Sealing Vent

[titmouse]

When I went to replace an over-the-range microwave, I discovered that the old exhaust vent vents into the attic so I will set the new microwave to recirculate (the amount of cold air coming down the exposed vent at 5am is surprising). What's the best way to seal the old vent? Plug it with insulation at the attic end? The vent is 4 feet tall with no elbows.

[HayZee518]

plug it with insulation and use real sticky duct tape to finish sealing it.