Flat roof insulation above window?

[Hugh1983]

Untitled

• Hugh1983
• 4 Sep 2011
• 1

The picture shows the inside of a flat roof which has been fibreglassed on the outside.

I removed the old plasterboard as part of gutting the whole of the top floor to re-do it from scratch. There was lathe and plaster below the plasterboard but no insulation between the rafters.

Should I now insulate this area in between the rafters or just re-platerboard it. If I insulate it with loft roll or sheet insulation I won't have any room to leave a ventilation gap between the insulation and the top of the roof. If there was a gap it would be 10-20mm max due to the restrictions I have with the current rafters. I can't extend the rafter depth due to the level of the ceiling and the window.

 

[^woody^]

Completely fill between the joists with quilt insulation leaving no air gaps

Then if you can't fit even a minimum 10mm celotex across the whole lot, or similar polystyrene, to prevent any pattern staining, then just plasterboard with some foil backed board. Or polythene before normal plasterboard

You don't need to ventilate

 

[Hugh1983]

Completely fill between the joists with quilt insulation leaving no air gaps

Then if you can't fit even a minimum 10mm celotex across the whole lot, or similar polystyrene, to prevent any pattern staining, then just plasterboard with some foil backed board. Or polythene before normal plasterboard

You don't need to ventilate

Thanks for your comment. I'm thinking of removing the plasterboard in the wall sections because I've made a hole in it to see whats behind and I can see the old lathe and plaster wood strips behind the plasterboard with loft roll behind it. Do you think I should just fill in the hole I've made and leave this section alone or remove it all and insulate how you have suggested that I insulate the top part of this section?

 

[^woody^]

Yes, I would do the same while you are at it

 

[Hugh1983]

Yes, I would do the same while you are at it

Ok thanks, one more thing.. would you advise me to staple a vapour barrier between the rafters and the plasterboard or simply fit plasterboard to the rafters covering the celotex without the vapour barrier?

 

[^woody^]

Any foil backed insulation will perform as a vapour check if all the joints are taped and the timber taped across too.

Otherwise use some thin polythene