Glass or acrylic sun tunnel dome: which suits your roof

The dome is the only part of a sun tunnel that meets the weather. Everything below it depends on how much daylight it lets through and how long it keeps doing so.

It sits on the outside of the roof, usually on a flashed base, and catches light from a wide arc of sky. A good dome is clear, so the tube beneath gets as much as possible. It is also tough enough to take hail, frost, a summer of strong sun and the odd branch. On a pitched roof it has to be small and light enough for the flashing to carry without strain.

That last point is why material choice is narrower than people expect. Most tunnel kits are designed around a moulded plastic dome. The mounting ring, the seal and the flashing are all sized for it. Swapping in a different material is not a free choice made at the last minute. It is a choice made when the kit is chosen, and it is worth settling before the order goes in. If you are still deciding whether a tunnel is the right product at all, our comparison of a sun tunnel and a rooflight for a dark room is the place to start.

Domes come clear or lightly frosted. A clear dome lets in the most light and gives the brightest spot of sun on a bright day. A frosted or prismatic one spreads the light before it enters the tube, which can soften harsh midday patches and is sometimes chosen for south-facing roofs.

The two materials behave differently in four ways that matter at home: clarity, weight, ageing and the sound of rain.

Acrylic is a clear plastic that transmits light very well. Good domes are made with a UV inhibitor, because sunlight is what ages the material. Over a long time an unprotected acrylic surface can haze, yellow slightly or show fine surface cracks, which is often called crazing. A dome with a proper UV-stable formulation holds its clarity far longer, but it is still a plastic living on a roof.

Glass does not yellow and does not craze. Its clarity on the day it is fitted is its clarity many years later. The price is weight and, in some designs, a different shape: a glass cover is often a flat or gently curved pane rather than a full dome, and a flat pane does not shed water or debris in quite the same way. For that reason glass tends to appear on flat-roof versions, where the tunnel sits on an upstand anyway. Our page on sun tunnels on flat roofs covers how those are set up.

In practice the brighter room is the one with the cleaner, clearer cover, whichever material that is. A dome that has gone dull will cost you more light than any difference between the two materials on the day they are fitted.

Sound is where the choice shows up most in a lived-in room, because a tunnel brings the roof surface very close to the ceiling.

A dome is a thin shell. Rain strikes it, and the shell can vibrate, passing a pattering note down the tube. The tube itself, especially a polished rigid one, carries that sound like a speaker horn. Much of what you hear depends on the tube and the ceiling fitting, but the top surface starts the noise.

Glass is denser, so it moves less when rain lands, and it tends to give a duller, softer sound. Acrylic is lighter and can sound brighter in a heavy shower. Neither is silent. A double dome, with a second cover inside the first and a pocket of air between, changes the picture again, because two layers that differ from one another do not ring together. We go through the quiet build in detail in our page on keeping a sun tunnel quiet in the rain.

Aircraft are a different matter. Low, steady engine noise passes through any thin cover and down a hollow tube. If that is your concern, read whether you can hear aircraft through a sun tunnel before you choose.

Start with the roof and the room. The material follows from them.

On a pitched tiled or slate roof, an acrylic dome on a flashed base is the standard and sensible fit. The kit is designed for it and the flashing sits properly. Our page on fitting a sun tunnel through a tiled or slate roof shows how the base is integrated with the tiles.

On a flat roof, you have a real choice. A glass top may be offered, and it makes sense if you want the clearest possible cover and the extra weight is carried by an upstand. Whichever you pick, ask what the cover is made of and whether it is UV-stable, because makers describe their products differently.

A few questions settle it for most owners:

• Is the roof pitched or flat?
• Is the room below a bedroom, bathroom, hall or office?
• Is the tunnel short and straight, or long with bends?
• Do you want the brightest spot of sun, or a softened glow?

A long tunnel with bends loses light on the way, so a clear dome matters more there. A short straight run can afford a frosted one. Our guide to tunnel length, bends and how much light arrives explains the trade.

Whatever we propose goes into the written quiet spec that comes with every quote, along with the reason for it. Work on the roof is installed to current Building Regulations, with a 10-year workmanship guarantee.

Does a yellowed dome mean the tunnel is finished?

Not necessarily. A dull dome is a cover problem rather than a tube problem. Often the dome and its base can be swapped while the tube stays. See whether an old sun tunnel can be replaced with a newer one.

Is glass safer if something lands on it?

Not automatically. Plastic domes flex and take impacts well, and glass is heavier if it breaks. Anything glazed overhead should be safe if it breaks, and a glass cover needs to be specified for that.

Does the dome change how hot the room gets?

A little. A clear cover lets through the most sun, heat included. Our page on whether sun tunnels make a room hot in summer covers the detail.

Can I clean the dome myself?

The outside is usually reached from the roof, which is a job for someone with the right access. How to clean a sun tunnel sets out what is sensible to do from the ground.

The whole range is on our sun tunnel installation page. If you would like the dome chosen for your roof and your room, get a quote.