Vaulted Ceiling insulation question ?

[hammerswinger]

Hi,
to insulate the vaulted ceiling in my extension I have to have 100mm kingspan type insulation in between the rafters, then 35mm over the rafters.

I have come across some 60mm foil faced insulation, is it ok to use 2 pieces of this between the rafters in place of the 100mm single piece?
I will still be fitting 35mm or maybe thicker over the rafters.

The rafters are 150mm (147mm) and the roof has a breathable membrane (felt).


Cheers.

 

[freddiemercurystwin]

Felt is not a breathable membrane! So is it a breathable membrane or is it traditional non breathable felt?

If a breathable membrane then only a 25mm gap is required. If non breathable felt then a min 50mm ventilated gap is required.

 

[hammerswinger]

Hi,
Yes it is breathable membrane, I put felt as I thought it was still referred to as this.
So 25mm is all the air gap I require, that's worked out fine then.

With regards to putting 2 pieces of the insulation in the rafter space, the boards don't seem to sit 100% together when laid on top of each other and I was wondering if sticking the 2 pieces together somehow may be an idea or will the slight gap between them be ok?

Cheers.

 

[freddiemercurystwin]

Pop some nails into the side of the existing rafters 120mm set back from the face, once you shove your 120mm insulation in between and fix your 35mm under and board it should all make a nice tight sandwich,

 

[hammerswinger]

Yeah I was thinking of using something to stop the insulation going in too far, I'll use some nails like you say.

Cheers.