It's a can of worms. Lots of so called breathing walls have an effective vapour barrier between them and the house.

Mostly with breathing walls only the walls breathe! A traditional solid wall house can breathe moisture out through the walls slowly.

Lime built homes can absorb large amounts of water vapour into their structure without damage and release it slowly later.

I think it is a poor turn of phrase especially as the majority of our homes actually do breathe -- in cold air and out warm every time the wind blows or someone opens a door.

Posted By: steveleighBreathing walls (permeable envelope) can never be an exact science because there are too many variables

Present common easy steady-state methods of assessing interstitial condensation risk (i.e. risk of water condensing out within the insulation layer or other unintended part of the fabric) are indeed mickey-mouse, not least because they start from thermal resistance k-values for materials, that are grossly over-optimistic in dynamically varying conditions. But proper dynamic thermal modelling, such as Tas, inputting actual year-round weather data for the site, or assuming future ditto (like UK's expected to acquire the climate of the Algarve by 2050), can reliably predict performance of breathing wall designs.

This from http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=127 should help:
"Airtightness is about movement of air in bulk under air pressure - wind, thermal buoyancy etc. Vapour tightness/breathability is about the movement of individual molecules of the vapour under vapour pressure. That may sound like the same thing but it's a quite different principle.

Vapours travel and pass through apparently solid things even when there's no air movement and no air pressure - and can even move 'upstream' against an air movement. Like humans, molecules of vapour 'see' only their own kind - and are repelled by them! So any local concentration of a vapour - a bathroom full of water vapour, or a tiny drop of evaporating perfume - wants to get away from itself, to disperse away from the centre of concentration, to even itself out. In a space, every single different kind of vapour that's present sees only its own vapour pressure - a measure of the local density of those particular molecules at any given point in the space. The molecules respond to their own private vapour pressure gradient and move away from high vapour pressure towards low vapour pressure areas. Volatility is a measure of how fast each type of molecule moves for given vapour pressure gradient - thus highly-volatile perfume reaches you with incredible speed, even when there's no air movement to carry it bodily.

Of course the many individual vapours that make up air are doing all of this - but air tightness is about the mass movement of air. The vapour pressure action happens within that air movement - the centre of concentration and the vapour dispersal pattern will be carried along bodily with the airstream, like a hot air baloon rushes overhead but to the occupants it seems they're in still air because they're moving with the breeze.

You can have membranes that do one or or both or neither. E.g. polythene is air-tight and water-vapour-tight (but more transparent to other vapours); Tyvek is fairly air-tight and almost vapour-transparent; anything open-weave is more or less transparent to both. As Biff says, an apparently impervious balloon puts up considerable resistance to many of the air vapours, but not much to CO2.

You can roughly assess air tightness by blowing through a membrane (I prefer to suck!) but that tells you nothing at all about its vapour resistance/breathabilty."

Posted By: albacorethe house itself losing vapour from the interior through the walls ......... seems to be asking for trouble

Not necessarily, if done with understanding or thermal modelling - or even both! There's a future prospect of (airtight) membranes being designed that will selectively promote the out-migration of common indoor air pollutants as well as water vapour, as an alternative to removing (or more accurately diluting) them by mass air movement. That would enable even lower ventilation rates, just enough to replace CO2 with fresh oxygen, so heat losses caused by necessary air-exchange could be reduced even more than with present HRV systems.

There is a useful code of practice - BS5250 Control of Condensation in Buildings. Worth reading if ou really want to understand best practice. It suggets in practice that any "breathable" wall with vapour permeabity 5 times greater on the exterior than the interior will not suffer from condensation or damp problems. Reverse flow, where moisture travels from the outside in requires specific circumstances- e.g. warm and wet outside, cold and dry inside.
I think the proof of breathable walls is the 10's if not 100's of millions of Europeans living in houses built from "Zeigel" style blocks - solid, single skin walls, with reasonably high breathability. They've been in use for decades with none of the problems people obsess about in this country. In fact, the much loved vapour barrier can cause its own problems - under reverse flow, vapour can condense against them. Perhaps the cause of much of the toxic mold often found behind stud walls?

Posted By: steveleighPeople lived in caves for years. Then someone built a house

And now it seems the key to zero-energy by means of storing summer-collected energy through to the winter, is to return to the cave principle. That doesn't now necessarily mean a buried house, except in the 'purist' no-machinery version, in which case ASMET could really come into its own as an underground tank to live in. However, ironically, the virtue of the buried version, whether using ASMET or not, is specifically no machinery, HRV included! - particularly valuable if we're looking forward to a future with shortages of everything, especially spare parts for old HRV systems.

Mass market? The Victorians managed to build several million houses without resort to MVHR or other arconyms. We just need to tweak it a bit so the walls and windows are better insulated.

Posted By: biffvernonseveral million houses without resort to MVHR or other arconyms

But utterly dependant, for the first time in cool climate history, on on/off powerful fuelburning, because of relatively low thermal mass, rather than slow steady trickle-feed (from cooking, bodies/animals etc) or strongly peaky (e.g. solar), which could be averaged, because of high thermal mass. So began the heating engineer's approach - burn fuel, regulate thermostatically - which is still the norm. Now, with the benefit of fresh scientific understanding, we're going back to the older principle, but MHVR is (so far) one of the most essential components - no natural or passive effective alternative has yet been found, unfortunately - please prove me wrong.

Steve,
what makes you think people wont buy a house with a massive cowl on its roof ? does it need to be massive ? sold properly it should increase the value of the property.

Steve -
was the specifier talking about the UK? I find it hard to believe that in this country we have the combination of high temprature and high Relative Humidity which would drive water vapour inside for any significant lengths of time. Even with "Breathable" walls, the movment is slow, what matters is the overall balance averaged out over many hours or days. In fact, this is one of the points of breathing walls. They regulate humidity by smoothing out the spikes - soaking it up and then releasing it .
I'd also very strongly disagree with trying to say that sealed systems don't have moisture problems - there are lots of pitfalls. A big problem is with internal air quality - odours, off gassing from VOC's - Internal climate is about far more than minimising heat loss. Boil a kettle in a sealed room with a plastic finish, and you have to open a window. In a carefully designed room with breathable finishes you won't. Which is more energy efficient?
I think TB could be more to do with jerry built slums and lack of medicine - I'd put a bet that they'd still have had TB etc even if they had built the identical houses but with some sort of vapour barrier on the inside.

It's been discussed time and time again here: passive ventilation is just not controllable or predictable enough to give the sort of air quality that's required in a highly insulated airtight building. There's a good reason that MHRV has been enshrined in the Canadian building codes for some years now. As much as people seem to have an aversion for mechanical systems, a MHRV is a pretty simple device that's easy to fix and could be run very effectively from a small PV array.

Paul in Montreal.

We are a bit behind the times here in the UK and we need your input to help us catch up Paul, Thanks.

Hi Paul,

I'm interested to know where you derive your certainty in this matter from, do you have access to any studies or data that show that a passive approach cannot work?

I appreciate that the Canadian system represents a lot of experience and knowledge, and I am not in possession of it. The trouble is that ,if we accept the point of view you have expressed, then we would all stop thinking and discussing ,and just do the things the Canadian way, but that cannot be correct can it?

There is also the matter, also discussed here before, that the climate in the UK and Canada are not the same, it may work for the UK, or parts of it? Would that not be a good thing?

Thanks for your help,

Peter

Paul,

Thanks for your explanations. I must admit for our build I'm still veering towards passive ventilation. The difference in UK and candian climates is significant (my grandparents had a ranch in Alberta when I was younger), and I tend to feel that absolute airtightness is more necessary in a country where the temperature gets down to -30C on a reasonably regular basis. As such, it seems to me that a controllably 'leaky' house in the UK climate can withstand our winters - or in other words, we don't need to reach complete airtightness to dramatically improve our buildings' energy efficiency. As our summers are more temperate, the simple option of opening the windows is largely effective whereas I have fond memories of hiding from biting insects in the Canadian summer (my Grandma offered me 1 cent for each one I killed!).

Anyway, surely this is a distraction from breathing walls, which are dealing with very slow diffusion of airbourne contaminants (including moisture), rather than ventilation?

I did say _controllably_ leaky! Putting leaks where you need them, rather than where your builders happen to leave them!

Sometimes even the doors open as well!

I'm still getting to grips with the topic, so forgive me... I'd be happy to be corrected where my thinking is 'woolly'.

In my head, breathing walls benefit the inhabited environment most usefully by acting as a buffer and path through which moisture can escape. Hemcrete looks like a good candidate as it provides for a natural, airtight wall that can still allow the passage of moisture.

Ventilation is a completely separate topic, and yes, is required to deal with CO2 CO etc. etc. It needs to be controlled to reduce heatloss during the heating season, which coincidentally when passive ventilation is most effective. Air flow windows seem to offer some benefits, allowing significant improvements in the efficiency of your building envelope.

I lean towards passive systems because:
1) I've seen how rapidly the efficiency of mechanical systems decreases as they absorb dust and debris
2) Mechanical systems are poorly understood in the UK and I have low confidence that my airtight building would be evenly and properly ventilated by design
3) As the technology is rare in the UK, I don't feel confident in long term support of systems - repairs and maintenance, where I would be dependent on a very small supplier base
4) Mechanical systems seem to require a significant allowance in your design for routing of ducts and airflow - particularly difficult in our case where we are going for a design with no loft space
5) We have always slept throughout the year with the heating off and windows open, so I'm unsure how that would sit with an airtight building and MHVR
6) In a smaller UK house, noise from heating and ventilation systems is much more invasive. This is particularly an issue for us because we live miles from anywhere, so noise pollution is particularly apparent.

We're still undecided, but in the mild weather of the south east the significant expense and practical difficulties of achieving full passive house standards seems hard to justify where we can achieve a big improvement over normal UK energy efficiency via other means. I fully accept that it works in Canada, but I'm not sure if it's the best 'first line of defense' here.