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Old 11-25-2008, 09:41 PM   #1
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Exterior Foam Insulation Help???

I am about to vinyl side the back of my house. I want to install ˝” foam board on the exterior of the house directly against the new plywood sheathing that I installed. I am concerned that the moisture barrier of the Dow foam board will trap moisture and cause mold and mildew between the sheathing and the foam board. Should I be concerned about this problem? Owens corning makes foam board without the moisture barrier however the r-rating is a little lower, should I use this? Also should I use Tyvec and should it go under or over the foam.

The house is from the 1950s and has very little vapor barrier on the inside of the house except from the fiberglass insulation with craft paper that is between the studs. I did contact Dow and Tyvec but did not get any real answer.
Thank you for all your advice!
Last edited by teachtech; Today at 05:16 PM.


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Old 11-26-2008, 06:42 AM   #2
Mold!! Let's kill it!
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Unless it is foil faced, foam board is moisture permiable and will allow vapor to pass through it. I've pulled one inch foam board off of houses that had blown in cellulose insulation and no vapor retarder and found no problems under it. The glue in the plywood sheathing serves as a vapor retarder as does the paper facer and the fiberglass insulation.


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Old 11-26-2008, 08:55 AM   #3
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we install Felt-paper, or Tyvek house wrap over the sheathing before we install the Dow Supper Tuff-R or any insulation board when we do vinyl siding, the extra protection from the elements is the main reason we install it. the siding is not water proof, BOB
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Old 11-29-2008, 10:19 AM   #4
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use foil faced and tape up joints
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