Rectangular metal duct connects the holes in my kitchen ceiling and exterior wall. I assembled the duct out of three-foot long flat sections, folding two pieces into ell-snapped walls and joining those to form a section of duct. To join the two sections I needed together, I used S-cleats on the long sides drive cleats on the short sides as outlined here. I then sealed the seams with mastic, which seemed to take forever to dry. For the long seams, I opted to use a high-quality foil tape, an approach I’ll use for any future duct work I do given its ease of use and instant gratification.
Once the duct was assembled, I carried it outside and slid it into the exterior hole, taking care to avoid disturbing the plaster ceiling. A vent cap finished the exterior. At the other end, I installed a 90-degree square to round adapter and ran a short section of 6″ round duct to the microwave. Venting to the outside makes a world of difference in the kitchen, removing humidity and odors in a way that our old recirculating fan couldn’t.