I need some advice: I am in the midst of building a 2000 sq ft Timberframe w/SIPs home in Vermont. Full 1500 sq ft basement with radiant tubing on "creteheat". 2" foamboard on ext. of foundation. Marvin Integrity windows, triple glazed on north and east. Excellent passive. I have had suggestions from a couple of minisplits and a hot water heater with my radiant to a gas boiler/heater combo forced air "Space Pac" AC at 50k.It may sound strange that I am here without a plan, but it is true. Any help would be greatly appreciated.

minisplits are likely to work, but someone has to do an estimate of the heat loss. Are you signed up for energy Star? If not, you should be, and the ES program - REM-RATE generates some of those numbers. I assume you insulated the floor; is it also isolated from the footings and walls? What are the R values of your enclosure? SIPS work well since they are tight, but R value is important.

Re values: 30 in walls ( 6" neopor ) 40 in roof ( 8" of neopor). 2" of foam under basement floor ( in creteheat) and 2" of blueboard on exterior of foundation. Do you think that a propane water heater can handle both radiant in and domestic? I will take your advice and get a heat loss estimate. Thanks

Yes heat loss calc is very important and generally done before you pour radiant tube floors. The lower your radiant design temp (i.e. 80 vs 130F) the greater your number of options in heat plant (and the lower your energy cost).

If the radiant is just for a (non-walkout) basement you probably don't need to run the load calculation before putting in the tubing, but anywhere else you would. The heat loads of insulated basements are pretty miniscule even with 1' spacing the water temp requirements well be well under domestic-hot-water temps, even on design-day.

You'll definitely need to run a real heat load calculation for zones intended to be heated with mini-splits though. Too much mini-split results in cycling and lower efficiently during the shoulder seasons whereas not enough leaves you cold. Ideally you'd like the thing to be able to meet that load at the 99% outside design temp, but still be modulating rather than cycling during the average winter load. They modulate, sure, but the turn down ratio isn't infinite, (3:1 is typical, but it varies by model.)

It seems odd that you'd spring for R30 SIP walls (probably R20-25 after thermal bridging) which is way better than code, but go with a sub-code R10 for the basement walls. The IRC spells out R15 continuous insulation min for basement walls in climate zone 6 (which covers all of VT.)

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

The 2" (R10) Crete Heat is on the thin side for a high-R building too, but as long as it's on well-drained gravel it's not going to be a huge heat loss.

How many square feet of window (at what U-factor), how many square feet of SIP roof, and how many square feet of SIP wall (don't count window & door area- just the SIP), how many square feet of above-grade foundation & band joist, and how many square feet of door, and what type?

With some estimated U-factors on the SIPs & doors etc, and a WAG on the air infiltration, with a ZIP code (to be able to look up your weather data to estimate the 99% outside design temp) we'd have a rough idea of what it'll take to heat this place. I'd be downright shocked if it needed anything like 50KBTU/hr combi HVAC unit. If it's an open enough floor plan and the room-by-room heat loads of the doored off bits aren't to out of line there are likely to be some decent mini-split solutions.

Heating with propane would cost more than twice what it costs to heat with mini-splits, at VT's average propane & electricity pricing. In fact, with some lower-cost electric utilities heating the slab with an electric water heater or electric boiler would be cheaper. If you cheap out and go the electric water heater route for the basement slab you'll want to run the load numbers for the basement to ensure it won't impinge TOO much on domestic hot water performance.