Hey everyone.

I have an unexpected window of time to continue a long term project working on re-purposing a Pennsylvania bank barn from the 1800s to have some additional conditioned work space (aka “livable”  I suppose) within the structure.  The building is board and batten sheathing/siding on timber frame construction.  I’m looking for advice on my approach to insulating, especially related to the ceiling and roof.  A little background info is below as it is all closely related.

There are two living spaces within the barn already and the space I’m currently working on insulating/conditioning may ultimately be combined with one of the existing living spaces that was previously insulated with Icynene spray foam.  The floor in the space I am working on is already insulated from the partially underground, fully enclosed, masonry walled, work space below with 9.5″ of Rockwool between floor joists, above the lower level ceiling.  Conditioning will be done with a mini-split but I may run PEX for radiant floors in the future.

I have two approaches for insulating the walls from the inside out based on my research so far.  In either case the walls for the space will be framed and clad with either 1x t&g boards or sheetrock.

Walls Approach 1 –  insulate the walls (all above ground) with several layers of reclaimed rigid foam board, applied to 3/4″ spacers attached to interior side of 1×12″ siding boards, then seal the joints with canned foam to create a tight air seal.  I have 25 sheets of 1.5″ fiber-faced Polyiso and the same of XPS I’ve already acquired for this purpose and may acquire more as needed.  I believe this will allow the siding to dry to the outside of the building as moisture penetrates the wall to keep the painted siding from rotting, but keep the moist air generated by people, inside the building.   If needed I could add rock wool or additional foam within the stud bays of the non-load bearing walls that will be framed within the timber frame and covered on the interior side with either sheetrock or 1x wood.

Walls Approach 2 – insulate inside of the existing board and batten on timber frame (8″ thick walls – posts are 8″ thick) using some combination of Roxul batts and possibly their Comfortboard product to allow the walls to breathe the whole way through.  I’m not sure on this approach as I know the air seal will be non-existent, but it may be a decent approach in this unconventional building to keep the inside comfortable while keeping the siding/timberframe from rotting due to trapped moisture.  Maybe apply a continuous vapor barrier inside beneath the sheetrock or 1x walls with this approach?

Ceiling/Roof – The space I’m currently working on insulating is actually two floors, call it floor 2 and 3 above the lower level workspace.  The vertical exterior walls of floor 2 will extend partially into floor 3, then floor 3 has a sloped ceiling from a knee wall up to 10′ along the underside of the roof, where it then flattens out to create a flat ceiling across rafter ties 7 feet below the ridge.  The existing roof is in great shape (1o years old) and is painted corrugated metal roofing panels applied directly over purlins with no continuous roof decking.  I know this is not ideal but it is what I have and its not in the cards to replace roof and deck it in any time soon.

To insulate the sloped and flat ceiling for floor 3, I am thinking of creating 1.5″ air gap beneath the purlins within the rafter bays for air flow from the vented soffits to the ridge, then packing the rafter bays with cut and cobbled rigid foam board, air sealing the joints with canned foam, then applying a full sheets of foam directly to the underside of the rafters to achieve the necessary R value.  Probably need 7″ build up.  Once the rigid foam build up is thick enough I can apply either sheetrock or 1x wood to create the finished ceiling for floor 3.

So, any critique, feedback and advice is greatly appreciated on this unusual adaptive re-use of this beautiful structure.  I have a decent window of time at the moment between contracts but limited funds.  That said I’d prefer to do this the right way with the soundest known approach possible.

Thanks for your time!