Advice on which side to leave air gap in garden office wall build

[Gjmiles]

Hi all,

New here and after some advice discussion on garden office wall insulation. I am building a garden office and the wall studs are 8x2 (overkill but they were a bargain). The build up from outside in is:

Timber cladding
Roof battens for cladding
Breather membrane/house wrap
18mm osb
My 8x2 studs with 100mm pir in the voids
Vapour barrier
Plaster board
Plaster.

There will be a 95mm air gap in between the insulation and either the outer OSB sheathing or the vapour barrier/plasterboard side.

I can’t find much information about which side to leave the air gap, and from what I have found people have done done one or the other.

So the questions are:

What side should I leave the gap
What are the pros and cons to both options

Thanks

G

 

[mrrusty]

You need someone with a band or table saw to rip those 8x2's down to 4x2's ! Or even a hand-held circular at a pinch.

 

[Gjmiles]

You need someone with a band or table saw to rip those 8x2's down to 4x2's ! Or even a hand-held circular at a pinch.

Hahaha, too late for that I’m afraid

 

[freddiemercurystwin]

It probably wouldn't cost much more to fill the whole cavity with 2 x 100mmm of EPS instead

 

[Gjmiles]

It probably wouldn't cost much more to fill the whole cavity with 2 x 100mmm of EPS instead

True, i already have loads of sheets of 100mm PIR though…..

 

[freddiemercurystwin]

Well in that case I would leave the air gap on the inside, useful for services too, I'd be inclined to fit a vapour barrier over the front of the studs, back into the cavity and then across the face of the insulation, or is your insulation foil backed? In which case you could tape it to the foil face.

 

[Gjmiles]

Well in that case I would leave the air gap on the inside, useful for services too, I'd be inclined to fit a vapour barrier over the front of the studs, back into the cavity and then across the face of the insulation, or is your insulation foil backed? In which case you could tape it to the foil face.

Great thank you. The insulation I have is foil backed on both sides. I was planning on also adding a vapour barrier over the front of the studs as you suggest, but I had not planned on wrapping it back into the cavity. Would that make a big difference? Thanks for the advice

 

[freddiemercurystwin]

Not gonna make a great deal of difference either way.