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Community and Q&A

Best way to seal seams in foam board?

Ine | Posted in Green Building Techniques on

We are building a vented roof over 16″ parallel chord trusses that will be filled with dense pack cellulose. The builder put 2″ tongue and groove XPS foam on the underside of the trusses as a thermal break. This will have 1x strapping as a base for a T&G wood ceiling. I want the foam to provide the air seal for the assembly but I’m not sure I trust tape to provide a long term seal on the foam joints. What is the best way to get this air tight? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    I've had good success with red contractors' tape like 3M Builders' Sealing Tape.

  2. Riversong | | #2

    I agree with your skepticism about the long-term reliability of any kind of tape. If it's possible to remove and re-attach the foam board, I would suggest either a foam-compatible adhesive caulk or low-expansion spray foam applied to each joint, particularly the non T&G butt joints. Using two layers of 1" foam board, with staggered and sealed joints is an even better way of guaranteeing a good air barrier.

    Fortunately, your vented roof (if it's done properly) will allow good drying to the exterior and, since dense-pack cellulose is highly resistant to air movement (which is why it qualifies as a fire-stop), the roof assembly should be able to tolerate a less-than-perfect air barrier.

  3. Ine | | #3

    Thanks for the responses but questions all around.. Is the 3M builders' Tape essentially the same type of product as Tyvek tape? I don't see the red stuff in my area. Would the PL polyurethane caulk I'm using be foam compatible? Being a belt and suspenders kind of person, I might caulk the seams and tape once the caulk is dry.

  4. GBA Editor
    Martin Holladay | | #4

    There are plenty of sources of contractors' tape online, if none is available at your local lumber yard.
    The Venture tape is similar to the 3M tape. It's available from Shelter Supply.

    Here's a link to 3M All Weather Flashing Tape -- that might be the new name for Builders' Sealing Tape.

  5. Riversong | | #5

    Tyvek tape is designed to work with one product: Tyvek (and similar) housewraps. The red Venture tape or contractor's sheathing tape ( is designed to be compatible with housewraps, XPS, polyethylene vapor barriers, as well as various sheathing products. If you must tape the seams, there's probably not a better product.

    But tests have demonstrated the vulnerability of tapes on housewraps. In tests under water pressure, half of them failed to maintain adhesion. It's anyone's guess as to the longevity of tape on XPS.

    The 3M All Weather Flashing Tape is a self-adhering flexible fenestration flashing, made to compete with the asphaltic and butyl flashing tapes. It's a different beast.

    I would be careful with polyurethane on XPS, unless it's specifically approved for that application. Polyurethane caulks don't bond with polyethylene vapor barrier material. PL used to make PL300 specifically for foam board application, and I believe Liquid Nails is foam compatible. PL Premium may be foam compatible. But caulks require a shallow gap in order to "key" into and maintain the correct section ratio in order to remain flexible. Simply smearing some onto the surface won't guarantee long-term adhesion.

  6. Ine | | #6

    The t&g joints are pretty tight but I believe I can run the tip of my foam gun between the ends of the panels at least a half inch in and seal with low expansion foam and then trust the Venture tape will do its best on the t&g sides.

    PL does still make a foam board adhesive caulk. Saw it in a shop last week but don't remeber if it is still labeled PL300.

    I really appreciate all the help I get from this site.

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