@mike_gessner’s experiences with attic heating here, plus @ixu’s thoughts on roof thermodynamics got me thinking about the relationship between attic temperature, ambient temperature and solar heating of a roof.
Ecobee came out with rev 2 of their temp sensors and I couldn’t be happier - they packed them with a far larger “coin” battery that now lasts for months, even when separated from the main thermostat by several walls. I finally instrumented my attic in May, so I could see how the temperature varied up there with outside temperature and time of day. Even though we get tons of sun (virually no clouds in the summer) my attic temperature stay relatively mild, even without a radiant barrier. You can see how the temperature in the attic cycles, typically starting out the warming day around 6-7AM, maybe 5 degrees warmer than the air temp, heating up to maybe 20 degrees higher than the air temp at the peak of the warming cycle, then slowly giving up heat through the late afternoon, then much more quickly in evening.
My newest instrumentation is adding a temp sensor in closest heating/cooling duct to the furnace blower, both upstairs and downstairs. No added functionality from these, but I’ll be able to get a better view on heating and cooling performance. You can see the sensor peeking out of my upstairs office vent, here. More fun thermodynamics to come. The unity line gives you a sense of how much heating come from solar absorption vs. conduction from the outside air temperature.