I took a college class back in the 90's that focused on environmentally sustainable energy use. The teacher was one of the few who had real world experience about what he was teaching. He told us about a house he built with an R-value over 100, and this was with tech that was available back in the 80's. He said you could heat it in the winter by turning on all the incandescent lights in the house. When he first built it, it was too tight though. Taking a shower resulted in condensation raining down from the ceiling in the whole house. He had to drill holes in parts of the structure to let it breathe. I think if it was cold enough that that level of insulation wasn't enough, the area wouldn't be habitable long-term and I'd move closer to the equator at that point.