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Old 05-27-2012, 08:56 AM   #1
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vapor barrier confusion


my understanding is that the vapor barrier should be on the warm side of the wall/whatever. around here, that would be the inside. correct ?
ok lets say we have that. but i see it recommended that rigid foam on the exterior should be used. wouldn't this create 2 vapor barriers ? = not a good thing.

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Old 05-28-2012, 12:00 AM   #2
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vapor barrier confusion


Many exterior foams are not class 1 vapor retarders.

The other benefit of foam on the exterior (if done to sufficient R-Values) is that it raises the temperature of the studs and eliminates the potential of condensation.

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Old 05-28-2012, 01:23 AM   #3
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vapor barrier confusion


http://docs.google.com/viewer?a=v&q=...4EyM707m0gPdXQ

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Old 05-28-2012, 09:33 AM   #4
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vapor barrier confusion


Quote:
Originally Posted by Windows on Wash View Post
Many exterior foams are not class 1 vapor retarders.

The other benefit of foam on the exterior (if done to sufficient R-Values) is that it raises the temperature of the studs and eliminates the potential of condensation.
even if that foam in on the inside of the exterior wall ?
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Old 05-28-2012, 02:54 PM   #5
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vapor barrier confusion


Your other post had a better description of the wall make-up, foamboard outside eliminates any vapor retarder inside. pp.16; http://www.buildingscience.com/docum...ting-sheathing

In winter the f.b. inside the wall leaves the sheathing (T&G boards) colder and possibly wet due to exterior leaks, and stops any diffusion (minimal) and air leaks (ADA the drywall) at the inside of cavity, rather on outside the sheathing, on the WRB/f.b. interface. In summer, with moisture drive reversed, the warm outside air would condense on the f.b. in the cavity, rather than outside sheathing; http://www.buildingscience.com/docum...apor-retarders

With the f.b. outside, the R.H. would be controlled by the HVAC, treating the cavity as conditioned-keeping any moisture completely out of the cavity - wetting the framing to rot, pp 3: http://www2.dupont.com/Tyvek_Weather...20Bulletin.pdf
In summer, the f.b. would keep the sheathing cooler (closer to inside temp.) and stop the moisture drive to the inside, if thick enough.

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Old 05-28-2012, 08:24 PM   #6
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vapor barrier confusion


so, what your saying is = i shouldn't put the foam board on the inside of the sheathing ?

btw, i am insulating for the winter. summer ac usage is not an issue.
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Old 05-29-2012, 12:33 AM   #7
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vapor barrier confusion


Putting the f.b. inside keeps the sheathing colder- last paragraph; http://www.greenbuildingadvisor.com/...s-and-ceilings

Here is the one I was looking for earlier: http://www.buildingscience.com/docum...r-requirements

As WoW said in the first answer, f.b. outside is safer, it's very hard to quality seal the cavity and the exterior against air/moisture; http://www.finehomebuilding.com/how-...de-or-out.aspx

It can be done if carefully detailed, some builders use it; http://www.greenbuildingadvisor.com/...id-foam-option

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Old 05-29-2012, 10:00 PM   #8
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vapor barrier confusion


well, i have a lot to chew on.

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