I totally agree, it's not.
In the kind that you're referring to, it's supposed to remove pollutants, without ventilation airchange.
In the kind that this thread's about, it marginally may do the same in the sub-case that inboard layer(s) are of hygroscopic material, meaning really only unfired clay, sheep/hemp/warmcel insulation or end-grain timber - people report subjectively sweeter air. Non-hygroscopic materials in 'breathing wall' construction won't have any such effect.

Bot

the system I think you are refering to is where the whole wall is pourous and they extract air from the house causing air to be drawn though the external wall inwards.

This does two things, creats a U value of nearly zero (apparently) and uses the pourous insulation etc to clean the air.

Totally different to the concept of 'breathability' in this case.

I have a case study here from finland that says that they experienced a 5% difference in RH when using a breathable wall vs one with a 'vapour barrier'.

They did caviat that quite a bit, but it could go some way to counter the arguement that moisture vapour passage though walls does happen.


Timber

Diagram of a Canadian bio wall...

Bot

Again I think there are many situations and ideas labelled with the breathable tag.

I think the concept of the origional article was refering a wall where moisture vapour from inside is allowed relatively free passage though the wall.

In the same finish document, they also stated that they did experience issues when using a poly vapour barrier of reverse condensation, i.e. bright hot sun forcing moisture back though the wall and it condensing on the poly vapour barrier.

Not sure that would be much of an issue in the UK, however, as I say that is what I think and not fact.

Someone needs to stump up some cash to get some of these things actually tested before loads of people waste loads of time (one way or the other) thinking about and building some of these types of construction.

The funny thing is that all of this must have been tested or poven in some way in europe, the more I google the more I find, but it is all so illusive I haven't yet found anything that I would hang my hat on!

Timber

More tennis anyone?

http://www.natural-building.co.uk/PDF/Breathability-Matters-response-to-Kingspan.pdf

agree

To get an answer to the question, "Can thick stone walls be safely and effectively insulated?" I found myself reading widely, at first anything I could find, then more systematically to understand the scientific basis of moisture movement across surfaces and different interfaces and the basis of calculating the size of these movements under various conditions. When you get down to it the principles are rather straightforward and uncontroversial (even if the algebra is a bit taxing compared with u-values). Not sure why some of you doubt Neil May's exposition which seems to me a model of straightforward clarity. Think of an all to familiar example of moisture between impervious surfaces, a shorthand for what may go on in walls. Take a double-glazed window pane. The seal goes - tiny gap, can't be seen but, Hey, no doubt about it, moisture is appearing between the panes of glass, a mist at first, then free water. Those timbers are not going to like it one bit. That is it really. That is what Neil May is describing when walls or roofs have impervious layers in the wrong place and no capacity to blow away, mop up and evaporate when the pin-holes occur - Ooops. Kingspan's exposition was none too clever really, better to concentrate on the proper use of their product than try to suggest that the particular qualities of insulation materials don't matter so long as we ventilate.

And by the way is double glazing really such a good idea? May be moveable secondary windows and good draft proofing have their virtues.

Posted By: bella Not sure why some of you doubt Neil May's exposition which seems to me a model of straightforward clarity.

I have said this myself on here in the past. The trouble is that the building industry as a whole is very conservative (small c) and with good reason. It may be simple for you to do the algebra and physics/chemistry, but for many people it is difficult or impossible. Builders know that they do not understand how buildings work, that is why they, quite rightly, resist changes to a model that 'works'.
But we need to make some changes because of the climate change/peak oil issue so the problem is one of convincing people of the need and of the best way to proceed, that is why I believe we need objective evidence about the best way, as well as some confirmation of important details. People need to be able to clearly see that the advice is free of vested interest.

A detail that bothers me:
If we build a wall that has a hygroscopic layer on the inside to buffer interior humidity changes(say 2 cm clay plaster), AND that is breathable throughout to protect against condensation problems within the wall, ought we to in addition provide a vapour resistant layer inboard of the insulation but outside the inner buffering layer?

Peter

Posted By: Peter ClarkIf we build a wall that has a hygroscopic layer on the inside to buffer interior humidity changes(say 2 cm clay plaster), AND that is breathable throughout to protect against condensation problems within the wall, ought we to in addition provide a vapour resistant layer inboard of the insulation but outside the inner buffering layer?

It all depends on the detail of the construction. There's no single answer to that question. It depends on what materials are used, in what quantities and how they are layered.

Posted By: djhHumidity buffering is not the same as removing water vapour from the building. Humidity buffering materials just don't have the capacity to do that, so some other mechanism is needed. The only effective means is air exchange.

For me this is where the proponents of breathable building materials often over-step themselves.

It makes sense to use breathable structures which can dissipate any moisture which finds its way in, whether this is due to build faults, extreme weather, damage to the vapour check or whatever. This requires vapour permeable materials which can dissipate moisture to the interior space &/or the outside air as conditions allow. As conditions do not always allow drying in this way, this in turn implies a requirement for hygroscopic materials which can buffer the moisture until conditions are right. With this I’m in agreement & when I get around to building my house, it will have a woodfibre sarking board on the outside of the roof structure & an OSB air/vapour barrier on the inside.

However, people often then go on to talk about indoor air quality & imply that hygroscopic materials can in some way reduce the need for ventilation. In the extreme case indoor air quality will be improved by not having damp, mold, etc, but this just shouldn’t be an issue in a building complying with Building Regulations & should be fully dealt with by heating & ventilation.

I can see how hygroscopic materials close to the inside space can help to regulate the indoor humidity, in the same way that thermal mass tightly coupled to the inside space can help to regulate indoor temperature, but is the former necessary or even beneficial? Assuming the space is correctly ventilated, the best it’s going to achieve is to delay/attenuate the climatic humidity change by/for a matter of hours or days. What is the practical benefit of this?

David

Posted By: fostertomIf we build a wall that has a hygroscopic layer on the inside to buffer interior humidity changes(say 2 cm clay plaster), AND that is breathable throughout to protect against condensation problems within the wall, ought we to in addition provide a vapour resistant layer inboard of the insulation but outside the inner buffering layer?

AFAIK the two issues are independent. Inboard hygroscopicity is great, and as a thin inner finish/layer greatly helps indoor comfort and freshness. What happens outboard of that can be the full range of solutions, from the fully breatheable, to the plastic/VCL. Neither particularly helps or hinders the other.

It gets more interesting when the 'what happens outboard of that' is also hygroscopic, i.e the insulation is hygroscopic e.g. Warmcel, Sylvactis, Calsitherm(?). In that case, the inner finish may not be hygroscopic, merely water vapour permeable e.g. plasterboard, and the insulation provides the hygroscopic benefits to the interior. But if the insulation is to be so in touch with the interior, that relies on having very low water vapour resistance between it and the interior, certainly no inboard VCL, not even a mild resistor. However breathing construction does require a mild inboard water vapour resistor - about 5x as resistive as the wall's outer layer(s). A conflict - what to do?

As far as I can see, with intersitial condensation checking/Euler software, it's OK if for example there's blown Warmcel between studs, plasterboard lined, then outboard of that the mild resistor e.g. OSB, then more EWI-style insulation outboard of that. That way, there's negligible resistance at the innermost layer; the resistance starts half way through and tapers off outboard.

Forget mass air movement through walls - this is about small repeated movement of warm damp air from inside (winter) or warm damp air from outside (summer) condensing on cooler surfaces, or free water entry from outside or anywhere else, anytime. It seems logical to stop all of these by using impervious materials on both surfaces but, but ------ ! Back to double glazing no matter what materials you use between the layers.

Now suppose there is just one impervious layer on the inside. If that is a plastered wall or ceiling, with plastic sheeting behind or shiney stuff on the back of the plaster board preventing all air movement (but it is nearly all -not all!) from the living space, and a well insulated, ventilated space beyond, is there ever going to be a problem with moisture?

It is going to depend on the materials between the plastic sheet and the ventilated space as djh says. Put fibre glass/mineral wool between and/or over timbers and any moisture getting through will blow away - so long as all of it is well ventilated. But what about that awkward corner by the dormer window? Or where the cold water pipes come through to a tank under the eaves? Just the place where the plastic sheet gets awkward, insulation incomplete and ventilation gets restricted by the insulation. Drips accumulate, pool, etc etc. Suppose instead you use no plastic membrane and insulate with sheeps wool or cellulose. Moisture, as in free water not just vapour, can be absorbed and can migrate to both ventilated surfaces and dry out. Now suppose you keep your plastic sheet and instead of mineral fibre you put in a layer of impervious insulation be it shiny or other wise. You didn't mean it to act as a vapour-excluding membrane but it is all the same, and it has the inevitable gaps, spaces that cannot be ventilated and moisture cannot migrate to surfaces that are. It is back to double glazing. Trouble folks. May be not this year or the next five or ten but if you live long enough it will happen. And you don't need a degree in physics to get the message.

Another study of a house with hygroscopic, vapour permeable walls and no vapour retarder, low levels of natural ventilation.

http://www.vtt.fi/inf/pdf/tiedotteet/2000/T2069.pdf

Pg 3/4
'these results show that it is possible to build a house with a porous and vapour permeable envelope that is moisture physically safe and improves the indoor climate'

It's a bit long, but interesting.

Peter

For a timber framed building, there MUST be some breathability in the wall to protect the structural timber. That is a MUST in my opinion. Think damp washing in a bin bag!

If you build a highly insulated timber frame building with good theraml bridging details and low air permiability with MVHR i think moisture migration though the structure will be minimal whether the external wall is breathable or not!

Timber

Posted By: fostertom Picture a plastic bag full of water. It's got a pinhole but someone says don't worry, it's only one tiny one. But overnight the bag's nearly emptied itself and the carpet's seriously wet. That's what a pinhole or hairline in a VCL is like. Any hole at all ruins it. Over a period that's very short compared with the length of a winter season, that pinhole will pass great quantities of water vapour - quite enough to cause real trouble to an un-robust design of wall-insulation sandwich, i.e. a design that relied on the (false) assumption that the VCL would function forever as intended.

The bag empties itself because the hydrostatic pressure on one face of the pinholed bag is higher than atmospheric pressure on the other face. The differential pressure drives a flow through the pinhole.

In the case of the pinholed VCL membrane, it's an indoor/outdoor differential in the 'partial vapour pressure' that the water vapour molecules 'see' - the water-vapour PVP is higher on the indoor face than the outdoor face of the pinholed VCL membrane. The PVP differential drives water vapour molecules to squirt through that pinhole. This happens even if atmospheric pressure is identical on both faces and there is nothing that would cause airflow through the pinhole.

You may say that such a PVP differential is small compared to the hydrostatic pressure differential in the bag of water example, so wouldn't drive such a flow - but then the viscous resistance of water vapour is also small compared to that of water. So that small PVP differential can drive a powerful 'squirt' of water vapour into where it may cause trouble, just as the drip-drip of water will empty the bag overnight and soak the carpet.

For each individual gas/vapour in a mixture of gasses, the PVP for each gas/vapour is proportional to (or a function of) the local concentration of that particular gas/vapour. Thus, in air, which is a mixture of gasses, nitrogen, oxygen, argon, CO2 and water vapour (and many more) each have their own PVP. Each gas/vapour molecule 'sees' only its own kind, and is repelled by them!- while ignoring the others.

So when some water evaporates to vapour, there's a local concentration of water vapour, which creates a local peak in water vapour PVP, relative to surrounding ambient water vapour PVP. In other words a gradient arises in water vapour PVP and the water vapour molecules are powerfully moved by that gradient, i.e. they repel each other, away from the centre of the local mollecular concentration. This continues until the local concentration is completely dispersed and evened-out with the surrounding ambient concentration. And while this is happening, all the other gas/vapour molecules are completely oblivious and unmoved by the repulsive drama that the water vapour molecules are playing out amongst and between the other molecules!

Water vapour movement is neither caused nor ameliorated by air movement. It's driven by PVP, almost completely independent of air movement aka ventilation. Once water vapour is created, in no time at all it's spreading and dispersing through airspaces and deep into vapour-permeable 'solids', blocked by VCLs but squirting through any pinholes. To repeat, water vapour migrates fast through any permeable medium, without disturbance to that medium. The air may be static or moving, x airchanges per hour may be happening, makes no difference, the water vapour 'flashes'outward until it's completely 'mixed' and equalised with surrounding ambient water vapour.

The only way that air movement can prevent that process is to create such a hurricane-velocity of airflow, that it exceeds the vapour dispersal velocity. This is attempted in chemistry lab fume cupboards and catering cooker hoods. Usually that results in a very high airchange rate but that's not the point - it's the velocity that does the job. And even so there are plenty of extra-volatile vapours, both in the lab and in the kitchen, whose dispersal velocity exceeds the air velocity and so still work their way 'upstream' and escape, causing smells.

No normal ventilation system attempts that trick. The idea that ventilation 'carries away' water vapour (and body odours!) as they're produced is completely false. What ventilation does is bring in fresh air of 'ambient' vapour content, thus slowly diluting the higher concentration that remains as post-dispersal equilibrium. PVP in the room slowly falls, the PVP gradient reverses, and the vapour that initially dispersed may return the way it came. Any learned paper that discusses water vapour movement as a function of air movement, is either ignorantly unscientific, or is written by the marketing department.

That's a great post Tom, its one for the archives!