Attic Temperature vs. Ambient (Outside) Temperature

@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.


I did one more experiment, trying to linearize the relationship a bit more. I paired earlier offset outside temperatures with attic temperatures based on the theory that there’s a delay in both outside temperatures and associated radiant energy making their way into the attic. Here’s the same chart with the attic temperature delayed by 5 min with respect to the outside temperature.

I turns out the best fit for a linear relationship is with a delay of 85 minutes, nearly 1 1/2 hours. Here’s the associated more linearized chart.

The fit line is below, so an good estimate of attic temperature is 1.56 * (Outside temperature 85 minutes ago) - 20.7 degrees.


Now that’s excellent stuff!

Would be great to compare such data, and linear fits, for different roof types and insulation types.

Something I’ve long pondered is “active” vs “passive” insulation/ventilation in relation to solar. Consider some of the biological methods/factors for cooling/heating (off the top of my hair-covered head):

  • Albedo (of skin), general passive/fixed but humans and some animals modify their outer layers.
  • Move to shade or grow within it; move to somewhere more insulated/warmer (migration); sunbake (rotation)
  • Transpire/sweat = active temperature regulation

In building thermodynamics we seem to have covered the first one pretty well.
The second, short of calling an RV home, extends to trees (biology) shading our houses – very clever of them to offer more leaf shade in Summer! Note that most people don’t have rotating homes but blinds/awnings have a similar effect.

The third is where I ponder most.
Solar cells like to stay cool for best efficiency and longevity.
At some point it would seem that some of the energy they generate would be best put toward cooling themselves. Assuming water is readily available: Solar sweat.
As the skins of our houses become more solar, more energetic, it stands to some sort of reasoning that they should become more like biological skin.

In vernacular architecture perhaps there is a point of maximum coolness when having completed your shady roof with large damp palm fronds, you relax underneath and let the sweat dry as your roof does the same.

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I tried to look for an analytic model for attic temperature, but the answer is an ugly and complicated set of heat and energy flow equations. You’ll find some answers in here.

As for “transpire/sweat” - In hot dry environments like Arizona or the Outback of Australia, you’ll find different forms of evaporative cooling from sprayers on the patio, to a spinning roller soaked in water AKA a swamp cooler…