We are lucky to have a late 19th century wagon house in our back yard: a timber-frame with vertical wood siding and an old Pennsylvania gray slate roof on skip sheathing that we recently had patched up and hope to get another 5-10 years out of. 

The building is entirely uninsulated but I plan to use the second floor as a home gym (I’m going to put a small climbing wall up there) that I will use in the evenings once the kids are asleep. The building itself is not worth weatherizing or insulating; while structurally sound the foundation has no footings and I don’t wish to invest much in it beyond the necessary maintenance keep it stable. We hope to build a new structure on its footprint once our town (hopefully) legalizes ADUs; perhaps in 8-10 years.  

My question is this: What do folks think is the best way to bring it to a comfortable temperature in winter (55Fish)? We are in central NJ climate zone 5a and the winter nights can be quite cold and summer nights quite hot and humid.  have lots of partial rolls of various breathable WRBs (Mento, etc) leftover from building projects (I am a GC). My thought is to staple or cleat that material to the inside of the timber frame walls on the second floor and the underside of the rafters to create at least some level of air/wind resistance and heat the area with a quart or ceramic electric heater (we have a 50 amp sub panel in the building). My only trepidation is that on the walls this could create a perfect cavity for bugs to nest in (unless I staple it to the backside of the siding/sheathing) and that it would make the roof an even bigger heat sink in the summer (all the WRBs I have are black; gray slate gets hot). In the summer my plan is to use a pedestal fan and open up the windows. Any suggestions or comments on this strategy? Thanks as always to the GBA community.