> One trick I've seen is filling plastic jugs full of water and spray painting them black and setting them in sunlight inside the house. They absorb heat from t

> One trick I've seen is filling plastic jugs full of water and spray painting them black and setting them in sunlight inside the house. They absorb heat from the sun during the day and release it at night. That's basically a ghetto version of the pasivhaus approach. In passive houses, you have one heavy interior wall, pained dark/black, facing a large, south-facing window. This wall gets to absorb lots of sunlight during the day, and then heats up the house 24/7. > Stop drafts (but make sure some fresh air gets in). It's good to get a CO2 meter for that, to make sure there isn't too much or not enough air exchange going on.