Insulating options for a barn home conversion range from simple fiberglass batts to super-insulated SIP panels. The main issue of course is that you want to have as much of the timber frame exposed and still have a good vapor barrier inside with a tight air barrier outside.
The two best insulation envelopes that I have found are sprayed foam and structural insulated panels(SIPS). Each has its own advantages and disadvantages and I will try to explain both without getting too long winded here.
The spray foam method is the cheaper of the two and it has a higher R-value at 7.5 per inch because I’m referring to urethane spray foam and most SIP panels are made with expanded polystyrene foam which around 4.6 give or take a few R’s after its aged. Spray foam sticks to almost anything and gets very rigid when it cures so it adds some strength. It does not require a vapor barrier inside and is great at sealing against air leakage from the outside as well. The main disadvantages are that it needs to be fire-proofed by covering it with an approved material. Also it must be applied in a heated environment or it doesn’t cure properly so this limits it’s usage in the winter time because we usually insulate between the old barn rafters and built a stud wall outside the barn wall.
Foam insulated panels offer many of the same insulating benefits as spray foam plus they have the added benefit of being a structural system. So in addition to the simple function of insulating SIPS can be designed to make up for structural defects in the barn itself such as the roof system which often fails to meet modern engineering standards which our building codes are based on. Their strength comes from bonding together two pieces of oriented strand board to a foam panel and can be increased by adding members in the seams between each panel. You can fasten roofing and siding directly to the panel however the wiring can be more of a challenge but if you invite the electrician to show up the day you start things go much better. The other benefits include less time to enclose the structure and the best energy efficiency if that’s important to you.