Re: ceiling of room is the underside of the roof--how to install ceiling?
Based on what I can see from here ;-) , it looks like the beams you describe comprise the supporting structure of the roof. In other words, they ARE the rafters. (BTW, the term 'joist' describes a horizontal member. Rafters are angled.) This sounds very similar to the house I grew up in.
My father solved the insulation problem by applying foam board insulation to the underside of the plywood, between the beams. An appropriate finish material (such as gypsum drywall, wood paneling, or boards) is then applied to the underside of the foam board insulation. The foam board should not be left exposed, as it is not fire retardant.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.