It's impossible to give an accurate advice without knowing your precise location and climate. As a rule of thumb, as an Architect (the real one), I would start

It's impossible to give an accurate advice without knowing your precise location and climate. As a rule of thumb, as an Architect (the real one), I would start by using the best insulation you can afford. In a good percentage of inhabited places this will guarantee you a livable if not a confortable home. Then, according to your climate and economic environment (wood and derivates, gas, fuel, coal and electricity prices are highly variable depending of your specific location) invest in an heating source.