[Strawbale] concrete wall vapor barrier

[Derek Roff]

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.  But in most cases, not enough can 
penetrate the wall, for it to be a useful contributor to a drying 
strategy for the basement.

This is a common building situation.  Many installed 
materials/systems can let enough moisture in to cause problems, 
without allowing enough moisture out, to cause significant drying. 
Another example important in SB homes is the interior air barrier. 
Small discontinuities are common in the interior air barrier (often 
plaster), at electrical boxes, posts, and floor and ceiling joints. 
These discontinuities allow air movement, which can bring a lot of 
moisture into a wall.  At the same time, by themselves, these 
discontinuities don't allow any significant drying of the excess 
moisture in that wall.

Derelict

--On Tuesday, November 27, 2007 4:03 AM -0500 
strawbale-request at listserv.repp.org wrote:

>> On 2007, Nov 26, at 20:22, GuyW wrote:
>
>> gee...millions of dollars has changed hands in SoCal over vapor-
>> emissions
>> thru concrete....There are standard sodium chloride dome tests to
>> quantify
>> vapor transmission thru concrete...guess not, tho...
>
> Speireag Alden replied:
> Looking in the ASHRAE Handbook, it appears that concrete is 3.2
> perms   per inch of thickness, which means that a quality four-inch
> slab is   about .8 perms.  Anything under 1 perm is a vapor barrier.
>
> An eight-inch concrete wall could be .4 perms.
>
> So, yes, concrete can be a vapor barrier.


Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu