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Community and Q&A

Penetrations in exterior foam and rainscreen

otter1492 | Posted in Green Building Techniques on

I will be installing 3 or 4″ of exterior foam with a 3/4″ rainscreen and installing new windows and siding. This will be my first retrofit of this type.

I have most of the details figured out for the windows and doors but I am having issues visualizing how exactly to handle penetrations. the locations that lights, outlets, hose bibb, dryer venting etc come through. Does anyone know of any good articles or have advice on these details. I understand that I may have to consult with my electrician for the electrical but he is kind of bewildered by the whole thing. Anything will help. thanks in advance. I am in st paul, MN.

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  1. GBA Editor
    Martin Holladay | | #1

    You will find this aspect of your design to be easier if you use trim pieces that are designed for walls with rainscreen gaps. One company with a good assortment of exterior trim pieces designed for rainscreens is Primex:

    Primex Manufacturing Ltd.
    20160 92a Avenue
    Langley, British Columbia
    Canada, V1M 3A4
    Tel: 604-881-7875

    Here are links to two relevant web pages:

    Primex - Outdoor wall caps

    Primex - Trim plates

  2. otter1492 | | #2

    Thank you for the response. In the case that I want to use the same trim as the rest the house, is there any links or anything that would point me in the right direction? Also, for the electrical, am I ok just poking the wire out and flush mounting an exterior electrical box to a trim piece assuming it's waterproof all the way to the foam?

  3. Keith92705 | | #3


    Take a look at Matt Risinger's blog. He covers many penetration issues. They may not be exactly the same, but you may see many similar applications (search "penetrations" on his site).

  4. otter1492 | | #4

    Thank you.

  5. DIYJester | | #5

    Check out Roxul's PDF install guide. It's a good 60 or so pages, with the last 30+ being finishing details for these sorts of things. Not all apply directly since they use the WRB at the sheathing. Most of the details simply use smaller pieces of 2x rainscreen furring around the penetration.

    You can use peel and stick to flash the penetration similar to a window at the WRB. I am doing 4" of foam also. For electrical boxes, I mounted them to the top of the rainscreen furring and mounted them flush to the trim. I then created another layer of peel and stick flashing similar to a window to shed water off of electrical box tops if it were to get wet.

  6. Expert Member

    Lucas Durand's blog is worth looking at too.

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