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

[Mark Bigland-Pritchard]

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
>
>
>
> 
>Yahoo! Groups Links
>
><*> To visit your group on the web, go to:
>    http://groups.yahoo.com/group/SB-r-us/
>
><*> Your email settings:
>    Individual Email | Traditional
>
><*> To change settings online go to:
>    http://groups.yahoo.com/group/SB-r-us/join
>    (Yahoo! ID required)
>
><*> To change settings via email:
>    mailto:SB-r-us-digest at yahoogroups.com 
>    mailto:SB-r-us-fullfeatured at yahoogroups.com
>
><*> To unsubscribe from this group, send an email to:
>    SB-r-us-unsubscribe at yahoogroups.com
>
><*> Your use of Yahoo! Groups is subject to:
>    http://docs.yahoo.com/info/terms/
> 
>
>
>
>
>  
>