[Strawbale] cement stucco problems?

[Peter van Balen]

Mark (and all other SB enthousiasts),

Next year I hope to start building our SB house in foothills (550m) of 
French Pyrenees. Quite wet (1000mm/yr, about 80mm/month in winter months), 
not too cold (mean Tmin/Tmax 0°/10°C in January), with quite a lot of 
clouded humid days in the winter. It's an exposed location (mainly from W to 
NW), so W/NW side will be protect by garage with wood cladding; N, E and S 
sides will all have 100cm roof overhang.
I'm planning to put earth plaster (3cm) on all exterior and partially earth 
plaster (3cm), partially wood panelling on the interior of these walls.
I cannot guarantee average internal humidity to stay below 50%, but will 
install small hygrostat-controlled extractor fans in bathroom, toilet and 
possibly the kitchen area. Some internal walls (e.g. in bathroom) will be in 
adobe so could store/even out humidity peaks, I suppose.
Does your study give any indications that I could be in trouble with the 
above settings?
And would adding weatherproofing (almost certainly decreasing the vapour 
permeance) to the finish coat (e.g. to better protect the N wall) 
significantly increase the risk?

Peter van Balen

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Message: 1
Date: Wed, 27 Sep 2006 10:10:54 -0600
From: Mark Bigland-Pritchard <mark at lowenergydesign.com>

I guess that what I'm saying is that (i) the vapour permeance of plaster
choices will not be an issue for most people in dry climates (though it
may still be in buildings with a high internal relative humidity - and
I've seen condensation damage in multiple-occupation buildings in the
semi-arid climate of Mongolia), but that (ii) it can become a
significant issue in temperate maritime climates.

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Message: 4
Date: Tue, 26 Sep 2006 20:01:07 -0600
From: Mark Bigland-Pritchard <mark at lowenergydesign.com>

Rob -
It's just not as simple as that.  When I've figured out how to get
graphs from enormous .xls files into .pdf, I will send you the relevant
bit of my PhD thesis.  What I found was that, while there isn't a
massive effect, the risk was definitely higher with
high-vapour-resistance plasters.  (I was comparing 10mm clay + 10mm lime
plasters on each side with 20mm cement plaster on each side.)
Now, my theoretical model showed that risk was minimal even in an
exaggerated version of the climate in Cardiff (think Vancouver or
Seattle with added humidity year-round), so long as internal relative
humidity was kept down to an average of 50%.  So I would be pretty
confident that cement stucco would pose no risk in a continental climate
for usual domestic conditions.  (The runs I did for the slightly less
humid conditions of Aberdeen and Bordeaux came out showing effectively
zero mould growth risk.)  But stucco does bring an additional risk, and
though that addition to risk is small it could be the factor that tips
the balance both in moist temperate climates and in buildings with high
humidity levels.
Mark