Draughts are easy, and cheap, to fix but we tend to learn to live with them. It’s just a matter of blocking up any holes which allow warm air out, and hot air in.
There are all the obvious places to seal – gaps around sash windows and under doors, floor to wall joints, ceiling to wall joints, gaps between floor boards, loft hatches, keyholes and letterboxes, and chimneys – but the real culprits tend to be where pipes and cables are brought through walls or floors, and there is often a gap around the pipe or cable.
Some draughts will be so bad that they are easy to spot. You can find the others by using a thermal heat detector or by walking round the house with a lit candle, and noticing where the flame flickers.
Simple draught prevention can be done through curtains and blinds, carpets, and draught excluders.
Otherwise, there are lots of products in your local DIY store for sealing round windows and doors; good old mastic will do the job at the junction of floor and walls, and even gaps between floor boards. Penetrations through walls for pipes and cables can be sealed with mastic, or expanding polyurethane foam for the larger gaps.
Chimney flues can be bricked up or there is a handy balloon-type thing called a Sempaflu that will do the job. It is important to cap the top as well as the bottom to stop rainwater getting in. Sealing the bottom will still allow rainwater into the top, with consequent damp problems now that the flow of air that used to dry it has been blocked.
For windows and doors you can use foam sealant for gaps, which comes in tape form or as spray foam, or metal or plastic brush strips, as well as installing letterbox plates and keyhole covers and attending to cat flaps.
Secondary glazing film can be used for a double glazing effect but, it will need re stretching from time to time, and can tear.
A note of caution; beware of blocking up air-bricks. Make sure they can still circulate air where it is needed, typically to the ground floor joists, under a suspended timber floor. No air means wet or dry-rot could get established. And make sure you don’t block trickle vents in windows or extractor fans.
For loft hatches, you can install foam sealant.