Glass or polycarbonate for your roof lantern

A polycarbonate roof lantern is lighter and cheaper than a glass roof lantern, but glass insulates better, stays clear and gives a real view of the sky. For a living room, kitchen or extension you live in, glass is therefore usually the better choice. Polycarbonate, often simply called plastic, has its place on a carport, shed, garden canopy or another unheated space.

What is polycarbonate?

Polycarbonate is a transparent plastic that is almost unbreakable. For roofs it is mainly used as multiwall sheet: two or more thin layers with small channels between them. Those channels insulate, but they also make the sheet cloudy. You see light, not clouds.

Solid polycarbonate also exists. It is as clear as glass, but barely insulates, so it is mainly used on unheated canopies.

Insulation: glass versus polycarbonate

HR++ double glazing has a U-value of about 1.1 W/m²K, triple glazing about 0.5. Multiwall polycarbonate of 16 mm comes in at roughly 1.8 to 2.4 W/m²K, depending on the build-up. The lower the figure, the less heat escapes. In a heated room you notice that in your heating bill and in the temperature under the roof.

It matters in summer too. Glass with a solar control coating keeps heat out without getting dark. Polycarbonate is often supplied in opal or with a heat-reflecting layer, but that comes at the expense of light. What a U-value actually tells you is explained in the U-value of glass explained.

View, noise and lifespan

Beyond insulation there are differences you notice every day:

Polycarbonate also expands considerably in the heat, much more than glass or aluminium. It is therefore fitted with play in the profiles, which can tick or creak as temperatures change. Glass is dimensionally stable and stays clear for years with ordinary maintenance. How to do that is covered in maintaining and cleaning a roof lantern.

• View: glass is clear, multiwall polycarbonate is cloudy
• Noise: rain drums audibly on plastic, glass dampens it better
• Scratches: glass is hard, polycarbonate scratches when cleaned
• Ageing: polycarbonate can yellow and dull under UV

Plastic roof lantern: where it does fit

In a space you do not live in, plastic can be fine. It is light, so a lighter supporting structure will do, and it hardly ever breaks from hail or a falling branch. On a garage or carport or an open canopy, a clear view matters less and insulation plays no role.

If you are unsure about a roof lantern for a canopy or veranda, think about how you will use it. Will you sit under it in spring and autumn, or will it later be enclosed with glass walls? Then glass is a choice you will not have to make again later.

What does the difference cost?

Polycarbonate sheets are cheaper per square metre than insulating glass. But the price of a roof lantern is not just in the infill. You need a frame, upstand, junction and installation with either one. That makes the difference in total price smaller than the difference in sheet material.

A glass mono-pitch roof lantern from DagLichtDesign starts at €1,532, with HR++ double glazing and a thermally broken aluminium frame. How the price of a roof lantern is made up is covered in what does a roof lantern cost.

Frequently asked questions