[Strawbale] 5 perms / no ventilation, and top-of-wall (Mark Piepkorn)

[Frank Tettemer]

Hi,
and here's another *two cents worth*, from the Algonquin hills of Ontario,

(I'm speaking from the experiences of post and beam, and strawbale 
plastered walls acting as the SIP's, adding the needed insulation and 
rack bracing.)

Every junction of plaster render with a post or beam, adds further 
complexities in vapour handling.  Of course a membrane for that 
transitional area is a good thing, under the plaster,
attached (and *sealed* to the beam or post of wood). 

But cracks are cracks, and in plaster renders, very hard to prepare for 
or plan for. Or prevent.
I visualize the home, with it's inner air pressures, and think about 
this whole question from that avenue of exploration.  And "typical house 
stack-affect" makes me tend believe that ( on a non-windy day), internal 
house pressures from stack effect (in a cold climate)
are busy moving *hot air* and moisture vapour *outwards*, through any 
passage available, towards the great outdoors, about anywhere on a wall 
from the neutral plane, on up. 

And from *below* the neutral plane, *cold, dry air* spends its time, 
slipping *inward* through any cracks.

I know, I know. There's rarely a still, breeze-less day. 
So when there's a wind, the neutral plane tips like crazy upwards 
towards the windy side, and that whole wall-side tends to move air 
inward, through whatever means available.  I'm not so sure that what 
you've said is true, Monsieur Tom.

Rob Tom:  "In predominantly cooling climates, air leakage would tend to 
be from the  
outside-in at the top plate wall/ceiling junction but the outside/inside  
differences in temp & humidity  (and hence the concerns WRT)  aren't as  
great in that situation."

I'm thinking: "airleakage goes from inside to outside, near the top of a 
wall assembly, and then one can address airleakage on that basis".

*** As far as the top-of-bale-wall question goes, all my structures have 
been plastered continuously from outside skin, over top of bale wall, 
continuing downward on the inside plaster skin.  I appreciate the 
discussion as far as moisture is concerned,

But in all cases, I've looked to those top-of-wall-plaster coats as
Fire-Stops. 

Yes, they are a horizontal thermal bridge, between straw and cellulose.  
And that's too bad.
But replacing these plaster coats with tyvec or tarred felt is not a 
good option, to me.
Because rodent-proofing and fire-proofing are a big concern as well, and 
that is certainly an area to address.

Okay, maybe that was a Loonie's worth, instead of two cents.

Frank Tettemer
Living Sol ~ Building & Design
www.livingsol.com