Garden brewery/workshop shed - best insulation option?

[norstar]

Hi all. I've been reading and re-reading articles on shed insulation and am now totally confused.

I'm building a workshop/brewery shed down the end of the garden - 3.5x4.5m. Plan is to build a 2x4 pressure treated timber frame on three courses of bricks to get the frame and cladding off the ground, hold off T&G shiplap on 25mm battens and panel the inside of the shed with Ply. EPDM roof on ply sheets.

It's the insulation that's giving me aggro. Brewing throws out a lot of moisture - just ask the missus about the time I stripped the wallpaper from the kitchen

Would a good compromise of budget and effectiveness be (from inside outwards) 12mm plasterboard/ply, then a vapour barrier, then 30mm air gap, then 70mm of rockwool, then tyvek wrap, then 25mm batten air gap then shiplap?

Or if I used celotex can I dispense with the vapour barrier? Is Tyvek necessary?

Grateful for any opinions. A bit confused.

 

[r896neo]

Thats a recipe for problems, You absolutely want to avoid air gaps between the wall makeup layers. This is where you risk condensation forming. You want to full fill the wall space with insulation and then use counter battens outside the tyvek and form the air gap here between tyvek and cladding.

If there us some reason you really want to use 75mm insulation rather than 100mm wool you need to keep it pressed tight to the inside face and then the place for a 25mm air gap would be at the outer edge of the studs. if you had an airgap here it would negate the need to counter batten the tyvek.

I would full fill with rockwool and use a vapour barrier on the inside behind your ply/pb or the pukka job would be 100mm wool in the studs and 25mm celotex across the inside face of the studs with the joints taped.

So inside to out

Pb/ply - vapour check - 100mm wool - tyvek - counter battens - cladding

Or pb/ply - vapour check - 75mm insulation - 25mm air gap - cladding direct to studs.

The tyvek is necessary but any cheap roofing breather membrane will do rather than branded tyvek

 

[norstar]

Thanks for that. Just to clarify with the Celotex option, the celotex is on the inside of the shed fixed to the studs, then ply fixed through that onto the studs, so no vapour barrier required as it's part of the celo sheets?

So order is ply/25mm celotex/rockwool/membrane/25mm batten gap/cladding?

Presumably the moisture doesn't accumulate on the barrier under the ply then and cause issues?

What about at the top in the eaves? With a vapour barrier, would it be sensible to leave an air gap between the top of the wall and the rafter insulation?

 

[r896neo]

Yes that's it, celotex across studs and then ply fixed through it into studs.

Your order is right for the wall make up.

Same rule applies to ceiling, dont have unventilated voids within the structure.

You can do the celotex over joists here too and that would help stop any thermal bridging. Insulation and vapour barrier should be continuous.