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Community and Q&A

Sealing edges of polyiso

Timothy Hargrave | Posted in General Questions on

I live in climate zone 6 and I am installing 2 layers of 1 inch polyiso on my house exterior and I will
also install 1×4 over the polyiso. Should the exposed edges of the polyiso at the bottom starter course be sealed and if so how should this sealing be accomplished. I have the same concern
where my porch roofs meet the main structure. Any drawings or advise would be great.

Tim Hargrave Upstate NY

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should the exposed edges of the polyiso at the bottom starter course be sealed?"

    A. Yes.

    Q. "If so how should this sealing be accomplished?"

    See these two GBA videos:
    Video: Superinsulating a Home With Rigid Foam

    Video: How to Install Rigid Foam Insulation Outside a House

  2. Timothy Hargrave | | #2

    Martin Holladay,

    Thank you for your prompt reply, the video links were helpful. However, they only show the easier areas with regards to starting out the foam layers. Can you provide me with a link that shows the sealing/flashing details where the polyiso transitions around the drip edge/facia area on a porch roof. Also, to seal the polyiso edges, would a peel and stick window membrane be an acceptable alternative to the bent flashing approach shown in the video.

    Thank you for your time,
    Tim Hargrave

  3. GBA Editor
    Martin Holladay | | #3

    I hesitate to describe details in writing, since I'm not sure whether you have the experience to understand basic flashing methods.

    Peel-and-stick membranes are not designed to be exposed to the weather. They must be protected from UV light.

    If you are installing polyiso on the roof, you need to install a lumber frame at the perimeter of the roof -- eaves, rakes, and ridge -- equal in thickness to the thickness of the foam. This lumber frame can be covered with a fascia at the eave and with rake trim at the rakes.

    Of course, every roof needs dripedge flashing at the eaves, and the dripedge should be installed in a way that prevents or minimizes dribbling down the fascia.

  4. Aj Builder, Upstate NY Zone 6a | | #4


  5. Timothy Hargrave | | #5


    My question was misleading or unclear, sorry.

    My house is already covered with Tyvek and the porches have been built and the roof has been properly shingled and step flashed. ( no polyiso on the roof ).The fascia has been covered with 5/4 Miratec trim. Flashing the the area where the porch fascia trim meets the house (at a right angle) was done according to an article in Fine Homebuilding.

    My area of concern deals directly where the polyiso, applied to the house, butts up against the porch fascia at a right angle. The water that may "dribble down the fascia" is what I want to seal the polyiso from. The bent flashing technique in the video appears "clunky" if used in this fascia detail area. Any alternative ideas?

    Thanks again fo your comments,
    Tim Hargrave

  6. GBA Editor
    Martin Holladay | | #6

    I'm still confused.

    A fascia is a vertical piece of trim.

    A soffit is a horizontal piece of trim.

    If "the polyiso, applied to the house, butts up against the porch fascia at a right angle," then it sounds as if the polyiso is horizontal. So what element of the house is the polyiso being applied to?

  7. Timothy Hargrave | | #7


    In the video link that you sent me, the workers are applying the polyiso on the outside of this house. My project is similar but my house was first sheathed with plywood and covered with Tyvek. I want to apply the polyiso over the Tyvek.

    Immagine the video workers applying the polyiso flat on the exterior and then working around a porch roof ( that has already been shingled and the fascia has been covered with trim), this is my area of concern. If I am still not clear, then disregard my question.

    Thank you,
    Tim Hargrave

  8. GBA Editor
    Martin Holladay | | #8

    Is this what you are trying to say?

    1. The polyiso is being installed on a wall, not a roof.

    2. The polyiso on the wall intersects a porch roof located below the wall with the polyiso.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Tim, best way, remove facia, roofing etc so as to properly bury foam behind all. Since you most likely will not do this then you will have to make up a system that somehow does the job of shedding water. You could tape over raw edge of foam to keep water out of it. You could cut into facia to add flashing that you forgot. If you have someone who is an expert chimney flashing type, he should be able to make all work out for you. Flashing is really the same in principle no matter what materials and location is involved.

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