[Strawbale] Moisture sensor redux (was Re: cement stucco problems?)

[Mark Bigland-Pritchard]

Rob Tom wrote:

>
>>I suppose the big question then is along the lines of:  I have had 80%rh  
>>in my walls every night for the last week, at temperatures of 15degC.   
>>What do I _do_  to reduce the risk of mould growth?
>>    
>>
>
>Well I can tell you what they wouldn't do and that is change the vapour  
>permeance of their plaster, which was what brought about this moisture  
>sensor redux discussion in the first place.
>  
>
No, but if I were a public-spirited individual in that position I might 
be able to help other people learn from the experience, including 
helping to develop guidelines relevant to my particular geographical 
location and building use patterns for the benefit of future builders.


>What they should probably do is look for the sources of the moisture  
>loading and do something about that.
>
>If the source of that moisture was excessively high ambient humidity  
>levels (ie in excess of the danger point for straw) then I would question  
>their choice of a moisture-susceptible cellulosic material for their wall  
>insulation.
>  
>
Well, there is now over 10 years of experience of sb building in 
Britain, Ireland, the Netherlands and Denmark, over 20 in Brittany, and 
probably more than that in the US northwest coastal strip.  There have 
been some damp problems, but they have been fixable.  Buildings in 
places as damp as Cardiff - and damper - have been staying up.  Though 
there hasn't been much monitoring data, and I'd like to see more of it.
So actually, Rob, people with "excessively high ambient atmospheric 
humidity levels" do seem to be getting away with it.  So far.  My 
theoretical predictions, based on hygrothermal modelling [Rob has a copy 
of the relevant chapter] are that they should get away with it if they 
use clay and/or lime plaster, but they may not if they use stucco.  Also 
they need to take even more care than everyone else has to with regard 
to architectural detailing and building quality control.  Fortunately, 
the local "traditions" of plastering sb in Europe do mostly involve mud 
or lime or both.  And fortunately all the designers that people listen 
to in Europe have thought about what they are doing.


>And I would question the efficacy of mechanical dehumidification for such  
>a scenario (as was suggested by someone else (Andrew Morrison ?) earlier  
>in another thread) since I think all that would accomplish is accelerate  
>the movement of moisture into the wall.
>  
>
I agree that mechanical dehumidification is a bad idea as a general 
practice - much better sort the problem than fix it.  Not sure I agree 
with Rob's reasons, though - dehumidifiers simply take moisture out of 
the (indoor) air and condense it inside the machine, at some expense of 
energy.  So they reduce or at least control indoor r.h. - something 
which needs to be done in all buildings in damp heating climates where 
the occupants expect a modern lifestyle (because the movement of vapour 
is almost always from the inside outwards).  But the same thing can be 
done by a controlled ventilation strategy - such as is in fact required 
by the Building Regulations in all 3 UK building regs jurisdictions.

Mark