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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

Autumn Skye
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3 Answers3

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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.

Ecnerwal
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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.

RibaldEddie
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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/

Fresh Codemonger
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