> If you can maintain shirtsleeves conditions inside an old farmhouse at that outdoor temperature

I don't know, can you? I would need an indoor temperature at least 18C to be comfortable in shirtsleeves, unless I'm doing something physical.

I'm comfortable in shirtsleeves at 24C (current temperature here) and I'm wearing wool and a scarf if I'm sitting at my desk at 18C. That's why they won't send me to the Moon or to Mars.

With a wood stove, yes. Gets so hot sometimes I take my shirt off entirely.