# where to put vapor retarder



## Tom Struble (Dec 29, 2008)

oo...if i was you i wouldn't do any of that:no:


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## biglurr54 (Jan 9, 2012)

How would you insulate an attic with polyiso foam. I already have the material.


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## Windows on Wash (Aug 30, 2011)

I am having a real tough time discerning what you are referring to here.

Can you provide a picture or try to be a bit more clear?


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## Maintenance 6 (Feb 26, 2008)

biglurr54 said:


> I am putting insulation in an stock that has no vapor barrier. I will be adding a vapor barrier before I add the insulation. I will be adding 8 inches of oily ISO foam that will rest on the attic floor jiosts. I was planning on putting the vapor barrier over the attic floor joists, staple and tape everything. Then I will lay the foam over it. Will this rot out my joists because the moisture will be held there by the vapor barrier.


Can you clear up the words in red? 
The iso probably has a facer on it that will be enough vapor retarder. You're probably going to get more bang if you cut the iso to fit between the joists and get it down to the ceiling level. Then you won't have that huge air gap.


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## biglurr54 (Jan 9, 2012)

Sorry the auto correct program changed the words on me. The first word is supposed to be attic and the second is Poly. 

The foam I got its second hand from commercial roofs so the foil membrane isn't in the best condition. Should I lay a vapor barrier down so it goes inside the bays between the joists, then over the joist, then into the next bay. Then cut the foam to fit and lay it in the bays? I'm concerned this will trap moisture on the joists and eventually rot them out.


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## Maintenance 6 (Feb 26, 2008)

I would get some foil tape and patch the facer on the iso. I don't know where you are, so I can't answer much on a vapor retarder. I know you do NOT want to go in and out of the cavity with poly. Where it runs across the tops of your joists, it will be uninsulated and on the cold side, so you'll have condensation along all the joists.


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## Gary in WA (Mar 11, 2009)

In agree, no poly up there at all. 
1. Will you be covering the joists or going inside? Fiberglass in the cavity?

http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

2. Where are you located?

Gary


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## Bob Mariani (Dec 1, 2008)

A vapor barrier only is applied to the ceiling joists prior to drywall (attic floor) on the warm side of this structure. No vapor barrier is to be used atop the joists. What is important is an airtight attic floor for any insulation to work. (read my how to doc on attic insulation posted in the how to section of this site). Any insulation placed in the attic needs to be tight against the drywall to be effective. If you want to cut your foam board and insert it between the joist bays, you need to cur around any wiring or plumbing and foam all the edges with spray foam. A very time consuming project and not very cost effective use of time.


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