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There is a heater on the other side of the wall. From that heater a duct goes into crawlspace under the house (pictured below). There is quite a gap, which mice/rats have been using to enter/lease the crawlspace. What do I use to close that gap that would also be mouse/rat-proof?

Elsewhere in the crawlspace I had been using duct seal. Should I just use a large amount of it here as well? Would it melt and slide down, opening the gap partially, when it is heating season and the duct will be hot?

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Loki Kriasus
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    For mice/rats metal is usually the best with concrete the second. Mice/rats will chew though most other materials. Thin sheet metal is usually easy enough to bend and nail screw into wood or ducts for tight fit. – crip659 May 19 '23 at 11:30

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Spray foam would work well. Specifically, Pest Block has an additive that makes it undesirable for pests to chew through it. It is also rated for duct work as long as the heat does not exceed 240 deg.

RMDman
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  • Even that product recommends adding metal screen or steel wool inside of the foam for mice/rats. – crip659 May 19 '23 at 11:53
  • @crip659, this is from the manufacturer: A: Hello TAJohnson - Our Great Stuff™ & Great Stuff Pro™ Pestblock Insulating Foam Sealant products contains a distasteful agent that strongly discourages consumption of the foam and protects the integrity of the physical barrier, keeping pests outside where they should be. by| Apr 26, 2023 – RMDman May 19 '23 at 12:10
  • I found nothing that suggested adding any other substance. Perhaps an idea from a user but nothing from the manufacturer. – RMDman May 19 '23 at 12:12
  • In the questions about the product on the second page, you will find the answers about mice from Dupont and Great Stuff about using screening/steel wool – crip659 May 19 '23 at 12:22
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    Kudos to you for digging deeper. Still this is the best and easiest way to block that opening. – RMDman May 19 '23 at 12:26