I would like to replace a traditional, split system with a ductless mini-split. I’ve got a Manual J, have large open rooms, and exterior wall access. It looks like a really good fit. But what should be done about the space previously occupied by the duct work, both in the living space and outside of it?

My home is a “typical” Texas, 2 story house: two air handlers/furnaces inside a vented, unconditioned attic along with most of the duct work. Insulation that I can inspect is R-19 fiberglass.

Since most of the duct work is in the attic, it is readily accessible. One run goes from the plenum to a single point in the attic. It then connects to a single interior vent on the other side. After installing the mini split, I could remove the duct work there, patch the vent hole in the drywall, then replace the insulation.

But some of the duct work is less accessible or for me, less obvious how to fix. One of the very large return ducts is in a wall, runs for a bit inside the wall, then comes out at a different point in the attic.

Another run is between the ceiling of the 1st story and the floor of the 2nd story, supplying conditioned air to a 1st story room through ceiling vents. I can see where the duct run disappears into the ceiling/floor, but that’s about it.

So I am asking what should be done to the old duct work with respect to closing the envelope? Are there any short term options? My count is 21 supply vents, 2 large returns, with most on ceilings 10 feet high or higher. That would take me some time even if I could patch the inside and restore the insulation on the outside in every instance.

Thank you,
Matt