Is it acceptable to run 6" oval supply duct in exterior walls or is there a problem with it - insulation or other wise?
Unfortunately, its done.
Worse place to put it.
Contractor locator map
How many times must one fix something before it is fixed?
If it's for heating, you could lose a lot out the wall. If it's for cooling, and touches the wall, you could ruin it eventually. If it's the only place you can run it, I'd recommend spray foam between the duct and wall.