Thermal images of most front doors, even when closed, show how leaky they can be. A 1 mm gap all the way round a door adds up to the same area as an A4 piece of paper – like a massive ever open letter-box – so it is well worth paying attention to draught proofing.
Doors containing panels of thinner material are always very apparent in thermal images, as are any gaps around the edges.
For sides and top, self-adhesive foam or rubber strips are effective. For the larger gaps, especially for outside doors, more robust rubber, plastic or part metal strips which need to be tacked into position are best. The bottom of the door can have a self-adhesive or tacked on plastic strip incorporating a brush. If you are keen to do the job as economically as possible, a strip of heavy felted material stapled into position works well. Letter box covers (or a bit of fabric hanging in front of the box) on the inside are helpful.
The most effective solution is to have an outer door to create an airlock effect and this makes a dramatic difference. If that cannot be achieved, then the old fashioned method of deploying a curtain across the aperture works well. This can be complemented by a “portiere” which lifts the curtain a little as the door opens allowing it to fall all the way to the floor. And if a curtain is a little clumsy all year, remove it for the summer.