Minisplit comfort in leaky house zone 4a

Hi,

I was able to go from about 5200 at neg 50 cfm to 4200 at neg 50 cfm by doing closed-cell foam flash and batt in my attic a few years ago. Finished area of the house is 1272 sq ft, but there is an unfinished basement and the walk-up attic. The recent analyst said the CFM is 13 per hour at that 4,200 leakiness number.

I had a 24,000 BTU hyperheat Daikin installed for the upstairs, but have not used it much for heating at all since all we have downstairs is radiators, so there were only a few nights where it seemed like it would be nice to have a little heat on but we wouldn’t be cold in the morning. It does seem to be able to warm up my North-facing room better than the radiator does. 🙂

The manual J for the upstairs done at the time said
Structure Btuh30468
area (ft²)599′
Volume (ft³)7490

I am going to have the rim joists airsealed with closed cell foam, and I am going to install an interior storm window on the one room that has an 100-year-old-plus original double-hung window that opens to an unconditioned little back porch area. (All the other windows have storm windows of various vintages; blower door guy said the ancient aluminum ones were actually more effective than the Larson Golds we bought four years ago)

My question is — with a house this leaky, do you think I could be as comfortable with floor mounted minisplits as I am with radiators? Our gas boiler is 1980s vintage, and I would like to get off gas eventually if possible. We have four radiators on the first floor currently, and the living room/dining room are open to each other, so I was guessing one where the radiators are in the living room and one in the kitchen would do the trick?

Or is my only option to change radiators to low-temperature radiators and get an air-to-water solution for the first floor? I know that’s much pricier.