[Strawbale] cement stucco problems?

[Mark Bigland-Pritchard]

It's taken me a while to respond on this one.

I used to do investigative work on public housing in London (England) 
which had damp problems.  I only got the ones that the council building 
surveyors couldn't sort out or found ambiguous, so it wasn't an 
unbiassed sample, but they were all real cases.

I think the building surveyors sorted most of the leaks, because I 
didn't see too many of them - in fact I saw none where a leak was the 
only problem (though I saw several where it was adding to 
condensation-related problems).  I saw _a lot_ of surface condensation, 
though - which in practice means that the surface r.h. was above about 
80% for significant periods of time.  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 using the steady-state method that 
was laid down in the British Standard of the time, I found that 
condensation was indeed to be expected on the basis of vapour diffusion 
through the wall in this one particular property.

Now, that British Standard (and indeed ISO 13788, which has now replaced 
it in Britain and presumably is in common currency worldwide) provides a 
pretty useless analysis method for walls made mostly of strongly 
hygroscopic materials (like straw) - because the adsorption-desorption 
process, not diffusion, dominates moisture movement in straw walls.  
However, to talk (as Rob Tom does in the quotation below) of vapour 
diffusion as a "non-issue" is misleading.

Moisture content in straw walls needs to be kept below the level at 
which mould spores can germinate and/or grow.  If, say, your walls are 
consistently at about 80% relative humidity (as was the case in some 
walls in the California winery monitored by John Straube some years 
back), then they are at risk.  If they are consistently at about 60% or 
lower (as will be the case for most sb buildings in most parts of the 
world), then they are not at risk.  Anywhere in between, the level of 
risk depends on the temperature of the straw.

In a humid climate, it is quite easy for normal living to get your walls 
close to the danger zone.  How close depends mostly on the climate and 
on your lifestyle.  But it also depends on the ability of vapour to get 
out of the straw, and therefore on the type of plaster used.  Yes, this 
is not an issue in continental climates, except for those who generate 
vast quantities of vapour inside their houses.  But it is an issue in 
west-coast maritime climates.  Even there my theoretical modelling 
results showed a relatively small effect.  And it's probably not going 
to be the immediate cause of the number of building failures that will 
be created by poor detailing or poor construction (including along the 
lines that Rene mentions).  But it could easily generate _some_ building 
failures (and just one building failure is a disaster for somebody).  
Especially as the diffusion/adsorption/desorption processes set the base 
level for moisture content - local effects resulting from water or air 
leaks will raise moisture levels above that base level, so surely we 
want the base level to be as low as possible.

Mark





Rene Dalmeijer wrote:

>Peter,
>
>The whole discussion going on now I think is giving you the wrong 
>impression. A lot has been discussed regarding permeance of plasters 
>and I think therefore many people reading this list are starting to 
>confuse the issue. I will attempt to put things back into perspective.
>
>Study has shown that moisture damage in house walls, whatever building 
>method, is mostly due to (in order of importance, #1 plain old leaks, 
>#2 air leaks and then a long way back #3 condensation due to moisture 
>traveling through the surfaces. In practice it means nearly all 
>moisture damage I expect at least 99% is due to leaks.
>
>There are exceptions though where correct management of moisture 
>permeance is of importance in preventing moisture damage. This is when 
>the structure in question is almost steady state cold on one side and 
>warm on the other and even more so when the relative humidity content 
>is higher on the warm side. This situation exists for instance in cold 
>storage wharehouses situated in for instance Florida. In such cases the 
>best way of preventing moisture damage is to ensure that the moisture 
>permeance of the surface after the insulation is higher then before 
>including the insulation. This is the reason why closed cell insulation 
>is so popular for this  type of structure. Correctly sealing joints 
>between the insulation panels is of paramount importance otherwise you 
>are back to moisture cause #2 and all your efforts regarding moisture 
>management are null and void.
>
>In most building envelopes the above steady state allways heating and 
>always cold on the other side do not exist. This means that the 
>envelope experiences drying and wetting cycles either every day or 
>longer periodes up to seasonal. The rational  for permeance of the 
>surface finishes is thus; if moisture somehow works its way into the 
>structure it can work its way out during a drying spell, the faster the 
>better.
>
>  
>

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.