Insulation is key
Insulation is often overlooked as a mundane part of a building, but if forms a vital function within any building in any weather.
In a domestic situation, insulation will help retain the internal temperature within the home. Basic physics applies: the more insulation there is in the fabric of the building, the more heat will be retained during the colder parts of the year.
The same physics applies in the warmer months of the year, when the sun is exposed to the walls and roof of your home, the walls and roof act like a thermal battery, locking that heat in and releasing it during the night time. With good insulation, that heat will be directed away from the house and enable the occupants to have a good nights sleep.
Insulation is seen as the low hanging fruit of the energy conservation world, insulating your loft can be done by rolling out Rock wool or fibreglass wool, available from your local DIY superstore. Cavity wall insulation (CWI) can be done by most local insulation companies.
There are a few buildings still around that are unable to have CWI due to them having solid walls or a prefabrication structure, there are external and internal options for insulation in these cases, generally costing more to install.