Glass or polycarbonate: which rooflight keeps the warmth in?

Heat leaves a room through a rooflight in three ways: through the material, through the cavity, and around the edge. Glass units give you far more control over all three.

A polycarbonate dome is usually a single moulded skin, or two skins clipped together with a pocket of trapped air between them. Multiwall polycarbonate sheet has a honeycomb of thin ribs inside. Both hold some still air, and that is the only insulation they have. There is no coating, no heavy gas and no purpose-made edge. The ribs and the thin walls also carry heat from one face to the other, a bit like a row of tiny bridges.

A sealed glass unit is built differently. Two or three panes sit a measured distance apart. The inner face of one pane carries a microscopically thin low-e coating that reflects heat back into the room. The cavity is filled with argon, and a warm edge spacer holds the panes apart without making a cold ring around the perimeter. Each of those features has its own page in this section, and together they do what a plastic skin cannot. Our guide to how heat escapes through a rooflight follows the three routes in more detail.

Thickness matters too, though not in the way people expect. A thick slab of plastic is not a good insulator. What insulates is the layered build of glass, coating, gas and spacer, and plastic sheet has no equivalent.

The table sets out what each material brings to a room you actually use. It is a general comparison of construction types, not a product rating.

The weight row is the real argument for plastic. A glass unit needs a sound upstand and sometimes a stronger frame. On a roof that can take it, that is a design task and not a reason to accept a colder room.

Energy efficiency is partly about how a rooflight performs on day one, and partly about what it looks like and does years later.

Glass does not yellow. Its surface is hard, so cleaning does not scuff it, and a coating sealed inside the unit is protected from anything that touches the outside. Polycarbonate is a soft plastic. Ultraviolet light slowly breaks down its surface, unless a protective layer has been applied, and grit from cleaning leaves fine scratches. The skin turns hazy and then dull. A hazy skin lets in less daylight, so the room needs its lights on earlier, and that is an efficiency loss of its own.

Plastic also expands and contracts more with temperature than glass does. Over many summers and winters, that movement works on the fixings and the sealant at the edge, and gaps open for draughts. Sealed glass units sit in gaskets that are designed to accommodate their own movement.

A cloudy dome with condensation trapped between two skins is also no longer doing the job it was sold to do. Where a sealed glass unit shows the same sign, the answer is to replace it, and our page on misted and failed rooflight replacement explains how that goes.

A plastic skin is light and thin, so rain strikes it like a drum skin. Hail is worse. Anyone who has sat under a polycarbonate conservatory roof in a storm knows the sound.

Glass lets you fix that by specification. A laminated acoustic inner pane damps vibration, a thicker outer pane moves less, and the cavity is sized for both sound and warmth. The same build that holds heat is the one that calms the room, which is the idea behind one glazing build for warmth and quiet. Around Crawley, where aircraft and road noise already reach the upper floors of many homes, the glass choice is worth making with both in mind. The written quiet spec that comes with every quote names the panes and the interlayer for your room.

Polycarbonate suits a place where warmth and looks do not matter, and where weight is the main constraint. Think of a cold store or a covered bin area.

For any room you heat, the trade is poor. A bedroom in a loft conversion, a kitchen extension or a home office all lose more warmth through a plastic skin, and they feel it on winter mornings as a cold patch overhead. That is why an upgrade from plastic to sealed glass is one of the more noticeable changes a rooflight room can have. Our page on upgrading an old rooflight explains what a replacement involves, and energy efficient rooflights in Crawley is the hub for the whole range.

If you are comparing the two for a flat roof, the flat roof rooflight installation page shows the glass options that fit the usual kerb sizes.

Is multiwall polycarbonate a proper insulator?

It holds still air between its ribs, so it does better than a single skin. It still has no low-e coating, no gas fill and no warm edge, so a sealed glass unit with all three keeps more warmth in.

Does a glass rooflight overheat more than polycarbonate?

Glass can be specified with a solar control coating to manage summer sun, which plastic rarely offers. Our page on solar control glass for rooflights covers the choices.

Can glass replace a polycarbonate dome in the same opening?

Often the opening can stay, but the kerb or upstand usually needs checking for the extra weight and for insulation. We look at this at the survey and record it in the quote.

Will glass cost more than plastic?

The unit itself costs more. Over its life it stays clear and holds heat, and the price depends on size and specification. Our rooflight and skylight costs page gives general market guidance.

To compare a plastic unit with a glass replacement for your own roof, get a quote and we will write the glazing build into your quiet spec.