I find myself agreeing with most of what Phil Heath says in his excellent article in the latest GBM. I suspect there are many who do not. Anyone like to present a counter argument in answer to the points made?

1. Yes he does, but then so do those purporting breathing wall 'benefits'. I think the Author is perfectly entitled to answer criticisms of his company's product.
2. Agree, but the important point for me is the circumstances under which moisture passing through fabric can cause damage. I have seen it suggested that some insulations exacerbate interstitial condensation problems, even that some cause it. This I think is wrong, as it is not any particular insulation which can cause problems, it is only when they are used or installed incorrectly, ie in a construction method not tested and approved by the manufacturers This point is well made in the article I think
3. Agree completely, but some products are promoted as having additional benefits which are, shall we say, unproven. I've not read the full Cambridge study yet but will be requesting a copy to see more detail of the conclusions.
4. Again, I agree that walls 'breathe' in the sense suggested, but would suggest that this is irrelevant, at least in new construction. It' the choice of components which make up the whole building element which is important.

Regarding internal water vapour production as the source of the intersitial condensation is a complete red herring. See the esteemed Mark Brinkley's blogs http://markbrinkley.blogspot.com/search/label/Ventilation especially 3.7.07 and 6.7.07. "there is about 100 times more bound water in the structure and furnishings in a house than there will ever be free floating water vapour in the air" "a detached house ... could be holding as much as 10 tonnes of bound water ... 10m3 in volume, about the size of a small bedroom" "All this bound-water doesn’t have to stay bound. When conditions dictate otherwise, it can either absorb more water or it can release water via evaporation. This does rather depend on the surfaces surrounding the materials".

It's the ins and outs of the water already in the structure that causes trouble - is it better to let it come and go unobstructed, or trap it in various cells and compartments, or worse still, force all the traffic to pass through the few permeable bits e.g. through the studs and rafters because the Cellotex in between them is impermeable? Plenty of evidence that that situation causes saturation of the timber, even if the overall humidity levels are nothing special.

I'll see if I can - it was a Canadian study on this subject, Grd Fl over 'crawl space', that you posted, that first alerted me. Do you remember what that was?

You might think 0.5 but more like .99 its shut in my experience.

Well, I've re-read the Green Building article and it's a most outrageous bit of commercial puffing. Can't think what Keith was thinking of, publishing blatant press-release stuff . It's uncanny how salesmen for the most dodgy products manage to find ways to genuinely believe in their tat. This respectable-sounding study is typical of the paid-for ingenious justifications that companies commission to provide their sales force with an excuse to 'believe'. Cambridge Architectural Research - their website looks good, offering project-by-project expert services to professional teams. So when did they start doing paid-for research in support of product manufacturers?

It's an old trick - set up a ridiculous proposition then knock it down. Don Quixote. The study proves what we all know - internal water vapour generation has to be removed by ventilation, or bulk air exchange as they call it, whether deliberately or by fabric leakage. No-one expects 'breathing' construction to achieve that, without ventilation. Well, there's a little speculation that it might help, indeed possibly even selectively diffuse-out pollutants - but that's not what breathability is meant for. So a research project to prove that bulk air exchange accounts for 95% of humidity reduction by water-vapour removal, and that 'moisture transfer by diffusion is an irrelevance' is not news. That's as far as the research project went - the second half of the article is just the author's 'common-sense' aka sales chat.

The whole risk of 'moisture may become trapped when some insulation products are fixed externally to a wall' is covered by the pious hope that 'it should generally be expected that the construction material in question is given time to dry out ...' The true purpose of breatheable construction, to allow the free passage, in and out through the fabric, in a fail-safe way, of the large seasonal flows of atmospheric (and residual constructional) water vapour, which dwarf the internal production of water-vapour, is glossed over, dismissed, and unreasearched. The article says nothing about the wisdom of weakly, patchily blocking such powerfully-driven bulk flows with impervious materials, leaving an unconsidered permeable remainder to carry the full traffic. My unresearched feeling is that it's a recipe for partial stagnation, pockets of high moisture accumulating by the familiar one-way pump mechanism, far from the possibility of re-evaporation.

Finally, nothing to do with the research project, we get the old scare of 'slump' and 'moisture ingress'. No mention of the brittle failure, shrinkage and blowing-agent loss that degrades the type of materials that Kingspan flog.

Let's not confuse this with the subject of hygrocopicity of insulants, which is something else again, and can definitely average-out and moderate peaks of internal humidity, by diffusion - but even then 'bulk air exchange' is still completely necessary, to remove the nett internal production of water vapour.

PS the PS to the article mentions an experiment that's supposed to have discredited the 'dynamic insulation' idea - yet another 'something else again'. The experiment appears to have grossly and ignorantly confused vapour permeability with bulk-air permeability. How could anyone expect a Findhorn 'breathing wall' house to provide the kind of bulk-air permeability (in-only because of maintained internal low pressure) upon which 'dynamic insulation' relies? As they say - 'no airflow occurred through the panel' No wonder the 'dynamic insulation' effect didn't show up!

This is really poor stuff.

Posted By: fostertom
This is really poor stuff.

LOL. Ditto my second post in reply to Tony

Theres plenty of articles on this subject area here: click on 'literature' then scroll down for english text
http://www.wufi-pro.com/

Also Neil Mays short article on breathability is here
http://www.natural-building.co.uk/art_how_to_prevent_water_damaging%20_buildings_and%20_health.htm

Tom - I think this is the article (or 'building science insight')
http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces
much much more here and on the links to the left
http://www.buildingscience.com/doctypes/digest

Posted By: skywalkerI remember the 'crawlspaces' document well

Can you remember any clues that wd allow you to search for it?

Tom.

It is in the links provided by Ali.

S.

Great Mike.

Posted By: skywalkerI remember the 'crawlspaces' document well. It provides a hugely positive endorsement for the use of PUR under floors

But not between the joists, as a simple substitute for the glass wool. They say put what you like between; the vital thing is foil-faced PUR right across beneath the joists. Actually, not even specifically that; it's any kind of insulation between, plus vitally any kind of insulation under, plus a VCL on the underside of all that. Whereas in Britain we'd automatically put the VCL on the topside of all that. That's because, for Washington DC, they deem the summer external high-humidity to be the crucial thing, not as in UK, the winter internal high-humidity.

The Article may set the scenario in summer but the issue is the same as here higer temp & humidity air from the inside meeting colder saturated air from the outside.

From the article:

"The average dew point of the exterior air during the summer months in Washington, DC is 65 degrees. Let’s bring this air into the crawlspace—so therefore the dew point of the air in the crawlspace is 65 degrees. Recall that the top of the floor joist is 75 degrees. The floor joist sees the dew point of the air in the crawlspace (we can ignore the vapor permeance characteristics of the fiberglass batt insulation since it is so vapor open—just pretend that we have air rather than insulation here—but not just any air—air with a huge temperature drop— “insulating air”), but because the floor joist is 75 degrees at this location, the relative humidity at this location is 70 percent yielding a wood equilibrium moisture content of 13 percent. The floor joist is “dry” at the top and “wet” at the bottom. Why wet at the bottom? The surface of the wood is cold, below the dew point of the air in the crawlspace and therefore condensation forms on the wood. At fiber saturation the moisture content of wood is 28 percent (go to Figure 3 again).'

So the vapour barrier in the solution is on the cold side.

S.

Suddenly had this image of us all doing handstands.

Sorry Made me laugh

I'll get my coat

S.

Posted By: skywalkerI haven't seen a construction type like this advocated by any PUR manufacturer. Do you know of any Tom?

Warm roof upside down?

S.

No, in a warm roof ALL of the insulation is outside of the structure.

Where insulation is between the rafters, the cold side usually requires a clear ventiated gap of at least 50mm, although some say this is unnecessary where a breather membrane and counter battens is used exernally - I always go for the air gap personally, as well as a VCL on the warm side of the insulation.

A warm 'floor' would be impractical in retrofitting floors in my opinion.

Can you check the link please sky, it doesn't work. Am very interested in that