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Detailing this Exhaust Duct Penetration

rondeaunotrondo | Posted in GBA Pro Help on

I wanted to see how folks here would detail this penetration. I am ducting my own ERV (CERV2) for both fresh/exhaust air. This will likely be fresh air intake. I would mount on a pvc block although finding one that will fit a 6 inch duct is challenging.  Layers from the outside include vinyl siding, a flimsy soil layer, cedar shakes, some sort of paper membrane, shiplap sheathing. I have 3M Liquid flashing in a sausage gun as well as foil tape. Not opposed to buying high quality tape but seems like liquid flashing would work best. Please see attached picture. There was an old dryer vent already here that I previously decommissioned.  I have my ideas about doing it but I wanted to see what other people thought. (Pics are rotated when published for some reason)

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  1. Expert Member
    PETER Engle | | #1

    There's no real way to make a 100% waterproof detail for a wall like that without stripping the whole area and replacing. It doesn't even look like there is a real WRB anywhere in the system. Your best bet is probably to form a Z flashing and tuck it up behind the shingle layer. I would make the square hole in the siding layers a bit larger with a high-speed oscillating saw to start, and make the horizontal leg of the Z flashing slope down to the exterior. Install the Z and caulk the sides to try to force water out of the wall system. Install a 8"-10" PVC block, bedded in caulk around all four sides. Use a holesaw to drill a proper sized hole for your duct. Finally, install the duct and hood, with an extension if necessary to get all the way through the wall. Another bead of caulk around the top and sides of the hood. If you can get to this area from inside, use spray foam to make the air seal. This isn't a great solution but it is probably better than the other penetrations in that wall.

    1. rondeaunotrondo | | #2

      Thanks Peter. Would you put the pvc block (tough to find ones this big) against the cedar shakes or against then sheathing?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        You can also use a piece of 2"x cedar or PT lumber instead of PVC. It should go on the sheathing.

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