Air gap needed ?

[marsaday]

View media item 72547
I have insulated this small bay window roof.

It had zero insulation in there and i have managed to get in 50mm. So far i have pushed it right up to the floor boards which are acting as the main skin of the roof. I still have a gap of 25mm to fill and was planning on putting in more insulation.

However, do i need to leave an air gap instead behind the insulation and between the wooden roof ? If so i will drop the insulation down 25mm, but i would prefer to get more insulation in there if possible.

 

[catlad]

I would not leave any gaps and use insulated plasterboard below the joists.

 

[Blagard]

It's not very practical to vent the space - so as catlad says, fill it with insulation. When you do the ceiling use either a vapour barrier or one of the insulation backed boards that have a vapour control layer built in.

 

[tony1851]

As the 2 above; Building Regs actually accept that it may not be practicable to vent small areas of roof, such as over a bay or porch.

 

[marsaday]

Excellent, thanks. Yes will use additional plastic membrane on the underside of the insulation and then plasterboard.