I have a 1940s detatched garage, 12’ x 20’ x 7’, outside of Boston. It’s 3 walls are 2 courses of brick with air gap, slab at grade floor, 4” concrete slab ceiling, 8’ x 8’ wood double carriage doors with large air gaps. 1 Harvey recent double hung window. 

Just guesstimating that the whole structure is R-2 across doors, walls and ceiling, it’s likely 400btu/degree hour + leaky infiltration puts it at (least) 25k @ 70/9F. 

Given I’d like to convert this to a workshop, keeping it at 50F throughout the winter and occasionally heat to 70 for work, curious peoples thoughts on heating it cost effectively. I can get away with a design temp more in the 20s as it’s unlikely I’d be working there on arctic days. 

How bad is working with dust with a ductless head? I have a Fujitsu AOU24k with an extra port available for up to a 12k head, so that seems like the easiest way to heat it. The rest of the heads are off for the winter so it would be running in 1 head mode, but likely never turn off at those levels. Dehumidification in the summer might be nice to have. 

Any other options if dust is a problem? I could work on better dust collection if it’s just a matter of cleaning the heads once or twice a year.

Resistance baseboard and just turn it on for the days I need it? 

Appreciate anyone’s thoughts. Thanks!
Kevin