Roof insulation performance, warm weather

The cathedral ceiling in the third-floor/attic, an unvented roof insulated with icynene foam and a double radiant barrier, seems to perform well in warm weather. We didn't install central air conditioning (too expensive both upfront and operating cost), and we both have office space in the third floor, so we depend on that roof insulation system for comfort both winter and summer. So far, in hot weather the attic temperature is always lower than the outside temperature, which I think is pretty good.

Boston's had our first heat wave the past few days, and I took some measurements today. We keep most windows closed during the day, opening them at night to pull cooler air through with a small "whole-house" attic fan.

Location	11 am	3 pm
Outside air	89 F	93 F	air temp
Exterior siding (south wall)	95	114	surface temp
Exterior shingles (roof)	132	152	surface temp
Attic ceiling	86	92	surface temp
Attic air	83	89	air temp
Second floor air	81	84	air temp
First floor air	80	82	air temp

The cathedral ceiling is only 6" thick (made of 2x6 rafters), so all that separates the interior drywall from the 150-degree shingles is 6" of icynene foam and the 1" radiant-barrier airspace.