Dometic Heki 1

PMMA (acrylic). It's very clear and easy to form and shape with heat. Dometic says it looks like glass at half the weight and is around 30 times stronger, insulates well against heat and cold, and stands up to weather from -40°C to +80°C.

Motorhomes and campervans, caravans and panel-van conversions. They come in many versions and Dometic describes them as dependable, simple to fit and long-lasting.

It varies by model - Dometic lists every rooflight's dimensions and specifications on its website. Before fitting one, measure your roof's thickness and check whether the cut-out (the gap between the inner and outer roof skins) needs reinforcing; if in doubt, ask the vehicle's manufacturer.

Look for the information printed on the pane before ordering. The PNC/SKU number and code identify when it was made and which spare parts it takes, and the letters and numbers along the bottom identify the glazing. Rooflights made after February 2016 also carry a label on the inside (on the Midi Heki, for example) - though there's no label on a Mini Heki.

Between about 2.9 kg and 22 kg, depending on the model.

400 x 400 mm for the Mini Heki S, Mini Heki Plus and AirQuad; 700 x 500 mm for the Midi Heki; and 960 x 655 mm for the Heki 4 Plus, Heki 3 Plus, Heki 2 and Heki 1.

Acrylic is an organic material that absorbs tiny amounts of water, so moisture can work its way through the acrylic, making the air wetter in the gap between the two layers of double glazing. When it gets colder outside, the air in that gap cools and the excess moisture condenses on the coldest surface - the inside face of the outer pane.

It isn't a fault. To clear it, ventilate the vehicle to bring the humidity down (good air circulation helps), or raise the temperature inside, since warmer air holds more moisture - then give it time to disappear.

Usually dust or grit blown against it, whether you're driving or parked. Brushing against tree branches on the road, or washing it with an abrasive sponge, can scratch it too.

Cracks in acrylic are usually caused by 'stress corrosion' - chemical attack on the material while it's also under tension. Over the years, changes in temperature and repeated wetting and drying create slight stresses in the surface; on their own these are harmless, provided the pane is cleaned and cared for properly. But cleaning products that aren't suitable for acrylic attack the stressed surface and can eventually destroy the pane.

So never use glass cleaner, general-purpose cleaner, abrasives, washing-up liquid or anything containing alcohol.

Only a cleaner made for acrylic glass. Never use harsh products - glass cleaner, general-purpose cleaner, abrasives, washing-up liquid - or anything containing alcohol.

Your caravan or motorhome dealer can order them for you - Dometic's website has a dealer locator to find one.