I am a DIY homeowner looking for a solution to a rather unique problem. My home was built in 1972.
My living room has a vaulted ceiling with exposed 4×8 beams on 48″ centers. The beams rest on a center 4×8 support beam below them. The wall framing is standard 2×4 on 16″ centers with double 2×4 top plate. The ceiling is a hot roof with no insulation. It is 1×4 tongue in groove covered by sheathing, tar paper, and shingles. The temperature in the room is about 4 degrees different than the other rooms because it loses heat/cool.

I would like to add additional ceiling joists between the beams to cut down to 24″ center, insulate, and finish with tongue in groove leaving only the center support beam exposed.

My problem is:

How should I tie in the new joists to the wall?

How should I insulated to prevent moisture/rot and achieve better temperature control?

Will the 24″ span be adequate for the tongue in groove to prevent sagging?

Thanks in advance for your help and input