We have a microhood, that is a microwave/exhaust fan, mounted over our range. Currently it is set to just recirculate the air. I would like to allow it to exhaust to the exterior of the home. The installation manual calls for 6" round duct.

First, can I use flexible round duct for this application?

Second, how frequently should the duct be supported?

I lucked out in that the center of the microwave is between two floor joists above, so running the ductwork shouldn't be too difficult. Also, the run should be less than 10 feet as the kitchen is relatively small. However, it is in good shape, so I'd prefer to do as little damage to the ceiling as possible (the kitchen is on the first floor of a two floor home, so no attic above the kitchen). Will it be necessary to open up the ceiling for the whole run of the ductwork or can I get away with just a couple of strategic holes?

Thanks!

First you should remove the hood/fan if it is a separate unit. If it is not a separate unit remove the microwave/exhaust. It may be built to exhaust out the rear or it may exhaust out the top. You may have an option. Remove the cabinet above the microwave. If it exhaust out the rear, remove the drywall on the wall and enough of the ceiling drywall to work. If it exhaust out the top only you will probably have to run the duct through the cabinet. If you exhaust out the rear, install 3.5 x 8 rectangular duct. You will have to cut the top plate. Hopefully there won't be a wall stud in you way. If there is, you will have to cut the stud and install a cross member to adjacent studs. Install the rectangular duct with a 90-degree transition. Carefully calculate where the opening through the exterior wall should be. That duct can be flexible or galvanized 6” flue pipe. You should be able to put the duct in through the exterior opening. However there may be electrical wires that you have to deal with. The ceiling drywall should be sufficient to support flexible duct. If you use the flue pipe you could support it with a metal strap nailed to the ceiling joist at the microwave and midway between the microwave and the exterior wall. Put you transition high enough in the ceiling joist so that you can reconnect you wall top plate with a piece of 2x4 or 2x3 nailed or screwed flat to the top plate to reconnect the plate.

If you exhaust out the top of the fan, cut out cabinet for either rectangular or round duct. Cut out ceiling drywall and install transition and duct to exterior. Install ductwork down through ceiling a few inches. Replace ceiling drywall. Reinstall cabinet then duct through cabinet