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
kavisiegel t1_iuhor56 wrote
Reply to Insulation around Microwave hood vent by RatArmy
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