We want to finish a 3rd floor attic that is approx 900ft sq. My question is about reflective coatings such as aluminum paint on the back side if the roofing substrate to reflect the heat. Venting consists of a perimieter soffit vent plus a thermostatically controlled power vent. Any suggestions on this or other methods to reduce heat gain?
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Many of those coatings are suspect.
Cellulose will absorb radiant gain. You could blow the rafter bays full of cellulose.
Another option would be to fill thye rafter bays with the insulation of your choice...cellulose of even <gasp> fiberglass batts.
Then install rigid foam board insulation over the faces of the rafters. Foil faced polyiso is my favorite, in attics I use 2" thick boards.
During the installation of the RFBI gap the sheets by about 3/8" or so, then come back later and seal those gaps with canned or gunned foam.
Horizontal furring strips over the RFBI, 16" oc, with the strips screwed into the rafters, will provide purchase for your drywall screws.
The 2" polyiso will give you an additional R-14 by itself, but even more important than the rated R-value is it is such an effective air infiltration barrrier and radiant energy barrier.
In attics I recommend 2" foil-faced pokyiso on the walls and 1" foil-faced polyiso on the gable end walls.