Dometic S4

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.

Polyurethane is a versatile plastic (foam) used everywhere from homes and cars to sports kit, and it can be made rigid or flexible, which makes it well suited to vehicle window frames. Dometic says polyurethane-framed windows are durable and environmentally friendly, and insulate well, preventing cold and heat bridges into the vehicle - which saves energy and cuts the CO2 from heating and air conditioning.

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.

S4 windows - top-hung, fixed and sliding versions - are made for motorhomes and caravans and meet their construction standards; they open to adjustable or fixed positions, insulate well, and come with a fly screen and a blackout blind for privacy. The S7P is a top-hung or fixed wall window with a flat outer pane for vehicles with curved sides, and suits panel-van conversions.

Its slim aluminium frame sits flat against the vehicle's wall, making it lighter than a polyurethane-framed window with the same opening size, and its frame corners have a different radius from the PU window's. A version with a 5000 radius is also available.

The S7 and S7P fit walls 22-44 mm thick, and the S7Z 28-42 mm, adjusted to the wall using the matching clamps. Before fitting, measure your wall's thickness and check whether the cut-out (the gap between the inner and outer wall skins) needs reinforcing; if in doubt, ask the vehicle's manufacturer.

Dometic's website has an online product configurator for each (S4 and S7P) showing the available sizes.

Check the information printed in the window's top right-hand corner 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. Windows made after February 2016 also have a label on the inside of the window hinge - in the same place on every window, and only visible with the window open.

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.