“They looked at the water vapour permeability of a whole range of products and estimated the following absorption ratings:
• Plasterboard painted with emulsion - score 40
• Concrete, unfinished – score 90
• Brick, natural finish – score 110
• Softwood, unfinished – score 150
• Strawbale behind lime plaster – score 240
• Durisol board behind lime plaster – score 250
They reckoned that anything with a score of 50 or higher would work as a humidity buffer. They also pointed out that such walls work automatically, don’t break down and require no energy to operate.”
“What you don’t so often hear — in fact just about never — is what proportion of your home is made up of water. I am not talking about water in the pipes and in the hot water cylinder and various tanks: I am referring to water bound up in the fabric of the house. There’s a surprising amount.... very possibly around 10% of the mass of a house will be made up of bound-water. ....
... Masonry materials, even concrete, are hygroscopic to a surprising extent and can take on and release moisture according to conditions.....
.... The fact is that a detached house, which can weigh anything between 50 and 200 tonnes, depending on size and construction methods (and that’s excluding all the foundations), could be holding as much as 10 tonnes of bound water within the walls, floors and roofing, and a lot more within the fixtures and fittings as well....
.... 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: some are very water permeable, others are not. Exactly how much water transfers between the solids in the house and the air in and around it is unknown - it’s never been measured, as far as I know – but it’s likely to be fairly substantial. If it amounted to a change of just 1% of water by weight, we would be talking about 100 litres of water....
... Because the water vapour is floating around in the air, if you swap the internal air for some external air, then you will also be swapping the water vapour levels as well. You are basically sucking water vapour out of the house.
Diffusion is ignored in all this. The fact that there is over a thousand times more water bound up inside the building than there is floating around in its airspace just doesn’t come into the equation, despite the fact that everyone agrees that moisture levels within a building are constantly changing. “
http://www.housebuildersupdate.co.uk/2007/07/how-should-we-handle-humidity.html