[Strawbale] stucco outside, earthen plaster inside?

[Mark BP / Low Energy Design Ltd]

Erica -

If you're in one of the humid bits of BC, I would stongly advise not to 
use stucco at all.  I did a hygrothermal modelling exercise on this for 
my PhD a few years back, and found that for conditions in Cardiff (a bit 
damper than the weather data I've seen for coastal BC, but not by much) 
there is a real risk of bale moisture content reaching the sort of 
levels that mould can grow at if the plaster is Portland cement based.  
There is no such problem with clay or lime plaster.

The reason for this is the low permeability to vapour flow of Portland 
cement based plasters.  Moisture that enters the wall can't get out 
quickly enough.  This is particularly a problem with the configuration 
you suggest - relatively easy for moisture to get in though 
high-permeability clay plaster (in a heating climate the flow of 
moisture is from inside to outside for the vast majority of the time), 
relatively difficult for it to get out again to the outside through the 
stucco.

The results of my calculations were, of course,  based on a whole series 
of assumptions, as is any research in building physics.  If you were to 
keep your house at lower average humidity levels than I fed into the 
model, you might be OK - but to do so involves greater energy 
consumption (by maintaining higher temperatures throughout and/or 
increasing average ventilation rates).

Lime render, if applied correctly, is more than adequate for coping with 
driving rain.  The British Isles are littered with mediaeval castles 
that used it (and it wasn't rain penetration that turned most of them 
into ruins....) - in many cases you can see the continued success of 
traditional plaster recipes not only against driving rain but also 
against salt spray.  It has been used successfully for centuries on cob 
buildings in the wet and windy west of England and in wet and windy 
Wales.  So far as I can see, the reason that so many Canadian sb 
builders use stucco has nothing to do with superior performance and 
everything to do with familiarity, availability, lack of skills & 
training (though it's actually not that difficult), etc.  European sb 
builders are, on the whole, a lot less inclined to use stucco.

Your proposed course of action would work perfectly well here in SK.  
But it's not a risk I would ever want to take in coastal BC.

atb
Mark Bigland-Pritchard



Erica Konrad wrote:

>Dear all - I was wondering if there is any problem with plastering the 
>interior of my strawbale home with earthen plaster and the exterior with 
>stucco. We are in British Columbia in a pretty wet, wind exposed site and I 
>would like to cover the exterior with stucco for this reason (won't be doing 
>the stucco until the spring, but will work on the interior plaster (with 
>heat) over the winter). Does is matter if both sides are different finishes 
>in terms of moisture?
>Thanks in advance,
>Erica
>
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