[Strawbale] On moisture

[Mark Piepkorn]

Oldish news with a friendly face.

http://markbrinkley.blogspot.com/2007/07/new-methods-of-handling-humidity.html
Excerpts from the webpage:

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How should we Handle Humidity?

...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.

What does 10 tonnes of water look like? Due to 
the miracle of metricated measurements, 10 tonnes 
of water turns out to be 10m3 in volume, about 
the size of a small bedroom, or perhaps 70 
bathfulls, if you prefer that. It also turns out 
to be 10,000 litres, which is the amount of water 
a typical household uses in about two weeks, or 
around 1500 flushes on a 6-liter low flush toilet...

...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. Doesn’t it 
seem strange that we should spend so much effort 
and energy expelling 10 litres of water vapour a 
day from our homes, when they are 10,000 litres 
of water already sitting in the fabric?

There are alternative water vapour management 
strategies out there. The best established one 
hails from Germany and it consists of fitting 
highly permeable materials to be used as a 
reservoir to store moisture, with a view to 
letting it be evaporated back inside when 
conditions allow. In Germany, it is seen as part 
of the Building Biology movement and they regard 
the use of humidity-buffering materials as one of 
their key principles. Needless to say, they 
regard the use of mechanical ventilation as an 
anathema. A lot of building scientists regard the 
Baubiologists as cranks, but there was some 
independent testing of their humidity buffering 
principles carried out in Canada in 1997 by Straube and Burnett ...

...They looked at the water vapour permeability 
of a whole range of products and estimated the following sorption 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...

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Mark Piepkorn
www.potkettleblack.com

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