[Strawbale] cement stucco problems?

[Mark Bigland-Pritchard]

Rob Tom wrote:

> On Sat, 14 Oct 2006 19:24:08 -0400, Mark Bigland-Pritchard  
> <mark at lowenergydesign.com> wrote:
>
>> I saw one case of interstitial condensation, and it was so far gone 
>> that  I couldn't tell you whether the culprit was diffusing moisture 
>> or air  leaks or both.  However, when I did calcs to predict 
>> condensation ... I  found that condensation was indeed to be expected 
>> on the basis of vapour  diffusion through the wall in this one 
>> particular property.
>
>
>> If, say, your walls are consistently at about 80% relative 
>> humidity(as  was the case in some  walls in the California winery 
>> monitoredby John  Straube some years  back), then they are at risk.
>
>
>
> Okay! I like this.
>
> In the first example above where moisture intrusion as a result of 
> vapour  diffusion was a suspect, it would be useful to know some of 
> the building  specifics.
>
> For instance,
> 1. What type of construction was it ?
> 2. Was it built in a manner that would ensure relatively air-tight  
> construction ?
> 3. What was the occupancy ?
> 4. What sort of conditions were on the two sides of the building 
> envelope ?
>
Bear in mind that I'm working from memory here, and I encountered the 
building in about 1990, so I can't be sure of all the details.  It was 
some sort of primitive panel system, built in the early 70s when British 
designers had little idea about such things.  What I do remember is the 
brick outer skin with some sort of extruded polymer insulation (probably 
polystyrene) immediately on the inside of it.  There was a cavity 
immediately inside that (except for the structural vertical timbers), 
and I can't remember the details of the inner skin.  By R-2000-type 
standards, it certainly wasn't airtight, and no doubt there were air 
leakage areas which contributed to the problem; but my point is that 
steady-state theory of vapour diffusion predicted the problem (or would 
have done if the designers had bothered to apply it, as they are now 
expected to do routinely).  I do remember noticing that the water 
pooling inside the bit of wall I managed to get into, and the 
discoloration on the inside of the wall elsewhere, didn't seem 
particularly to be associated with the joins between polystyrene 
sheets.  I don't remember what the occupancy level was (actually it was 
empty by the time I got to it), but it was I think a 3-bedroom terraced 
house [that's a row house in North American], which means typically a 
family of 4 or 5 (I would remember if it wasn't a typical number), and a 
floor area of about 50 or 60 square metres.  So most North Americans 
reading this will think it horrifically cramped, and most people from 
say, Mongolia, for example, would consider it luxuriously spacious.  The 
outside of the building envelope was subjected to the London climate - 
humid all year round, low rainfall by British standards but probably 
more than Ottawa, very little driving rain, low solar intensity probably 
made even lower by overshading.  I can dig out climate data if you 
really want it.  On the inside, I don't know what the tenants' 
moisture-generating behaviour was like.

> In the second example of the California winery, was the constant 80%  
> relative humidity:
> 1.  as a result of ambient outdoor humidity levels being that high or
> 2. as a result of extreme conditions created by occupancy/building type ?
>   (ie tall building and/or large temp differentials between 
> indoor/outdoor  creating a condition where moisture was driven into 
> the wall)
>
It was the result of the high indoor humidity apparently necessary for 
the winemaking process - the barrel room was kept at a constant 80% rh 
according to the report.  The wall location with the highest rh values 
actually was lower than 80% for most of the time, but still at values 
that were high enough (and for a long enough duration) to be in the 
danger zone for mould attack (though the report authors didn't report 
any signs of mould or deterioration).  Have a look at the document - 
it's on the EBN site.


>
>> and Rob Tom wrote (with rather less care):
>>
>>> As has been mentioned numerous times on these Lists recently, the  
>>> plasters used on SB walls , at the thicknesses which they are 
>>> typically  applied, are sufficiently vapour impermeable and non-air 
>>> permeable to  make moisture movement through the plasters via vapour 
>>> diffusion a  non-issue.
>>
>>
>>> Bulk moisture transport via leaks OTOH is another matter entirely 
>>> and  has nothing to do with relative humidities.
>>
>
>
>
>