Insulation above bay window

[Magrah]

Hi All

Quick question, I have a bay window in a second floor bedroom with a lower ceiling to the main one. The bay is about 2.0m x 0.6m, the plaster had gone in that section of the bedroom so I took the old plaster down. I will put a vapour barrier on, board it and then plaster. The void is built of 3" x 2"s and is not insulated (it is a flat roof covered in led).

I am thinking of insulating it but there is no airflow, it's a sealed void, I know that normally you need airflow above the insulation but will this matter in such a small space?

Thinking of using 50mm of Celotex, mineral wool or polystyrene.

Any comments or advice would be appreciated.

Thanks

 

[oldbutnotdead]

I doubt very much you've got any ventilation in that void now. Your vapour barrier is a good plan (it'll stop warm humid air getting in from the room), if you've got the headroom (300mm) then pretend it's like any other roof and put 250mm of rockwool in. If you don't have the headroom then stick 100mm of rockwool in and put your 50mm celotex between the battens holding the plasterboard up.

 

[Magrah]

Many thanks, I don't have any headroom due to the double glazing going right up to about an 1" of the ceiling once boarded. So the only option for insulation is between the 3 x 2" battens.

 

[^woody^]

Fill it completely with celotex the depth of the joists, tight against the deck, ensure no gaps at all and then either foil tape all joins and over joists or fit your membrane/polythene or use foil backed plasterboard, and it will be ok.