[Strawbale] cement stucco problems?

[Mark Bigland-Pritchard]

Rob Tom wrote:

> On Sat, 14 Oct 2006 21:36:44 -0400, Mark Bigland-Pritchard  
> <mark at lowenergydesign.com> wrote:
>
>> 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.
>
>
> Thanks for that Mark. I've just logged on to check email before 
> shutting  down for the night so this will be brief (no doubt to 
> everyone's relief).
>
> If I am visualising this correctly, from the inside out we have:
>
> 1. Interior skin
> 2. Framing cavity (presumably filled with insulation
> 3. rigid foam insulation
> 4. Brick veneer exterior cladding
>
> ?
>
> AngleLand, so presuambly heating situation rather than cooling ?
>
>
> So what we have is a layer of relatively vapour-impermeable rigid 
> foam  insulation on the cold side of the framing cavity insulation , 
> with an air  barrier on the warm side of the framing cavity that is of 
> questionable  integrity.
>
> At this point with the sketchy picture formed above, at this point, 
> it  sounds to me like a classic case of  poor air-sealing on the 
> interior with  a vapour impermeable layer on the wrong side of the 
> insulation, or worse,  vapour impermeable layers on both sides of an 
> insulated cavity.
>
> ie Bulk moisture transport via air leakage from a heated interior into 
> the  framing cavity where it bumped up against the backside of the 
> rigid  insulation, condensed and probably resulted in a mess of mould 
> in the  framing cavity.
>
I'm sure air leakage contributed to it - but even without air leakage 
the calcs indicate (for non-hygroscopic building materials, which is a 
good approximation to what we had there) that vapour diffusion processes 
would be enough to produce interstitial condensation in the winter.  
Assuming, of course, that the standard values put into the calcs are 
correct (and those standard values derive from standard tests which may 
not themselves have been immune from air leakage effects).  Fwiw, the 
patterns of damage didn't correlate with the joins in the plasterboard - 
which is where I would have expected them in a primarily 
air-leakage-driven situation.  But I have to admit my memory is hazy as 
to what was immediately behind the plasterboard - and specifically as to 
whether there was a polythene vapour retarder or not.

>
>> Have a look at the (CA winery) document - it's on the EBN site.
>
>
> I'll have a look tomorrow but right now it sounds like a situation 
> not  unlike the problems that one would expect of a refrigerated 
> storage  facility (ie not exactly a low-rise residential single family 
> dwelling  especially if it's taller than two or three storeys)... 
> refrigerated  buildings and tall structures being special cases where 
> vapour diffusion  would need to be addressed , but neither case would 
> be typical of SB  homes, which was the original topic of discussion.
>
The winery appears to be single-storey.  And the temperature in the 
barrel room is controlled to a constant 60 deg F, which I calculate to 
be about 16 deg C - yes, cooler than a typical home, but not 
refrigerated.  So not a special case on either of those counts.  (Going 
off on a tangent for a bit, the low temperature may help to explain the 
lack of mould growth - though most of the thresholds for mould growth 
differ only by a few percentage points between 16 deg C and 20 deg C.)
And, yes, most residential buildings are not at a constant 80% r.h.  But 
I've been in some - in more than one part of the world - that came 
close...  In a damp climate, I would always look for a way (humidistat 
controlled fans are the easiest though not necessarily the most 
ecologically attractive) to maintain r.h. in wet rooms (kitchens, 
bathrooms, etc) consistently below about 65% so that overall it averages 
out at maybe about 50%.

>
> ===* ===
> Rob Tom
> Kanata, Ontario, Canada
> <archilogic at chaffyahoo dot ca>
> winnow the chaff from my edress in your reply
>
>