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.
I’ll challenge your cubic feet comment. Adding that attic volume really adds very little envelope load, the incremental roof area due to pitch and the gables. Windows are really your big envelope load. Getting that ductwork inside the insulated envelope is worth a LOT!
Some further comments -
I really like BSC. I came across that web site a year or so ago, when trying to figure out how to best insulate a basement. I’ve got enough of a technical background to generally follow along, find it worthwhile reading.
And ducts DO exist in attic spaces, it’s just the way things were done for many years, and insulating up at the roof joist level with foam is one of the ways to retrofit that existing condition for much improved energy efficiency.
Regarding DOE, Washington bureaucrats are horribly divorced from reality. You shouldn’t be surprised at the lack of research with practical results.