I appreciate there is lots on "breathability/permeability" on this forum and I have tried to read as much as I can, but I am needing to make decisions quite quickly and don't want to make a giant balls up. My extension build up is now most likely going to be single skin block work with EWI. Thicknesses and exact products still in discussion phase, but I am thinking of using Graphite EPS as the EWI product. On top of this will be an acrylic render. The renovations I've carried out to the existing house have been done with a view to allow breathability through the walls, should I be trying to do the same with this extension build up? Lets say the build up is

Render - 150 EPS - 215 Block - Plaster (poss plaster board on battens).

Will this be OK or do I need add a vapour control layer?

Many thanks again

Andy

Completely classic construction - no vapour barrier needed - indeed will be less robust with one.

With 150 EPS outboard of it, the whole blockwork will be maintained at nearer to indoor than outdoor temp, well above averaged dewpoint.

However, any transient condensation in it should be free to dry out inward as well as outward - hence no inboard VCL which would obstruct that.

Put another way, an inboard VCL, unless superhumanly free of pinholes and hairlines forever, would act as a one-way pump sucking internal vapour into the wall under some transient conditions but not allowing it back out again inward.

Thanks everyone for speedy help!

@Fin199 - you've noted PIR externally. FT uses EPS. None the less, you will need a VCL to pass the building Warrant application unless you can demonstrate that it is not required by means of an approved interstitial condensation analysis.

Just be careful interptreting some of the things described on this forum, which may not be based on the Scottish Warrant approval system, which can in parts be more onerous than the rest of the UK.

In terms of written info in a formal manner, that would be Scottish Building Regs, specifically 3.10.5.

You'll want an air tight barrier anyway, which the VCL will provide if installed correctly. If it's a new build you'll need to pass airtightness testing.

If you're looking for an opportunity to save something on costs, question the need for a wall Uvalue of 0.09W/m2K (that's what I've calc'd it at). Building regs require 0.17 (existing building) or 0.15 (new build SAP10 TDER value). Not to say you shouldn't improve on minimum regs, but the lifetime carbon footprint of insulating to that level may be increased, in light of a green electricity system - others are more expert on that than I (Fostertom, WillinAberdeen).

May be you've loads of glazing, and so need to compensate to pass SAP10??

PIR on the outside acting as vapour barrier is just the problem. Any vapour barrier, if you must have one, must be internal or not more than say half way out through the sandwich. With PIR externaly, then yes you'll need a strong internal vapour barrier, in the (imperfect) hope the PIR won't see any vapour originating from inside. Fundamentally rubbish (but common) way of going about it. With adequately vapour permeable EPS instead - great, less expensive, less embodied-CO2e-heavy, and you can afford to give away a bit of your 0.09.

Posted By: GreenPaddy... the Scottish Warrant approval system, which can in parts be more onerous than the rest of the UK ... you will need a VCL to pass the building Warrant application unless you can demonstrate that it is not required by means of an approved interstitial condensation analysis

Yes, so the Warrant application should be satisfied with an approved condensation analysis, just like in England. I don't think you'll find a ready-made Certification for a VCL-less construction, so will have to be a fresh WUFI study - the flawed Glaser method won't give you what you need, and is expected to be dropped as 'approved' next time it's all revised.

Isn't there any ply/OSB3 sheathing, inboard or outboard of your stud kit-frame? Esp if outboard, and sealed at all its joints, that can be your airtight barrier.

Welcome @Fin199!

I share FT's concern about the PIR and its position - effectively this means you have a VCL at the wrong point in the 'sandwich'. I would use graphite EPS which, as FT suggests, I think, is alleged to have a degree of breathability.

I am sceptical (always) about VCLs, even when installed with obsessive attention to detail - I have never claimed 100% 'tightness' for my VCLs, but I feel it is important to have them in the (alleged) 'right place' in the sandwich - conventional wisdom says immediately behind the plasterboard, while some say a tiny bit into the sandwich (say behind a final 20mm in a fatter sandwich of insulation) is OK.

My concern for your lay-up is that even if we persuade you to have a VCL in the 'conventional place' there is a risk that, if that VCL is less than 100% tight, your 'secondary VCL' of PIR foil, will trap moisture in the sandwich.

If that is as clear as mud I apologise!

**Note*: I typed all that before your last post (above) popped in. I take your point about %age contributions to the insulation, and re the +ve effects of MVHR, but effectively this seems to be expecting MVHR to make good a reasonably significant ?deficiency/ ?unusual-ness? in the VCL position.

I am with the Bldg Warrant person, I think, in preferring, in your particular situation, a VCL in the 'normal' position, and my own 'taste' would be for Pro Clima Intello or similar. 'Tain't cheap, though, especially at this late stage when you had not expected to need anything there. Out of interest who drew and specified your lay-up?

And note I am not in Scotland nor au fait with Scottish Regs - sorry!

<blockquote>6x2 (filled with Mineral wool), 150mm Continuous PIr,</blockquote>

So in terms of clarification, where is the VCL on that PIR? I realise there's foil on both sides, but unless I am misinterpreting this, you could not tape it on the 'warm' side, as that's against the frame, so I am assuming that it's taped on the cold side, which if I am right gives you a 'VCL' (or vapour 'check') even further from where it 'should be'.

The building world is wonderful, and we probably all have examples of lay-ups which should not have worked, and did, and vice versa. On the face of it, though (if, as I surmise, your PIR is taped on the outer face) you could have a 'vapour stop' much too far out in the sandwich. For this reason, if it were me, I'd use a VCL in the conventional position to 'cover myself'. I'll post now, but I'm going to search for a Grand Designs episode where a cowshed conversion (??) used PIR externally. If it could be tracked down you could maybe try to contact them for feedback? 95% certain it is not in Scotland, though.

Posted By: Fin199I feel this is the way forward for small timber framed buildings

Agree, but using vapour-permeable EPS or wood fibre outboard, rather than impermable PIR (with or without foil facing) or in fact any of the 'aero bar' foamed plastic insulations.
Then, if so using EPS or wood fibre (subject to WUFI study to your satisfaction, and in southern England at least), WUFI tells me that no VCL at all is needed, in fact works very much better without, interstitial condensation-wise, than with any kind of VCL, 'dumb' or 'intelligent'. So no inboard battened service void needed either - your internal plasterboard can be punctured at will by electrician etc.
The ply or 11 OSB3 sheathing on the outside of the studwork can be your airtight layer, and I prefer to achieve that by gapfilling glueing and screwing/power nailing incl to all unsupported board edges, rather than taping, which I try to avoid.

Nick is right

Posted By: Nick Parsons... a VCL in the 'conventional place' there is a risk that, if that VCL is less than 100% tight, your 'secondary VCL' of PIR foil, will trap moisture in the sandwich

(or even PIR without foil). In fact will function as a one-way pump, steadily accumulating moisture in the sandwich - slow & steady in, no effective way out. Having said that, countless buildings are fitted in this dangerous way, but seem to get away with it!

It's a myth that most of the water vapour originates inside - it's a constant in/out traffic, certainly seasonal and also diurnal, mainly of the inexhaustible outdoor water vapour, driven by vapour pressures that push vapour independently through even still-air conditions, and which though weak are 24/7 remorseless. So it's futile to try to seal 100% perfectly against that vapour movement. Best to let it flow freely, to model what then happens, and to arrange for the inevitable transient interstitial condensation, outboard in the sandwich, to a) occur where it can do no harm, an b) can freely re-dry at the first opportunity, unobstructed.

Yes, exactly. A one way pump, steadily accumulating moisture. What is the way around this that also satisfies building control?

I have never used them, but Greengauge (https://ggbec.co.uk/wufi-part-3-options/ do WUFI and are not a materials supplier AFAIK, so will cost, but won't insist on you using a given product.

Posted By: Fin199It's based on 'the perfect wall' concept by Joseph Lstiburek.

I usually have a lot of respect for Lstiburek, but I think this is just wrong. Maybe intended for parts of the USA that are warmer than here. It's poorly produced with no explicit indication of where the insulation is and much too little. Plus poor choice of insulation for cold climates. In a cold climate, any VCL needs to be on the INSIDE!