Welcome to the forum :)

A vapour barrier near the outside is generally considered a complete no-no, and some don't even like them in their usual place on the inside of a wall. What led you to such a decision? What does condensation analysis show you?

Personally I wouldn't use PIR anywhere, because of the pollutants it can produce in a fire, but each to their own.

Thank you for the feedback :)

I must admit, the PIR route stemmed purely from past experiences and the analytical view of U value performance. I think I came across a detail somewhere that illustrated this build up, did the basic maths and moved on. I assumed so long as the warm room air didn’t enter the wall there was no risk of condensation.

Having played around with a WUFI calculator it is fascinating how different materials have such different behaviours , how moisture moves through the wall structure and how you need to think of the complete construction not just the U values. Also the change in behaviour with vapour barrier and where they’re located.

In theory the current wall does hold up, but is very dependent on that interior vapour layer to allow no interior air through. I do much prefer the idea of a moisture permeable wall, that allows movement and has no risk due to lack of sealing or puncturing.

Funnily, the best performance for drying and moisture movement, comes from moving the OSB internally, using rockwool (or similar) between studs and then wooden batts directly to the outside of the studs and render. You do have to increase the thickness however to achieve the same results. I’d have to do a cost comparison also.

Thank you for the suggestions Tom, I’ve seen you suggest similar elsewhere, and the calculations do stack up for condensation. I’ve a preference for fibreglass over warmcell as I can DIY, and less mess. Presumably you glue and mechanically fix the EPS?

EPS does seem to be the way to go, I like the idea of wood batts, but they’d have to be thicker, and nervous of using it only 150mm from ground level.

Whoo, there's a thought

Only a few points from me in response to others. I started a response yesterday but it fell off the cliff...

Of all the mainstream insulants PIR gives close to the best 'headline' figure but it's only as good as the fit and the 'cloaking' of any thermal bridges. As FT highlights, anything with no thermal bridge mitigation is less than ideal, so anything that's 'all between studs' is, in my humble view, worth avoiding. For the OP's original construction, full (140mm) fill with Frametherm 32 (lambda 0.032W/mK) comes v close to the U value you'd achieve with PIR, and the fit will be better, because of the 'squidge'. *But you still want to cloak the bridge of the studs (see FT's suggestion).

''I like the idea of wood batts, but they’d have to be thicker, and nervous of using it only 150mm from ground level.''

I used a lot of WF, but XPS (or, for my preference but not necessarily the comfort of system providers, EPS) below ground and above ground to at least the height of 'local splash-up' (the industry suggests rain splashes up 150mm. I have a 300-ish 'tide mark' on my shed...). In my view (and contrary to 'conventional wisdom') a set-back (plinth layer not as deep as main EWI layer) is not essential.

Try that for size and I'll come back later if any 'supplementaries' req'd.

How high rain splashes depends on what the surrounding surface is like. We have a gravel topped French drain all round our house for that reason. The downpipes from the roof gutters discharge onto the gravel and we haven't seen any problems.

Posted By: fostertomwhat software is that screenshot? Doesn't use the obsolete, flawed Glaser method, does it?

It does - it's Ubakus, so not very helpful.

Posted By: Oj0001EPS does seem to be the way to go, I like the idea of wood batts, but they’d have to be thicker, and nervous of using it only 150mm from ground level.

A big advantage of wood (or hemp) batts is that they significantly reduce the decrement delay - effectively the time taken for heat to reach the inside from the outside; once heat reaches the inside then the building starts to heat up, and you want to delay that in a heatwave. Wood beats hemp for decrement delay, hemp beats wood for its moisture handling properties, both beat non-biological insulations (EPS, mineral wool, etc). You could use both though - EPS on the outer face, wood / hemp between studs.

For various reasons, look to achieve good levels of airtightness. OSB3 could form part of an airtightness layer if the joints are sealed, but not all OSB3 is airtight, so check the manufacturer's data for the brand concerned.

Posted By: fostertomIf the stud above is 95 wide, I don't see why not 100 block base wall - significant economy of construction, smaller footprint. But I've had BldgInsps and Engineers insist it must be min 190 thick (meaning 215 or 2x100 cavity wall) but they can't say why.

I'd guess that's due to Building Regs structural stability requirements (Part A, 2C6) Solid walls constructed of coursed brickwork or blockwork should be at least as thick as 1/16 of the storey height - although most of wall is not in brickwork or blockwork, the part that is would be subject to similar forces.

Posted By: fostertomOj, what software is that screenshot? Doesn't use the obsolete, flawed Glaser method, does it? You've previously used WUFI?

In your cross section drawing at top, have you met any resistance to base wall 'only' 140 thick? If the stud above is 95 wide, I don't see why not 100 block base wall - significant economy of construction, smaller footprint. But I've had BldgInsps and Engineers insist it must be min 190 thick (meaning 215 or 2x100 cavity wall) but they can't say why. Anyone?

It is Ubakus, apologies that's my ignorance, I hadn't fully appreciate the differences between the two methods. Although I don't fully understand why this method is considered less accurate from thermal and vapour gradients standpoint? I presume your actual WUFI build up of the above suggestion didn't show condensation risk?

Posted By: Mike1
For various reasons, look to achieve good levels of airtightness. OSB3 could form part of an airtightness layer if the joints are sealed, but not all OSB3 is airtight, so check the manufacturer's data for the brand concerned.

Is there a specific difference between air tightness (I presume to reduce thermal losses due to air passage) and Vapour diffusion through the wall? Since vapour barriers are designed to prevent air movement, but still allow vapour transport, why is there such an objection to them?