BTW: I ran a quick LoadCalc on the 250' room in Orlando assuming
-- R13 in the walls, under the floor, & above the ceiling with 10' ceilings
-- 140 square feet of framed R13 north wall, 140 square feet of south wall, 125 square feet of west wall (which has all the window area subtracted out)
-- 55 square feet of west facing single-pane with no blinds, no exterior shade/overhang
-- 10 cfm of ventilation/air leakage plus two people and it still came in at 8125 BTU/hr. With zero people it's 7275 BTU/hr
You're totally safe with a 9K head there.
If that room is leakier than that or has lower R-values than indicated, from a comfort point of view it's worth fixing those deficiencies rather than just throwing a 12K head at it. An R13 ceiling under a flat roof radiates a lot of heat at the occupants even if the air temp is reasonable.
How deep are the rafters in the flat roof?
-- R13 in the walls, under the floor, & above the ceiling with 10' ceilings
-- 140 square feet of framed R13 north wall, 140 square feet of south wall, 125 square feet of west wall (which has all the window area subtracted out)
-- 55 square feet of west facing single-pane with no blinds, no exterior shade/overhang
-- 10 cfm of ventilation/air leakage plus two people and it still came in at 8125 BTU/hr. With zero people it's 7275 BTU/hr
You're totally safe with a 9K head there.
If that room is leakier than that or has lower R-values than indicated, from a comfort point of view it's worth fixing those deficiencies rather than just throwing a 12K head at it. An R13 ceiling under a flat roof radiates a lot of heat at the occupants even if the air temp is reasonable.
How deep are the rafters in the flat roof?