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Old 03-09-2011, 08:57 PM   #1
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Vapor barriers and wall design

At the site there is a paper titled 'Vapor barriers and wall design'. Here is a link to the paper.

Vapor barriers and wall design

I have read, re-read, and re-read this but still can't figure out if my wall design for my climate is ok, or if I need additional insulation on the outside of the sheathing, and whether or not I need a class II or class III vapor retardant paint over the drywall.

The climate is western Arkansas, a mixed-humid climate.

The wall from the outside in consists of:
Vinyl siding, Huber Zip system wall sheathing, 6" batt of Roxul insulation, drywall, vapor retardant latex paint.

I will be heating with wood and don't have any idea what the inside RH will be in the colder months, otherwise I might be able to use their formula and figure out what the dew point will be.

Also, the house will be air conditioned in the hot months and I do know the the outside humidity gets rather high sometimes.

I would rather not have to put any foam board over the sheathing to keep the inside surface from reaching the dew point, but will if it's really necessary.

Can anyone shed some light on this ?



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Old 03-10-2011, 12:57 AM   #2
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Is the Zip a water resistant barrier and air barrier? I did not see a WRB in your description, nor a rain channel (drainage plane) behind the siding, in case it leaks. I really don't think you want a class I VR with air conditioning and high humidity. The outside moisture may well condense on your cold VR, on the outside, and soak your insulation. I think it was the BS site that discussed a business in southern Ohio that ran into that problem on many new homes; folded the company right up. Keep the inside vapor open and air tight. The exterior should be 5x as vapor permeable as the inside; don't know much about Zip. Study that one.


Last edited by jklingel; 03-10-2011 at 01:05 AM. Reason: added info for clarity
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