated glass takes a different route, cutting the heat that gets through while still showing a recognisable, if slightly toned, view of the sky and cloud above. Solar control glass for skylights explains how those coatings work, and rooflights and overheating: Part O covers what the current rules ask of a new extension or loft room where summer heat is assessed alongside winter loss.

Trapped air keeps the inner face of a multiwall panel a little closer to room temperature than a single sheet of glass would manage, which can mean less surface condensation on the inside face in cold weather. That advantage sits alongside the flute problem already described, and it does not apply to the edge of a glass unit in the same way, where a warm edge spacer rather than trapped air does the equivalent job of keeping the edge of the pane from turning cold. Skylight condensation: causes and cures covers where condensation forms on any rooflight and what stops it, whichever glazing material sits in the frame.

Part L of the Building Regulations sets a limiting U-value for a new or replacement rooflight, and it does not care whether that figure is reached with polycarbonate or glass, only that it is met. What Part L of the Building Regulations asks of a rooflight sets out the figure that currently applies and how it is checked on a real job. We install to current Building Regulations on every job and handle the Building Control notification where the work is notifiable, so that side of a polycarbonate-to-glass upgrade does not fall to the homeowner to sort out. Our energy efficient rooflights page sets out the full range of glazing this reasoning draws on, for anyone weighing a polycarbonate panel against a sealed glass unit on a specific roof.

Can multiwall polycarbonate ever beat a sealed glass unit on U-value?

Rarely for a like-for-like opening. A very thick multiwall panel can approach the figure of a basic double-glazed unit, but a low-E coated, gas-filled sealed unit with a warm edge spacer usually still reaches a lower U-value than any polycarbonate build.

Does polycarbonate glazing yellow or dull over time?

Clear-grade polycarbonate is more prone to UV yellowing and surface scratching over years outdoors than toughened glass, which affects how much light and how clear a view passes through it as it ages.

Is opal polycarbonate warmer than clear polycarbonate?

The two grades of the same thickness insulate similarly, since the trapped air inside the flutes does the work either way. What differs is the view: opal diffuses the sky into an even glow, while clear polycarbonate shows more of an actual image of it.

Can an old polycarbonate rooflight be replaced with a glass unit in the same opening?

Often, yes, where the existing kerb or frame opening suits a standard sealed unit size, though the frame itself usually needs replacing alongside the glazing to get a proper thermal break and a sound seal at the edge.