Insulation board size

[bennettp23]

Hi,

I’ve had a little search but would still like some advice please.

Got a new extension with a pitched roof and breathable membrane. About to order the insulation board for between the rafters but want to make sure there is enough air ventilation.

They are 7 by 2 inch rafters. Was looking at 100mm board but was considering thicker but wondering what the limit is for circulation- is it 50mm above the board so the rafters have the void?

It’s then being overboarded with 25mm insulation board.

Thanks for any help!

Paul.

 

[catlad]

You could easy go for 120mm

 

[foxhole]

Used 100mm plus 50mm insulated plasterboard on mine.