off. If I wanted more warmth, I'd add more insulation. People don't give enough credit to having a well insulated house.
I'd love to build a house or oversee the building of a house from scratch. Insulation and vapour barrier would be one of my top priorities. Probably double up the outside walls with staggered studs so that every last inch of house can be insulated. Traditional insulation methods are actually quite sub par because of the way studs, window/door headers etc take away from insulation area. Even if you do spray foam, if you're only spraying each individual stud cavity you are not getting a continuous air seal. Wood is porous. Traditional plastic vapour barrier is better in that sense, but it is also much easier to install improperly, or cut corners. Acoustic sealent at joints is a must, the tape can get undone over time. You also want cuts to be pushed on by the drywall or other, as a cut mid stud span will potentially get undone over time.
I'd also try to find a way so the insulation and vapour barrier can link properly to the ceiling, and even floor. I don't believe basement floors are even insulated even in modern construction though, and not sure how easy that would be to do, can you pour concrete over foam without it compressing? Though if basement is dug deep enough and the walls are insulated as part of the same envelope as the upstairs walls, then it would probably be pretty good.
Key is to have a continuous insulation and air seal envelope for the entire house, and this is not done that greatly, even in modern construction. Of course you want some kind of heat recovery air exchanger so you can still get fresh air in the house, but at least then it's done in a more controlled way. The air seal is probably almost more important than the insulation actually. Insulation without proper air seal is just a filter.
That said, my thermostat setting tends to vary. When I'm not home, or sleeping it's set to about 12-15C. It's a range to cause less short cycling of the furnace. Basically I just keep it at that temp so pipes don't freeze. When I'm home but upstairs I tend to keep it at about 18C, if I'm downstairs (split level house) I keep it at 22C or so.