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Should horizontal joints on EPS be sealed or not? Tyvek Bulletin

cloves2016 | Posted in Green Building Techniques on

Hi everyone,
Currently residing the house. I am using 1.5″ of EPS (fiber facing), Tyvek (wrb), 3/4 strips; a rain screen, and then the siding.

I have taped up all the plywood seams with 3m all weather flashing tape. Then installed my 1.5″ inches of EPS. I then taped up all the vertical joints with 3m flashing tape again.

However, I have not done the horizontal joints. I ran across a technical bulletin from tyvek that says that you should not tape horizontal joints because it allows any water that gets behind the foam a way to escape.

So my question is should I follow the bulletin or not?

Link to the pdf is on this page and titles below:

Weather Barriers are a Must with Exterior Foam Sheathing TECH TALK

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  1. GBA Editor
    Martin Holladay | | #1

    Before making your final decision, you should read this GBA article: Housewrap Tape Problems.

  2. GBA Editor
    Martin Holladay | | #2

    I don't think it matters too much whether you tape the horizontal seams in your EPS. There are arguments in favor of taping the seams, and arguments in favor of leaving the seams untaped.

    As long as you have a ventilated rainscreen gap between the back of your siding and your EPS, this assembly will be very forgiving.

  3. cloves2016 | | #3

    Hey Martin,

    Thanks for the reply. Great read, and I will be applying the tape personally so I will adhere to everything mentioned in the article to avoid the "reverse lap" water pocket.

    Consequently, after reading this article, if water did make it past the Tyvek (WRB) and somehow past the foam, even if the horizontal were not taped, wouldn't there have to be some sort of flashing to get that water to the front of the EPS? Thinking out loud, only to then meet the Tyvek. But then the Tyvek by design should allow the water vapor to pass thru correct?

  4. charlie_sullivan | | #4

    That article was a year and a half ago, and it concluded with the idea of double-sided tape, which wasn't available in the US at the time. Anyone know of any progress in making it available?

  5. GBA Editor
    Martin Holladay | | #5

    When I posted my first two responses, I was under the mis-apprehension that your rigid foam was installed on the exterior side of your Tyvek. Now that I have re-read your original question, I realize that the Tyvek is on the exterior side of the EPS.

    I think that you should tape the horizontal seams of your EPS. The Tyvek is going to intercept the (tiny) amounts of water that get past your siding, so you won't have any liquid water between the Tyvek and the EPS. If (through some strange circumstance) any water ends up there, it will easily evaporate, because Tyvek is vapor-permeable, and because you have included a ventilated rainscreen gap between the siding and the Tyvek.

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