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I recently had a tankless water heater installed in my basement. The 4" exhaust vents through the rim joist. I'd like to seal the gap around the PVC, but I'm uncertain what type of putty to use. I'm in climate zone 5b, so I need something that won't crack to about 10 degrees below zero.

What kind of putty should I use to seal this gap?

tankless water heater vent

view from outside

My original plan was to use the same putty that the electricians used for the weatherproof conduit into my basement, although I'm not certain what type of putty was used, and whether it can be used on wood in addition to concrete block (see image below).

weatherproof electrical conduit

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    I would try to reduce the gap first. Start with a 8"x10" (depending on the size of your joists) piece of plywood. Cut a 4" hole in the center of it. Then saw the plywood in half down the center of the hole. You now have two pieces of wood with a semi-circle in each one. Place them around the pipe and fasten them to the rim joist. You should have a much smaller gap that can be more easily filled with a range of materials.
    – SteveSh
    Oct 3, 2022 at 16:03

3 Answers 3

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That kind of putty is called “duct seal” and you can get it pretty much any hardware store, big box hardware stores, electrical supply shops, etc. it will work fine.

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    Yes, this is the stuff to use. Roll it into a long cigar shape, wrap it around the vent and stuff it in as far as you can. Do this numerous times. +1
    – JACK
    Oct 3, 2022 at 16:05
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So the water heater installers had a beaver or squirrel on the crew to gnaw a hole, rather than using hole-saws like competent workers? Ugh.

I'd wrap that tightly in a scrap of plastic (housewrap or whatever) and sprayfoam it, perhaps after taking @SteveSh approach on the "show" side. The plastic will let the pipe slide in the foam, rather than being stuck to it. The foam will expand, stick to the gnawed wood and one side of the plastic wrapping, and seal the gap.

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  • "So the water heater installers had a beaver or squirrel on the crew to gnaw a hole..." Well, when your only tool is a pick axe...
    – SteveSh
    Oct 3, 2022 at 17:55
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    If your only tool is a pick-axe, I expect to see better pick-axing than that, unless it's also your first time using your only tool...
    – Ecnerwal
    Oct 3, 2022 at 22:39
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    While I don't appreciate the snarkiness of the comments, I am disappointed with the result. I had arranged for a 4" hole saw to be used for this, and I don't know exactly what they did here. I added a view from outside for reference.
    – Parker
    Oct 4, 2022 at 2:07
  • @Parker - ask for a discount, that is a horrible job.
    – JonH
    Oct 4, 2022 at 19:40
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    @Parker It looks like the collar of the right-angle joint is slightly inside the wall. I'll bet they used a hole saw large enough for the pipe, later discovered they cut the pipe a smidge too short and now the hole had to be large enough for the collar as well, and then "enlarged" the hole using whatever tool they had on hand, with the pipe still in place and limiting their working space.
    – bta
    Oct 4, 2022 at 19:49
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You should seal it differently on the inside and outside. The outside your primary goal is to prevent bugs and liquid water. The inside you want to maintain your air barrier. 475 has good passive house gaskets that can be used to seal around pipe penetrations.

https://foursevenfive.com/blog/5-steps-for-sealing-intentional-holes-in-your-air-barrier/

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  • Thank you for the detailed response. I will definitely refer to this when I seal it up.
    – Parker
    Oct 4, 2022 at 2:09

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