One problem is that many so called breathing walls have a vapour barrier somewhere inside the insulation so only the wall can breathe in and out from outside and not the house. Traditional masonry constructions do breathe moisture in and out -- more out than in. Timber frames usually have a vapour barrier so cannot do this indeed it would cause them to prematurely decay if they tried to transport moisture outwards. Many homes have so many drafts that they literally do breathe in great gulps of cold air all winter long and blow out great mouth-fulls of warm -- expensive. True breathing walls can work -- most lime and natural constructions can buffer out peaks of high internal moisture concentrations and release them slowly as the levels of humidity reduce.

What do you mean by a breathing wall?

Tony, is the reason why breathing walls don't work in the real world because of what you've said, ie. they have a vapour barrier installed, thereby stopping the walls breathing?

My understanding of a breathing structure is where you have an internal membrane which has a high resistance to vapour to stop vapour getting into the structure. Any vapour that does get in is then encouraged to pass through to the outside by having an outer layer which has a low resistance to vapour. A breathing structure is not one where there is a regular transfer of air through the building fabric
The inner membrane also doubles as an airtight (not vapour tight) layer, which implies ventilating to deal with moisture build-up etc. Lime as well as fermacell/sasmox board manufacturers lay claim to helping to deal with moisture changes as Tony describes.

Biff's right, Terry - your with-membrane description is just the industry-standard of the last 20yrs. With an inboard vapour membrane, whether it's a full vapour barrier (very challenging to achieve) or just a gappy so-called vapour check, its resistance is so relatively high that you automatically can hardly fail to have outer layers of lower vapour resistance.

'Breathing' as described by Warmcell etc is different in principle - the powerful inner vapour resistance is avoided - the rule-of thumb is the inner-layer resistance is about 5-10 times that of the outer layer, ideally a steady gradient of resistance-values outward. Some say the ratio can be as low as 3:1. Ply/OSB inner sheathing and MR plasterboard or breather felt outer sheathing will generally achieve this - but calc those values every time! Don't go and spoil it with e.g. cement render - that will probably have higher resistance than the inboard layers.

An inner membrane isn't going to function as an airtight membrane without all sorts of non-standard detailing and unless consistently careful site work can be guaranteed.

Go on then tell us what you mean by a breathing wall please.

Thx Biff but i was asking Pingy!

Since you answered Biff, what about Victorian terraced? 1960's masonry cavity walls? modern masonry? I think they all breathe do you?

I don't think vapour permeability should be muddled up with airtightness. They are very different issues. One is about stopping the draughts (in some folks eyes to such an extent that humans can't breathe). The other is about diffusion of water vapour through apparently solid, air-tight, structures. It's this latter aspect that is the matter of a building's 'breathability'. Unfortunately the science its pretty tricky, and not made simpler by incomprehensible units that are not standard between different countries. Even a thin thing like a layer of paint can be controversial. So called 'breathable' paints may be better than a standard alkyd resin but aren't nearly as vapour permeable as limewash or linseed oil paint.

Yes, doubtless there are lots of vapour permeable modern materials, but I don't know a lot about them. Old houses, typically, are mortared and plastered with lime, and the way the structure handles moisture is very different from the way a 1960s or modern wall, made with Ordinary Portland Cement, does.

Suddenly started to get interesting -- so a breather membrane only lets water vapour through? -- a lime plastered wall will let moisture in and out but not air or oxygen?

Sounds very much to me like Biff should have long since suffocated! Yet we hear him talking nice stuff all the time -- can someone pleaseexplain this phenomenon.

Then why is Biff scared of suffocating in an airtight house?

Posted By: tonyThen why is Biff scared of suffocating in an airtight house?

I would be too - but then I'd arrange controlled ventilation (as it is I can see plenty of daylight round my farmhouse's windows!)

Posted By: biffvernonA lime plastered wall would, I imagine, let pretty much anything in or out, if slowly.

Not necessarily - sure to be selective, like anything permeable. Can only help, if we get to understand better how this works, then maybe tradition and 'modern' can work even better together than they can apart. Don't play the 'don't know much about', Biff - too much good sense there!

To answer Tony's question.....

<blockquote><cite>Posted By: tony</cite>Go on then tell us what you mean by a breathing wall please.</blockquote>

I have read a lot of articles about how important it is to continue to allow old buildings to "breathe". Many of these articles use examples of how people have "improved" these buildings by coating the walls either on the outside or inside (or both) with impervious materials resulting in damp problems. This not only is bad for the building but also the health of the occupants. Okay I admit there could be a host of other reasons why the damp problems arise but the articles always seem to suggest that when the buildings are restored (improved?) using traditional materials such as lime, clay, etc. then the damp problem go away and the buildings become more pleasant places to be.

The ability of these natural materials to regulate humidity within the building is how I imagine a breathing wall to be. From a health point of view this sounds a lot more preferable than the 'plastic bag' house which seems to be linked with the increase in allergies.

The thing is when these old buildings were built there wasn't any thought about insulation. To insulate to a high level and still maintain this breathability is why I'm asking questions on this forum. If you can convince me it really isn't necessary then that's fine, please feel free to suggest how best to insulate an old building to ensure the building fabric is preserved and the living environment is a healthy one.

What we are talking about here is essentially the interplay of water, air movement and heat within the building fabric. We know they are related, but we don't yet fully understand how. This leaves the way open for dozens of "theories", which all sound plausible, but are mostly unverifiable. IMHO, we still have a long way to go before we have definitive answers to these questions.

Think about it. Breathing walls, humidity buffering, vapour barriers, breather membranes, lime v cement, natural materials v synthetics, even Actis and the multifoils. They all sit in the gap that exists between theoretical physics and our limited understanding of what actually happens on site.

Nice summary, Mark. Add to that, as well as U value, thermal massiveness, decrement ............

Tony, it would be great to externally insulate the building but unfortunately I would not be allowed by the planning department. The building has some historical interest (not listed though) and the whole reason for being able to get planning permission was to preserve it's external appearance.

The building is red brick with a welsh slate roof.