[Strawbale] 5 perms / no ventilation; and top-of-wall

[Rob Tom]

On Mon, 12 Mar 2007 09:46:38 -0400, Mark Piepkorn  
<duckchow at potkettleblack.com> wrote:

> 	So, assuming that an air-permeable top seal is not a good idea in
> this climate... how vapor permeable should the air barrier across the
> top of the wall be? When there's no box beam or other full-width
> plate creating a condensation nightmare, people generally seem to
> lean toward smearing on some of whatever they're plastering with.


I don't think that box beams or full-width plates necessarily need to be  
condensation nightmares.

The techniques developed for superinsulated wood-frame construction to  
deal with such can be transferred successfully to SBC. After all, a  
thermal bridge is a thermal bridge (and elimination of) no matter what the  
materials used to make the walls.

And I don't think that simply "smearing on some of whatever they're  
plastering with" effectively creates the necessary air seal since plaster  
does experience dimensional changes with changes in temperature and  
humidity and the resultant cracks are sufficient to enable significant  
volumes of moisture to pass in typical Cold Climate conditions.

Quite frankly (and shirley too) , I think that the issue of vapour  
permeance of the air barrier across the top of the wall is a non-starter.

As always, it's a matter of the effectiveness of the air barrier, "air  
barrier" being a whole-house strategy rather than any particular material.

The air barrier is either effective (ie air-tight) or it's not.

If it's not, then juggling the vapour permeance of the little strip at the  
junction of the ceiling and walls isn't going to make any difference.

For that joint, gaskets and/or flanges, even if made of vapour-impermeable  
materials are effective and more reliable than any sort of goop in the  
long term.


=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
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