So, I couldn't help but tinker a little more. I've added another bit of code to my temperature logger based on the above calculations, so now I'm plotting heat loss against the exterior-interior temperature differential:
The blue curve is the temperature difference between exterior and interior (scaled by a factor of 100). Positive values mean it's warmer inside than outside. The red graph is the calculated heat loss in BTU/h. Negative values mean heat loss, positive are heat gain. I'm taking temperature differences within 5 minutes, calculate the BTU heat loss from them, and multiplying the result by 12 to get BTU/h.
This is today's data, so unless the forum caches the images, this plot should update live as long as my Raspberry Pi is online. The data is a little noisy despite smoothing, because sometimes between two 5 minute samples, the interior temperature doesn't change at all, which makes the heat loss graph spike towards 0 for that interval.
It shows one very interesting thing: The house gains heat much more easily than it loses it. Observe the temperature differential in the left half (about 10F) with a heat loss of about 1800 BTU/h on average. On the right, we're looking at a temperature differential of about -5F, but a also heat gain of roughly 1800BTU/h on average.
That tells me that I should probably invest in some attic insulation and radiant barrier foil to keep summer temperatures more comfortable
