I need to replace a large plywood window cill in a bay window 2.5m x 0.8m because it has delaminated due to damp. Looking up from underneath I can see the underside of the old cill – its just 10 mm ply! (Proved by a drilling a test hole) It’s fixed to 50 x 20 strips of softwoodfixed to substantial beams which protrude from the wall. I am told I could use any material I fancy for the upper surface, leave a gap for insulation then a lower surface for which best to use upvc cladding boards to keep out damp.

Two questions occur to me on this

(i) is upvc a good idea for the lower surface or should it be something that ‘breathes’?

(ii) what insulation should I use and should there be an air gap on top? I was thinking sheet polystyrene leaving a modest gap (2mm) between it and underside of top surface but then I was offered enough Thinsulex offcut to do the job. This stuff is made up of alternate layers of thin man-made felt-like fibre and very thin silvery plastic film and is said to offer 8 times the insulation of same thickness of polystyrene or rockwool. But it’s impermeable and manufacturer’s info relates to use in a roof where arrangements must be made for vetilation to prevent condensation.

While worrying about the cill there is also a flat roof to this window, the top surface is ok and has not leaked but the ceiling within the bay goes mouldy. There is no sign of any vents in the facia board and I have no idea if there is insulation present.

The window area tends to get very cold compared with the room. I am getting double glazing to replace the windows themselves.

Any advice re what best to do re cill and roof would be greatly appreciated.

Yoggo

Yoggo

We have a square bay window. From the top down it's something like this..

Oak venered MDF cill
Vapour barrier
50mm (80mm?) Celotex
Ventilated timber structure
T&G boarding (similar to that used for a shed).  

Fill any gaps between the sheet insulation with expamding foam.

The mold is probably due to a combination of no insulation and no ventilation. Consider adding trickle vents to the windows.