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Community and Q&A

Caulk Window Trim/Extension Jam?

scottwoodward | Posted in Green Building Techniques on

I’ve added 2″ of polyiso exterior insulation on top of ZIP sheathing, which is my WRB. The windows are flashed properly with beveled shingle and ZIP stretch tape as the sill pan. The polyiso overlaps the window flanges and flashing tape.

I have framed out 1″ channels that will be for picture frame trim/extension jams (windows are installed in “innie” configuration). Even though I’ve got the windows flashed properly, I’m wondering if I should caulk the inside joint/seam between the exterior insulation and where it meets the flashing tape?

On the one hand, not caulking it would allow water to run down behind the insulation and along the designated WRB. On the other hand, my gut and inclination is to always keep water away from the windows flanges.

What advice would others out there offer? (see photos)

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  1. Expert Member


    Though I think I'm looking at the right thing, could you throw a big red ellipse or point an arrow to the joint you're talking about in one of those pictures?

    1. scottwoodward | | #2

      I uploaded an edited photo that shows the joint I'm talking about -- but it would be all the way around the window where the exterior insulation meets the flashing tape that's over the window flange.

  2. maine_tyler | | #3

    I've never built with exterior rigid foam, but when the WRB is inboard of the foam, doesn't the foam effectively need to be treated as a secondary wrb in terms of water management? In other words, all water needs to be pushed exterior of the foam at any seam/opening as there is in your situation. No? I wouldn't be comfortable letting water enter that space which has significantly reduced drying potential compared to outboard of the foam. Your drainage plane in this case is outboard the foam, not immediately adjacent to the wrb. So it seems to me.

    In the below linked article, Martin suggests sloping the foam at the sill and then using peel and stick to seal the edges of the foam. Peel and stick seems a better option than trying to caulk foam, but obviously you want something that will stick to foam.

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