[Strawbale] concrete wall vapor barrier

[Robert Tom]

On Tue, 27 Nov 2007 09:47:39 -0500, Derek Roff <derek at unm.edu> wrote:

> Installed building materials seldom do anything at 100% or 0% levels.
> As Joshua (Speireag) points out, an 8" thick concrete basement wall
> will transmit some moisture and yet it is still ASHRAE-classified as
> an effective (but not perfect) vapor barrier.
>
> Enough moisture can penetrate a concrete basement wall, that it can
> contribute to a moisture problem.

[snip]


As with any other type of wall, moisture transport via vapour diffusion  
through a cast-in-place concrete wall is seldom if ever an issue even if  
it weren't vapour impermeable.

Concrete will suck up and hold moisture, whether that moisture be in  
liquid or vapour form and once in the concrete, since Ma Nature is very  
democratic, She will of course move that moisture around to even things  
out.

Wet earth on one side of the concrete ... silly humans (who are  
incidentally ~97% water themselves), trying to stay dry on the other.

And up until quite recently here in Ontario, the only thing that the Codes  
required to prevent that moisture movement through the concrete was a thin  
coating of bituminous goop sprayed (or on rare occasions, brushed) onto  
the exterior surface of the concrete.

And as we all know, concrete likes to crack every 10 feet of so no matter  
what you do.
(You can stick rebar in the concrete to provide tensile reinforcement but  
that tensile reinforcement does not come into play until after the  
concrete has cracked.)

Control joints will of course alleviate the stresses induced by  
temperature & shrinkage movement but when was the last time you saw  
control joints in a residential cast-in-place concrete foundation ?

Leaks through cracks in foundations (with that Code-required  
"waterproofing" in place were one of the most common issues that the  
Ontario New Home Warranty program had to deal with in years past and the  
Codes finally smartened-up and required that a drainage layer be provided  
to relieve hydrostatic pressure on the damp-proofing membrane.

I can say with some certainty that the old homes in Nova Scotia where Kim  
was contemplating basement retrofits (and old homes anywhere else for that  
matter) will not have that drainage layer present.

(At this point maybe it would be useful to mention that our very own  
beloved Don Fugler was known as "Mr. Leaky Basement" in building science  
circles (and squares, hexagons etc) for quite some time but you'll have to  
talk to him about that.)

While the amounts of water moving through the apparently-dry concrete  
walls may not be sufficient to cause puddles or flooding, I suspect that  
if trapped by a vapour-impermeable layer somewhere in the retrofitted  
interior insulation profile, accumulations would likely be significant  
enough to cause rot of any cellulosic materials in the construction, quite  
possibly rust of any rust-susceptible metal components and quite likely,  
compromised R-values of any insulation that does not have a capacity to  
self-drain.


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
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