3C is a lot if you're talking Celsius--but I do this at night (set thermostat to 60F-61F) as cold temps under layers of blanket is actually beneficial to sleeping.

Older houses sometimes still can benefit from insulation improvements.

So go in this order:

1. effective insulation

2. conscientious and smart use of energy and avoidance of very inefficient heating systems. avoid heating large areas when they don't really need to be at very warm temperatures. (careful though to avoid damp cold range of temperatures where wood rot and permanent damage to homes will happen). Avoid unburnt natural gas, eg in badly maintained or cheap cooktops. The ideal household kitchen has ~ 4 gas burners, and one or two induction burners flush with the counter, eg https://www.amazon.com/Countertop-Portable-Induction-170-Min... This minimizes unlit gas for those very short heating needs in the kitchen.

3. think about upgrading to very efficient systems. These most promising could be (depending on your area and scale of heating needs):

-- electric-heat cogeneration

-- heat pumps that use the large volume of ground underneath the basement as geothermal heat-mass reservoir. Heat the ground mass during the summer to provide AC, and squirrel away the cold thermal mass during late fall and late winter+spring.