[Strawbale] convection and humidity in straw bale walls

Mark Bigland-Pritchard mark at lowenergydesign...
Fri Jan 6 23:50:24 CET 2006

So far as I am aware, nobody has done the necessary experiments to 
confirm that it actually happens. This would be a quite expensive 
process, involving test rooms built to the appropriate 
European/international/American standard, and used for multiple wall 
assemblies (plus probably use of a radioactive tracer for some tests). I 
think somebody should do it, but it is entirely understandable that 
nobody has as yet.
Convection remains, however, the most plausible explanation for the 
differences in results between official tests to measure conductivity of 
a straw sample and official tests to measure R-value of a wall assembly. 
(The residual differences after adjustment of conductivity values to 
allow for moisture content, that is)
I don't, however, see any benefit in full barriers between each course, 
as this would most likely result in thermal bridging; and you will need 
to explain to me the mechanism that is being suggested for convection 
currents leading to rot.
What might work, however, is partial barriers - and this is something 
I'd like to see put through the aforesaid standard test procedure. 
According to fluid mechanics standard texts, convective flow through a 
porous medium that is taller than it is wide is almost exclusively in 
the outer layers. It may be that small strategically-placed inserts of 
non-porous material could break up this flow rather than diverting it. 
If there is anyone out there with good cfd software, I would be very 
interested to see the results of modelling this one.
Borden, Saskatchewan

Michael lough wrote:

> Hello there from cool but getting warmer Canada…
> A little while ago I read some information on the subject of 
> convection within straw bale walls and would like to know if this is a 
> resolved issue in Northern Europe ?
> Hasit been established with support from testing or evidence of 
> problems that this phenomena exists in the straw bale world ? I am not 
> a scientist so I cannot reach a conclusion from scientific data myself 
> but I could understand simple terms…8)
> I hope this will be possible as there is much debate here on the 
> subject and there has been no testing here as far as I know. The idea 
> that bale walls arein fact likely to rot without there being barriers 
> in between each course is worrying some people!
> Thank you all for your consideration
> Michael Lough
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