Another quick update: After building a temperature logger with a Raspberry Pi, a couple of digital temperature sensors, a bit of wire and a few dozen lines of Python code, I now have proper temperature data to share!
This is currently all without heater, and it shows a few interesting things. First of all, down to about 50F exterior temperature at night, we can comfortably do without heating at all. We've got about a 4-5F interior drop over eight to nine hours at night, which I'd consider pretty decent.
It also shows that there's about a two hour lag between exterior temperature drop and the interior following suit. That means, that when I measure an exterior drop, I've got plenty of time to turn on the pump keep interior temperature at target. Of course, that time will shorten as exterior temperatures get lower, but it gives me a good indication that it'll be fairly easy to keep the interior comfortable.
Here's some graphs from my custom built logger:
Exterior in green, interior in red. All temperatures are in degrees Celsius. Please ignore the downward spikes to 0, those are just failed readings from the sensor. In the current iteration, I'm sampling temperature every 5 minutes.
(I'm not sure how this forum handles the image links, so it's entirely possible that these immediately hit the webserver on my little Raspberry Pi. Please be gentle!)