Martin Holladay: some vapor barrier questions for Martin
Martin, First, let me thank you for some excellent discussion on a number of salient building issues.
I’ve read as much of your discussion on vapor barrier location as possible, but I remain puzzled by 3 comments that continue to resurface:
1) You believe that polyethylene film is not to be used as a vapor barrier: I could not disagree more; after 10 years of work in Minnesota, I’ve seen what can happen when VB’s are left out. All of the expert seminars at the AIA Convention insist that VB’s are essential in cold climates. After viewing their photos of buildings turned icicle, I must agree.
2) Some of your readers have implied that an exterior VB location is correct. This position is only correct in the lower Southern states where humidity outside the building is the culprit. Any northern or cold climate should have VB’s at the interior side of studs. Since the dewpoint in any wall system is within the system, moisture-laden air from the interior will drop when it hits that point. Thus any VB not to the interior is inviting moisture to condense in the assembly.
3) Some of your readers have referred to “moisture barriers” (Tyvek) as “vapor or air barriers”. This is not correct. Tyvek allows vapor and some air transmission. For this reason it can, and should, be placed on the exterior side of the sheathing without creating the dreaded “double vapor barrier”.
Thanks for the discussion,
Stephen Thompson, AIA
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