Dear Mike and Viking, I left this post for a while got fed up with the personal comments not aimed at me directly until the last few, without getting dragged back into it to much, my background is clear to see I am not a plasterer and my knowledge is not limited to EWI from the age of 15 I worked on sites as a carpenter did give you an idea if time it was when Yankee drivers came out, so a fair way back, the brackets us a side line and my life does not rely on this. I do not see myself as basis far from it other methods work but wood not sure on that, are experts further up the food chain wrong I think not. Let's try and recal this years form the line when wood fails, good luck all and lets play nice, agree to disagree for those wanting advice ask a system providers like SPS, Alumasc, Permarock etc etc

This cross posted from
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13524&page=4#Item_24

Posted By: willie.macleod
"So with wind driven rain, a modern thin coat polymer modified render could be allowing moisture through within a few hours of exposure. Eye opening. A lot comes down to the details of course and thickness & top coat."

Not only with driving rain; in breatheable-rendered (I hope no one would use non-breatheable) EWI, WUFI shows that at many times/conditions the outermost bit of the EPS or whatever will be at or below dew point of the water vapour that's in it (whether that vapour is currently driving inward or outward) and so it will be full of liquid water - just that outermost bit. But then it soon enough 'dries' out again.

Timber in that outermost zone might survive that alternating wet-dry treatment, but I wouldn't risk it. My guideline is to have at least 40% of the insulation thickness outboard of any timber, OSB etc.

Note that rendered EPS EWI is extremely reliable despite this frequent wetness of its outermost thickness - it's inherent, and none the worse for that.
Even in freeze conditions - it doesn't cause spalling, presumably because the water is held in thin capillaries between compressible granules of EPS - plenty of space for freeze-expansion, harmless.
And yes EPS loses 22% of its insulative value when completely water-logged - but a) in these conditions it's far from water-logged, barely more than surface-wetted; and b) the insulation is only degraded in its thin(ish) outermost bit - and WUFI takes that degradation into account.

Posted By: Mike George‘In reference to your question, we do not advise using any timber grounds within or hidden behind insulation in EWI schemes. The reason is there are perfectly good fixing solutions without the need to use timber.

Did he happen to explain what those solutions are? The case I'm thinking of is securing two layers of insulation without thermal bridges.

Presumably if timber grounds within insulation behind render are a no-no, then timber in the render itself is an even bigger no-no? In which case, how long would we expect a wattle and daub wall to last? A decade at best?

Found this a good site for fixings :
https://www.amaroc.co.uk/collections/post-installation-fixings

Every Condensation Risk Analysis carried out on an externally insulated wall shows the dew-point typically 5-10mm in from the external surface. I've pushed the probes of a wood moisture meter into external insulation a few times in the middle of winter when partial vapour pressure is at its highest and never recorded moisture at that point. I pushed the probe into the wall near a downpipe bracket where there was plywood grounds and it was bone dry.
While the CRA analysis shows there's a risk in 5mm, but there's probably never moisture present at that point. This is probably why the following bodies disregard water vapour diffusion and concentrate on airtightness: PassivHaus Institute, Fraunhofer Institute who developed WUFI and the Canadian House Building Association.

EWI system suppliers are always likely sell you extra products to compliment their system and part you with your money.

So... are we saying that no timber should be used in *any* type of EWI? Or just with particular insulants?

Where does that leave retrofit larsen truss or Beattie Passive ("TCosy")?

I'm a complete newbie. What does "grounds" mean? Anything that touches the ground, or anything that can be used as a fixing point? Or something else?

I can't see any particular problem with glueing a timber ground onto the face of the render and screwing to that, if its just a garden tap.

More of an issue to me is how to mount windows in the insulation layer if marine ply sleeves in the insulation are an issue.

That's because insulation is returned onto the frame, burying the cheeks, I assume.

<blockquote><cite>Posted By: converse</cite>I can't see any particular problem with glueing a timber ground onto the face of the render and screwing to that, if its just a garden tap.

More of an issue to me is how to mount windows in the insulation layer if marine ply sleeves in the insulation are an issue.</blockquote>

Glueing timber to the render?

Not me - I've made myself a rule to have no timber within the outboard 40% of a near-PH standard insulation sandwich - but that's subjective after looking at the animated 'films' of the ebb and flow of the 95% RH zone in several variants that I have modelled.

Standard 1D WUFI can't model 2D sections like a window jamb (unlike 2D Therm - which can't do 3D!), but I'd apply the same 40% rule and keep the 'cheek' at least 40% back inboard along the short curved heat-flow line around the end of a window frame.

Of course the window frame itself occupies the whole heat-flow line, from inboard to outboard, so its outer 40% lies in that danger zone. Don't know why but somehow that seems more OK because it's not just an isolated outboard piece surrounded by cold condensing insulation, but is homogeneous right back to the warm face. Comments?

Posted By: ringiI think windows are assumed to be replaceable if (when?) they rot. However we don't have a replacement method that works in air tight buildings.

The details in the Passivhaus Details book describe how to replace the windows in their details. So it's not an unsolved problem, just the usual issue of manufacturers and trades not keeping up with best practice. The usual strategy is to remove the window board and the plaster reveals on the inside and replace the window from in the room. That's also a useful strategy for high-rise buildings.

Paint at best resists liquid water ingress. If, as with many modern paints it also resists water vapour, then that's very bad for the window. The window survives by re-drying what it inevitably absorbs. A good 'paint' aka opaque strain, resists liquid but is highly vapour-breatheable. That being the case, paint or not will have no bearing on whether the 'outer 40%' of the timber is liable to interstitial condensation.

Most people don't know the difference between stress and strain!

Condensation isn't a concern in and of itself. There's a thin film of condensation on pretty much every surface in the world. The problem comes with a mixture of water, oxygen, some living organism and food. Paint keeps them apart.