If I want to expose the beams of my ceiling, is it sufficient to just install roofing insulation?
Your climate is similar to mine...never too cold and never too hot. That’s why not having insulation in your attic to date, isn’t critical.  However, any insulation is a plus, but you’ll have several issues to overcome: 1) fastening vapor barrier to roof, 2) fastening of “roofing” on insulation board, 3) added structure height at eave and rake, 4) finishing of interior beams and ceiling material, 5) added weight to roof structure, 6) cost, and 7) Environmental issues. 

1) A “hot roof” could mean a built-up roofing system. This means you have a low-pitch roof and you don’t have roof shingles. If so, tearing off the old roofing membrane and installing a new membrane on rigid insulation is easy and the vapor barrier is part of the system, including how the vapor barrier is secured to the roof. If not, it becomes a bit more complicated because you’ll need to add the proper vapor barrier on the old deck and secure it to the roof, then add rigid insulation, then add plywood on the insulation to hold the shingles, without any fasteners extending through the ceiling material. 

2) The easiest method to fasten shingles to roof is with plywood. However, you may need wood stripping if you live in a “high wind climate.”

3) If you raise the height of the roof by a few inches, you’ll need new edge metal. Also, if it’s a historic district, like the mission district, you’ll need Planning Department approval for increased height. 

4) After the ceiling is removed you’ll need to finish the beams and ceiling. All those nails / holes / etc. will need to be addressed. 

5) You’ll need to calculate the weight of what is being removed and subtract the weight of what is being removed. Then, verify the structure is adequate. 

6) $$$$$$$$ vrs. Potential savings from heating and cooling. Check with you local power company to see if they have rebates for adding insulation. They have people who can calculate the savings for 2” thick or 4” thick insulation and various kinds of rigid board. 

7) That 1930’s plaster ceiling could have asbestos or those 900 coats of paint could have lead. You’ll need to have it tested. 

Sounds simple...oh, and you are getting a Building Permit, right?