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kavisiegel t1_iuhor56 wrote

Spray foam it with low expansion foam (sometimes called low expansion foam) then come back the next day and trim it flush to the wall and the inside of the vent.

Then use some foil flashing tape from the pipe to the wall.

Then put some thick self stick foam gasket / door seal on top of the foil flashing tape

All 3 together gets you insulation, an air seal from the outside to the inside to prevent air blowing through the gap between the drywall and the vent, plus a seal between the vent and the microwave, to prevent air blowing through the damper and then down behind the microwave

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RatArmy OP t1_iui3k84 wrote

After looking at my set up, I'm not sure if the stick foam would fit around the vent without impeding it. Unless I found some thinner stuff. But I'll poke around.

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RatArmy OP t1_iuia8wu wrote

I assume non foil flashing tape will suffice? My local store has a bunch of 3M flashing tape, non foil I think.

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