Re: foaming
I’ve owned a well-renovated house of about 2kSF for the last 2-1/2 years, done 7 years ago. They spray foamed the heck out of this thing. I have been *very* impressed with the energy performance. I am now a spray foam believer / advocate. They didn’t replace the HVAC when they did the studs up renovation, one of a few things I still find to be head shakers. The renovation was architecture-driven, infrastructure was not on the radar, other than the spray foam and cementitious siding. I’m planning on an HVAC upgrade to get zoning and efficiency, probably involving ducted mini-splits. Perhaps one unducted.
Anyway, I think the foam stopping air leaks plus getting all the duct into the insulated envelope are huge. Old house had ductwork in the attic above the fiberglass insulation and did poorly there. I predict you’re about to be very pleasantly surprised.
The other factor is to build for ease of replacement and most say that must be done in pairs of outside and inside units since so much time will have passed before failure that there will be incompatibility. I figure replacement time will come when I am so old and feeble someone else will have to do it.
Not a fan of the encapsulated attic owing to moisture and adding it to the building envelope goes against my sense, just more cubic feet to pay to climate control. Building Sciences Corporation has shied away from encapsulation for that reason and because it is a solution to a problem that should not exist, that being ducts in the attic.
There is a lot of opinioneering in dwelling insulation with very few instances of controlled testing. For all the DOE money spent since the 70s you would think there would be more or any.