[Strawbale] cement stucco problems?

[Mark Bigland-Pritchard]

a partial response - more later
Mark


Rob Tom wrote:

> On Wed, 27 Sep 2006 12:10:54 -0400, Mark Bigland-Pritchard  
> <mark at lowenergydesign.com> wrote:
>
>> I guess that what I'm saying is that (i) the vapour permeance of 
>> plaster  choices will not be an issue for most people in dry climates 
>> (though it  may still be in buildings with a high internal relative 
>> humidity - and  I've seen condensation damage in multiple-occupation 
>> buildings in the  semi-arid climate of Mongolia), but that (ii) it 
>> can become a  significant issue in temperate maritime climates.
>>
>
>
> I'm afraid I have to disagree Mark.
>
> I don't doubt that you saw condensation damage in some buildings (just 
> as  there are thousands if not millions of buildings with such damage 
> here in  NA) but I would venture that that condensation is a result of 
> moisture  movement via bulk moisture transport due to air leakage 
> through  discontinuities in the air barrier, not via vapour diffusion 
> through the  broad surfaces of the walls.

I should clarify - I only saw one instance of mould growth on straw, and 
there were several possible factors that could explain this.  But I saw 
several instances of surface condensation damage in stone- and 
concrete-built apartments in central Ulaanbaatar.  This was in 
July/August, so it wasn't current - just the evidence that condensation 
+ mould growth had happened in the winter.

> Moreover, the worst possible conditions that would encourage moisture  
> movement via vapour diffusion (in scenarios with a high internal RH) 
> would  occur in heated interiors in Cold Climate regions (ie say 7500 
> HDD/yr or  colder) during the coldest portions of the winter when the 
> driving  mechanism (ie temp difference and difference in moisture 
> content of the  air, bewteen indoors and outdoors would be the 
> greatest) in structures  that are two or more storeys high.

Yes, the driving mechanism is strongest in the winter.  But it depends 
on high moisture generation rates inside - apartments in this part of UB 
receive district heating from a set date in October until a set date in 
I think May.  (Heat is distributed by radiators rather than blown air 
systems.)  But of course the abominable insulation standards in these 
Soviet-era buildings make surface condensation much easier.  And no, the 
2-storey issue doesn't apply - each apartment was on one storey only, 
and there was no lobby heating.

> That eliminates a good majority of SBH on this planet WRT concerns 
> about  the vapour permeance of plaster choices.
>
> Further, in the extreme conditions mentioned above, the vapour 
> pressure  would be greatest in the upper half of the interior volume, 
> increasing  with distance from the mid-point (or neutral plane in 
> geekspeak) so that  the maximum vapour pressure would be at the ceiling

I've done rh and temperature monitoring in quite a few houses and flats 
over the years, and have not found this stratification to be a 
significant effect.  Though to be fair most of those dwellings used 
water circulation (i.e. radiators) to distribute the heat, and I suspect 
that results almost always in less stratification than the N.American 
practice of using blown air.


> , with the walls being  only a minor player in any vapour diffusion 
> that may occur ... and the  typical plaster thicknesses being what 
> they are in SBH, which yeild  relatively low vapour permeances would 
> tend to further minimise the  amounts of moisture passing through the 
> wall plaster as a result of vapour  diffusion.   No ?  Which brings us 
> back to bulk moisture transport (rather  than vapour diffusion) being 
> the issue of concern.
>
>
> The other major source of water in walls as a result of vapour 
> movement  would be as a result of rain wetting of the plaster where 
> the plaster is  allowed to get soaked (due to poor architectural 
> design or lack of use of  a sealer in the absence of appropriate 
> geometry) and the moisture gets  driven into the wall via inward 
> vapour drive (powered by the sun) and  subsequently condenses ... and 
> the high insulation value of the thick SB  walls is not conducive to 
> permitting heat transfer through the walls to  provide the energy to 
> turn that liquid water back into vapour to allow it  to escape via the 
> process of diffusion. No ?
> Which brings us back to the "Avoid moisture problems by keeping the 
> straw  dry" comment".
>
> So I'd maintain, it is that simple.
>
>
>> I spent ages last night converting all the individual files to .pdf; 
>> now  I just have to either merge them into the main chapter document 
>> and send  you that, or else select the relevant bits and send you 
>> them.  Any  preferences? - besides the items under specific 
>> discussion here, the  chapter includes a lot of theoretical 
>> background (the equations forming  the basis of the model) plus runs 
>> for other variations.
>
>
> I would say that you might select what you think would be most  
> enlightening to the general SB populace and put it up in the FILES 
> section  at SB-r-us along with a note that more is available if 
> desired, by  contacting you.
>
> As for me, you're welcome to send me the whole schmozzle, if by email, 
> say  in small chunks of no more than what it takes you to upload in 5 
> minutes  or so assuming that you, like me are on a slow rural dial-up. 
> (Or not, if  you feel you have better things to do than upload huge 
> files).
>
>
> ===* ===
> Rob Tom
> Kanata, Ontario, Canada
> <archilogic at chaffyahoo dot ca>
> winnow the chaff from my edress in your reply
>
>