Re: vapor barrier theory

[Speireag Alden <Joshua.M.Alden.91@alum.dartmouth.org>]

Sgrìobh Dewey Lindstrom:

>I've often wondered if humidity will seek and flow to holes in
>a vapor barrier, much the way heat will seek and flow to a
>void in your ceiling insulation or an area of the wall/ceiling
>with no thermal break.  Or let me put it another way, if you
>have a hole in your vapor barrier, like a door, which accounts
>for 20% of the wall, will 20% of the existing humitity pass
>through that "hole" or will the hole actually "draw" a higher
>percentage of the vapor?   (Does this question make sense to
>anybody?  It's a bit difficult to phrase correctly.)

     This is a very important question.  In cold climates or hot 
climates, you really need to know your vapor barriers from your air 
barriers.

     Vapor will not *diffuse* through small holes in a barrier, at 
least not in sufficient quantity to worry about.  A vapor barrier 
with a few pin-pricks is still well over 99% effective.  However, 
*air* will move through such holes, especially in the upper portion 
of a house, because of the difference in temperature between inside 
and outside, and stack effect.  And that *air* will carry vapor with 
it, in great quantity.  So an *air barrier* with a few pin-pricks in 
it is a compromised air barrier, and can lead you to moisture 
problems.

     John Straube pointed me toward a very useful article:  NRC 
Building Practice Note #54:  "The Difference Between a Vapour Barrier 
and an Air Barrier" by RL Quirouette.

     A few months ago I posted it in its entirety to this list, so you 
should be able to find it in the archives.

-Speireag.
-- 
Speireag Alden, aka Joshua Macdonald Alden
Usually found somewhere in the wilds of New Hampshire.
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