Buy the sheet metal boot that converts from 3x10 rectangle on the top of the otr microwave to 5 or 6 inch round duct.
Venting microwave through wall stud in way.
If a floor joist or a beam is massively in the way you ll need some custom ducting or cancel the job entirely.
My microwave oven is mounted to an exterior wall above the stove.
If the two rectangular ducts have a combined area greater than that circular one you wont have issues with flow restriction.
Total distance is about a foot performance will be barely compromised.
A stud in a wall supporting or otherwise can be cut and boxed.
Your proposed opening for the exhaust is pretty close to the underside of that cabinet and the header must go above the exhaust vent.
I fixed it so that now it vents the kitchen smoke and odors through the wall and outside.
Re install trim or siding.
From microwave up into the cabinet is what you have now.
Think of an turning into a line with a box in the middle.
Install exterior vent hood utilizing proper exterior installation techniques applicable to your siding material.
It appears when this house was built provision was made for venting out the rear of the microwave into the standard 2 4 wall up the cavity inside the wall then up through duct work in the attic through the roof.
Use 3m flashing tape or vycor or tyvek tape to assist ensure that water will not enter the building envelope.
It is designed to work like a hood.
The manual for the microwave says to absolutely not use flexible ducting.
I saw this done on this old house when they installed a new microwave hood on an outside wall.
Photo of unit installed it was heavy and bulky.
Basically i would start by breaking out the drywall around the proposed opening and that stud in the middle.
Just use flexible dryer vent and be done.
First double check that your floor joists are lined up well enough that you can get a 10 duct through the floor where you need it.
I d cut a generous square at the bottom of the wall so you can cut out the plate in the stud bay.
Option 1 results in a very twisty exhaust path while option 2 is obviously a ton of work.
If you don t want to cut the stud install 2 rectangular ducts on each side of the stud and split the original duct into two.
This usually takes two people.
Sacrifice the cabinet above the stove by snaking an exhaust duct up from the hood over to one side of the stud and then through the wall.
Get a dryer vent going from outside to inside of the house.
Basically tear down the wall to install a header saddle jack studs etc.
Can anyone think of any other options.
Attach two flex elbows and steer around the duct and straight out the wall.
But the builder just left the microwave s fan blowing back into the kitchen.
Connect it all with a piece of flexible dryer vent hose.
Line up microwave to duct and connect by placing it on the wall bracket or connecting bolts above.