[Strawbale] stucco outside, earthen plaster inside?

[Erica Konrad]

Thanks for your thorough response Mark. We are actually in Nelson, BC and 
not on the coast. Nelson is not as humid or as wet as the coast but it's 
good to hear that about lime plaster as I think we may go that direction.
Loving this list,
Erica

----- Original Message ----- 
From: "Mark BP / Low Energy Design Ltd" <mark at lowenergydesign.ca>
To: "Erica Konrad" <strawbale at netidea.com>
Cc: "SB REPP" <Strawbale at listserv.repp.org>; "SB Yahoos" 
<sb-r-us at yahoogroups.com>
Sent: Tuesday, November 13, 2007 11:23 AM
Subject: Re: [Strawbale] stucco outside, earthen plaster inside?


> 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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