[Strawbale] cement stucco problems?

[Rob Tom]

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