Does anyone have any experience of living with or building breathing walls?
How does this idea chime with air tight construction?
Are they two contradictory ways of building?
is air tight the same as "water vapour tight"?
Biff
posted on 11-05-06
>is air tight the same as "water vapour tight"?
No.
Air tight means no big flows of air through holes leading to heat loss by mass transfer. You heat the air. The air moves out of the house carrying the heat with it.
Breathable means that water vapour can diffuse through it. It's what happens in a vapour-permeable material such as lime plaster/mortar. There is no associated loss of heat.
Peter Clark
posted on 11-05-06
Hi Biff,
Thanks for replying.
So when people talk about making a house air tight, that house might still have breathable walls?
But do houses that are being made air tight nowadays have breathable walls or are they also being made vapour tight, hence the need for mechanical ventilation?
If a house is air tight to stop heat loss, but also has breathable walls, would it require a separate ventilation system?
Sorry, loads of questions, i hope they are of interest to other people.
Tony
posted on 11-05-06
Quite a lot of modern so called breathing walls actually have a vapour barrier - foil back placterboard or actual polyethene!
Solid masonry construction can beathe depending on the paint used inside.
Victorian terraced houses with lime plaster and lime mortar will do this too and most probably are better at it.
There is a danger in trying to get vapours to pass out through your wall that you could finish up with interstitial condensation in winter and big problems.
Paul in Montreal
posted on 11-05-06
I think many have a fundamental misconception of the purpose of breathable walls, air tightness etc.
There's two things you want in a wall (besides insulation): air tightness and moisture resistance. In other words, you don't want the rain from the outside getting to the interior of your house. You also don't want water condensing inside the wall (and associated insulation). For the occupants of the building, they require fresh air and the removal of excess moisture - four people generate a surprisingly large amount of water vapour per day - far more than could be transpired through a breathable wall.
The outer skin of the building should be air tight but vapour permeable. This is to allow any moisture which gets into the wall structure to escape to the outside. Brick is permeable and, even with a cavity, moisture will end up in the wall structure. If this can't escape, all sorts of problems can result. This is why air membranes (such as Tyvek etc.) are similar to GoreTex: they're air tight, waterproof but vapour permeable.
On the inside, you don't want moisture from the warm interior ending up in the relatively cold wall structure where it will condense and cause problems. So you have a vapour barrier on the warm (interior) of the insulation. In hot humid climates the vapour barrier may actually be on the outside since it is cooler on the inside - which is the opposite of heating climate like the UK (and Canada for that matter). With an air tight structure, mechanical ventilation is required, not just to remove moisture generated by the occupants, but also C02 and other gases. No breathable wall will have anything like a high enough transpiration rate to remove sufficient moisture from the occupants without it condensing in the wall structure so. If the building is leaky, though, enough fresh air leaks in to provide the required ventilation, but a lot of heat leaks out. For an airtight building (which is energy efficient), heat/energy recovery ventilation is not a "nice to have" but a "must have". Of course, out of the heating/cooling seasons, just opening the windows works well too. In my climate in the summer, it is hot and humid outside and so it's essential to keep windows closed to prevent the hot and humid air getting into the building.
Paul.
Peter Clark
posted on 11-05-06
Hi Paul,
Thanks for taking the time to type all of that.
I have two main dislikes in house design I think:-
Firstly, mechanical things. I would like to do without gas boilers, water pipes, fans and anything else with moving parts. I recognise this is a practical impossibility, I'd just like to minimise them.
Secondly is wrapping the house up in a poly bag, like an old fashioned waterproof coat.
So, if I must make it airtight to try and do without a central heating boiler and wet system, and I can't get rid of moisture inside the house by transpiration through a breathing wall, how about 'passive stack ventilation' - no fan you see!
