My DH and I are building a home in Montana and appricate any recommendations for mixed heating needs in an atttached garage.

The garage will be 24'D x 45'W x10'H overall and the walls are spec'd at R27 and ceiling at R50.  There are 4 insulated roll-up doors and two 12 sq. ft. insulated windows.  The garage will have one stall walled off from the rest of the area to use as a workshop.  The soil type is powdery sandy-loamy stuff and ground water is about 100' down.  There will be 2" thick polystyrene insulation around the slab perimeter and 4' inward. 

A 95% gas mod/con boiler with floor radiant will be used for the house heating plant. It can be upsized for the additional garage load. 

So...

What we need is enough heat in the whole garage to sustain it around 35-40 degrees during those cold snaps.  Maybe some minimal amount of radiant tubing for the entire slab??  The workshop area will be infrequently used but a means of quick warmup would be nice.   So will some kind of separate hanging heater with forced air gas or a boiler-driven hydronic/fan unit work best or would a ceiling-mounted gas radiant-tube unit work better?

What are our heating options, should the slab be continuously insulated underneath and is the insulation package balanced for our needs?  What tube spacing is recommended for the slab?

Thanks in advance,

T