[Strawbale] [SB-r-us] Re: cement stucco problems?

[Mark Frazier]

Thanks for the [data qualified] response Mark. Since it hasn't even
threatened to rain here in 6 months, I think I'll stop worrying about  
the
plaster choice on our building.

Back to dealing with the planning dept. and planning which jobsite  
tablesaw
to get.... ;-)

On Sep 26, 2006, at 7:01 PM, Mark Bigland-Pritchard wrote:

> 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
>
>
> Rob Tom wrote:
>
>> Some Californicatin' biker wrote:
>>
>>
>>
>>> Maybe I should rephrase that
>>>
>>>
>>
>>
>>
>>> Have there been any problems with bale walls failing when covered  
>>> with
>>> CEMENT STUCCO as a result of BREATHABILITY issues (not cracking,  
>>> poor
>>> roofing etc).
>>>
>>>
>>
>> Dear Mark's other brother Mark;
>>
>> Moisture transport into the wall assemblies of low-rise buildings  
>> as a
>> result of vapour diffusion through the broad surfaces of the  
>> claddings has
>> never been and never will be an issue and as such, I would say  
>> that the
>> issue of "BREATHABILITY" such as it is, is a non-issue ... that is  
>> to say,
>> fretting over whether lime plaster is more vapour permeable than  
>> Portland
>> cement plaster or less vapour permeable than earthen plaster is  
>> just that,
>> needless fretting.
>>
>> Something else that needs to be understood is that vapour  
>> permeance varies
>> inversely with the thickness of a material and that vapour  
>> permeances are
>> cumulative.
>>
>> That is to say, even though one plaster may have higher vapour
>> permeability numbers than another (for a given thickness of test  
>> samples)
>> a thicker plaster with a high permeabilty number could actually be  
>> less
>> vapour permeable than a thinner plaster with a higher vapour  
>> permeability
>> number.
>>     note: Permeability = value for a unit thickness
>>           Permeance = value for the assembly or overall thickness.
>>
>> Similarly, if a series of materials that individually have very high
>> vapour permeability numbers are combined, the cumulative result of  
>> the
>> assembly could be one that is not very vapour permeable at all.
>>
>> Properly mixed/applied/cured Portland cement stucco is perfectly  
>> fine as a
>> skin for SB walls.
>>
>> Improperly mixed and/or applied and/or cured Portland cement  
>> stucco can be
>> a heartbreak and/or a butt-ugly and/or an expensive-to-fix skin  
>> for SB
>> walls.
>>
>> Bales that are allowed to get wet because of poor architectural  
>> design (ie
>> unprotected plaster) and/or poor construction detailing  (ie poor  
>> flashing
>> , sloppy workmanship) will likely suffer some deterioration as a  
>> result of
>> microbial activity no matter what plaster was used.
>>
>> If any drying of the wetted straw is to occur, that drying is  
>> likley to
>> occur more readily via discontinuities in the plaster rather than  
>> as a
>> result of vapour diffusion through the broad surfaces. Evidence of  
>> the
>> latter can be seen in the Pilot Moisture Study that CMHC did about 10
>> years ago. (see LINKS section at SB-r-us)
>>
>> The bottom line is that if you want to avoid moisture problems  
>> with SB
>> walls, don't let the straw get wet.
>>
>> ===* ===
>> Rob Tom
>> Kanata, Ontario, Canada
>> <archilogic at chaffyahoo dot ca>
>> winnow the chaff from my edress in your reply
>>
>>
>>
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>
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