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Taping Exterior Polyiso Foam-Board Joints

George_7224612 | Posted in General Questions on

I’m using 1″ foil faced polyiso exterior insulation.  Sheathing, WRB, polyiso, plastic siding.  Is it reasonable to use Tyvek tape to tape the joints or should I consider something else?  How good is that stuff?

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  1. Expert Member
    BILL WICHERS | | #1

    I like using foil duct tape for this purpose, partly because it works well, but also because it gives a clean look (my inner engineer coming out I suppose, since it won’t be seen for long before getting covered up with something else). I’ve occasionally also used a 3M flashing tape.

    All you really need on foil faced polyiso is a good tape that is made to be permanent and sticks well to the foil facing. If you were using XPS your tape selection would be much more critical, since tapes don’t like to stick to XPS as well. With foil faced polyiso, taping is usually pretty easy — just make sure the surface is clean. Nearly any good quality tape made for permanent applications will work well.


  2. Expert Member
    Akos | | #2

    I wouldn't worry too much about taping the foam. The important item to tape is the sheathing underneath. Make sure to use a quality tape for that.

    With thin foam, generally it is best to leave the sheets un-taped as the small gap allows for a bit of drying of any water and condensation that makes it behind the foam. Taped foam is more important for roofs and walls with thick rigid insulation.

  3. George_7224612 | | #3

    Thanks, gents.

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