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Are tar paper and Tyvek a vapor trap?

CKN | Posted in Green Building Techniques on

Can you please set me straight because I am getting different points of view on this. My home is 40 years old, has the old type of sheathing, a layer of tar paper and then T1 11 siding. In patching one side of the house, I have put 1/2 inch foam board over the T1 11 and then Tyvek. Is this a vapor trap? If so, what part of it is?

Is the use of tar paper and then a layer of Tyvek on the outside of the T1 11 a vapor trap?

Is the use of rigid foam board a vapor trap?

Thanks so much!

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  1. GBA Editor
    Martin Holladay | | #1

    In most cases, including yours, there are no problems associated with two layers of housewrap (for example, asphalt felt and Tyvek). Both types of WRB are somewhat vapor-permeable. (Asphalt felt is a so-called "smart" vapor retarder with variable permeance.)

    A problem might arise, however, from the use of exterior foam -- depending on your climate zone and the type of foam. In your case, a vapor-permeable foam like uncoated EPS would not be a problem; however, a foil-faced foam would be a problem, especially if you live in a cold climate. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. CKN | | #2

    Thanks for the response. I live in Chicago. Because I was patching areas of my home, I have already put the 1/2 inch Foamular product and then Tvek, both over the existing T1 11. Foamular claims their product has the same permability as OSB board.
    I did read that post and am still not sure if I should tear the siding off my house. There are areas that have a plastic vapor barrier on the interior walls, over Roxul (which is what they recommend). If it is definite that I should tear out the Foamular rigid board, I would love to know so I can get that done. Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    C.K. North,
    No, you don't have to remove the siding and the 1/2-inch Foamular. Your wall will be OK, as long as you do a good job installing flashing to keep rain out of your wall.

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