Posted By: TullichI've read this in various places.

In timber frame wall construction 80% of the insulation should be outside of any airtightness layer, and 20% inside.

I think you're misquoting. I don't think the position of the airtightness layer matters to any great extent.

What does matter is where the dewpoint is within the wall. The dewpoint is where condensation can start to form. So you want anything that can rot (i.e. organic materials like timber, wool etc) inboard of the dewpoint so condensation doesn't form on them and start decay. You also want the vapour control layer (VCL) inboard of the dewpoint if you have a VCL, otherwise it will have condensation forming on it.

So I've seen things saying that you want 50% of the insulation outboard of all the structural materials like timber, and I've seen other things saying 2/3 and so forth, which are all just rules of thumb. The only ways to have real confidence are to build something the same as what has been built before many times successfully or to have a proper dynamic condensation analysis done. The subject is fairly complicated and squirly if you dive down the rabbit hole.

Or you can be a lunatic like me and have organic material directly behind a permeable rainscreen with no VCL. (straw bales with lime render). Time will tell.

The rules are followed if it's necessary to get a warranty or a mortgage or if you're the kind of person that follows rules.

In countries that are very cold in winter they are very fussy about vcl being intact and on the warm side.

I always advocate vcl like that and have been doing them on ceilings since thirty years ago.

With lime things are different as it attracts any moisture in the wall and disposes of it to outside.

I don't think it is mad to omit a vcl in straw bale

If the vcl is too far out in traditional construction (and the dew point moves about a bit with temperature gradients when cold outside) you are at risk of mould, smells, fungus and silverfish, why take a risk when it is near impossible to fix.

Having all of the insulation outside the VCL works just fine (eg 100:0).

I think the 80:20 rule is more important for people tempted to add internal wall insulation to an existing house or put a lot of insulation inboard of the VCL. If you do that then the temperature of the VCL drops and there is an increased risk of condensation occurring on the inside of the VCL.

The thinking is that you can probably add upto 20% inboard of the VCL without the temperature at the VCL dropping too much. If in doubt perhaps get one of the insulation companies to do an interstitial condensation risk analysis.

Posted By: numeniusThis has done my head in - So my plan was/is real stone outer, 55mm gap, breathable waterproof fabric stuff they use on SIPs , 142mm SIPs panel, extra 50-75mm Kingspan type insulation inside, then plasterboard. Still undecided whether direct fix the plasterboard or on battens though. I was speaking to some guys from one of the big SIPs firms about getting the thicker SIPs or the possibilities of adding insulation inside instead - they said the latter option was far cheaper than getting the extra thick SIPs. This would place the "inner" OSB side of the SIPs panel in effect sandwiched between insulation, which I presume would mean it should reach the same temp as the surrounding insulation so no condensation issues, and VCL then to be either vapourcheck plasterboard, or a VCL on the inner surface of the additional insulation? Does that make sense?

If the SIP supplier is happy for you to add extra insulation inside then I would ask them where they recommend placing the VCL, since you'll presumably be relying on their warranty. I'm not criticising their proposal in any way; I'm just saying that in the circumstances it might be best to follow their suggestions all the way.