My attic is finished; 3 rooms, wood floors, plaster ceilings. I've gotten conflicting advice: some say insulate the floor, others the roof. The attic will not be used as living space. There are no heating ducts in the attic. Any advice?
A new 2-story house under construction in my neighborhood has under roof insulation--spray on open cell foam. The builder told me he expects the attic temp to be about 85 F on a 100 F day. The natural gas fired furnace and tankless water heaters will be in attic space and will get combustion air from outside through vents. I can see the water heater vents in the roof and these are the concentric type where the exhaust flows out the center and the combustion air comes in through the annular space. I assume the furnace will be the same. The house also has a natural gas "fireplace" on the first floor and I must assume the flue is designed and performs appropriately.
I asked him about retrofitting an existing house using that same foam sprayed under the roof decking. He said this is not advisable because air flow and exchange is restricted in the attic in that system and restricting airflow in an attic designed and built for airflow can lead to code problems and performance problems.
His concern was the exhaust flue systems for natural gas or other combustion furnace and water heaters in an open attic house are not rated to be sufficiently tight for a restricted attic. Carbon monoxide (if produced) could leak from the flues and build up in the attic.
Since your attic is finished it might be used as living space. My ideal suggestion would be to remove the ceiling plaster, add ventilation from the soffits to the ridge (with appropriate soffit intake and ridge exhaust), insulate the roof and knee walls, cover the ceiling, seal any gaps/leaks in the knee walls & ceiling, and add supply & return vents to the attic. This would bring the attic into the conditioned living space.
If you're not willing to remove the plaster and insulate the ceiling as I described, you can insulate the entire attic floor and access door, and add venting to remove hot stale air from the attic. Note, you may have to remove/reinstall the existing attic floor to insulate it properly. Since the existing ceiling is plaster this may be the easier way to go but you're essentially sacrificing an already finished space and if it's not vented properly you run a risk of trapped humidity causing mold or dry rot down the road.
This is an active topic of debate in the building and energy communities. The tradeoffs are hard to evaluate.
Your choices are either to air seal every little crack from the home to the attic (including any attic stair). Or, insulate between the rafters in the attic, and make the "thermal envelope" the attic.
Your question can't be answered without considerably more details including photos, drawins and most importantly your roof type and climate zone.