Massive angled skylight - secondary glazing or replacing? - Skylights & Roof Windows

August 29, 20233 yr

I have a large (c. 5.8 x 1.6m) skylight, on a low pitched roof (I'd guess c. 15 degrees). This appears to be made of two sheets of 10 or 12mm polycarbonate. I suspect it was built in around 2006/2007. The polycarbonate has since turned somewhat cloudy – so a little translucent now, rather than clear (clear would be preferable).

The roof is an unusual and lightweight construction – see

Since it’s a single sheet, thermal performance is terrible – there a lot of heat loss, and a lot of condensation in winter months. So it either needs to be:

1) Replaced with a glass double glazed unit (quotes seem to be c. £12k-20k), and a structural engineer would be needed to confirm whether the loft can bear the weight of a glass unit, so quite apart from the huge cost, it might not even be viable. On the plus side, this would be professionally installed, stay clear, provide decent u values, and could be done in UV control glass, etc.

2) Secondary glazed with acyrlic. I suggest with a sheet of 12mm Perspex (clear cast acrylic), either on the inside or outside, with a between the two sheets. It might make sense to replace the existing polycarbonate at the same time with new, clear sheets. Vastly cheaper (perhaps £1k-2k, including labour and materials).

Thermally not quite as good as a proper double glazed unit with inert gas between the layers, but should make an appreciable different for a much lesser outlay.

I’ve had a few thoughts on how this might be done by building a frame on top, or a frame underneath, but ideas would be much appreciated; also on how best to re-seal the outside of the skylight to the roof (flashband would probably work … though it seems a bit temporary!).

I’m a bit concern that 2 might be a false economy in the long term. But since we may ultimately need to replace the Decra roof, I’m equally concerned that replacing this with a glass unit (1) should only really be done once the roof’s (re)done ….

Step 1 of course is getting a structural engineer in, but in the meantime, thoughts welcome – as would suggestions of inexpensive glazing companies for sky lights.