[Greenbuilding] re: Another debate rages on Vapor barrier or not in 8700 hdd

[Robert Tom]

> building near Ottawafrom the exterior:
> 1" board and batten wood siding (cut from our own trees)
> 	3/4" air space with horizontal furring
> 	1 3/4" EPS foam R 7.5 	generic air barrier well taped and sealed
> 	7/16 OSB
> 	6" wood studs with 6" Roxul  batts unfaced
> 	vapor barrier?????
> 	3/4" strapping
> 	Gyproc
> 	Latex paint (as a vapor barrier???)

Stewart;

If you were to install a polyethylene sheet on the inside of the 2x6  
studs, its intended function would be as part of an air barrier strategy  
and in order for it to be effective in that situation, it should have been  
planned for from the outset with detailing to ensure its continuity. (ie  
strips of vapour-permeable barrier material at sill plate/rim joist to  
join onto basement and 1st floor wall barrier materials, strips between  
top plates top plates etc).

Further, the poly sheet needs to be fully supported to prevent damage due  
to changing pressures as a result of wind, something that is not  
accomplished with the 3/4" furring on the interior.

In single family residential structures , moisture migration into the wall  
materials as a result of vapour diffusion never has been and never will be  
an issue and two coats of a suitable vapour diffusion-retarding paint on  
the drywall will adequately deal with the non-existent issue.

However, in an 8700 HDD/yr climate, an effective air barrier strategy is  
of paramount importance.

Me ? I would have omitted the styrofoam and replaced the 3/4" interior  
furring with a 2x3 stud wall which would be used to carry services.

Hanging 1" thick wood siding on 1x4 furring that is fastened via fasteners  
cantilevered through 2" of styrofoam has not in my experience, been very  
satisfying.

I would look at pre-fabricating the siding into demountable panels that  
would be clipped to the building.

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Rob Tom
Kanata, Ontario, Canada
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