Cold air entering a room is felt some distance from where it gets in, because it falls and travels along the floor. That is why draught proofing applied to the obvious candidate often changes nothing: the actual opening was a gap at a frame edge, a worn cill seal, or an unfilled void beside a later replacement window.
Finding the real entry point takes one cold windy evening and costs nothing.
Locating it properly
Work on a cold day with the wind on that elevation, and move the back of your hand slowly around the whole perimeter of the opening rather than across the middle. Check the joint between frame and wall, the underside of the cill, the hinge side and the latch side separately, and any trickle vent.
A smouldering match or an incense stick shows the air movement visibly, which is more reliable than feel and quickly tells you whether air is entering at the frame or at the glass.
Timber windows and doors
Sticking that comes and goes with the season is the timber swelling as it takes up moisture, and the correct response is to find out why it is getting wet rather than to plane it. A planed door that was swollen becomes a draughty door in August.
Look at the paint on the lower rail and the end grain, which is where water gets in. Sound paintwork, a working drip detail and clear drainage holes in the cill keep a timber window stable through the year.
Replacement units and their seals
The rubber gaskets on a modern window perish, flatten and shrink back at the corners after fifteen years or so. Where the gasket has gone hard and no longer compresses, it can usually be replaced as a strip without touching the frame or the glass.
Misted glass between the panes is a failed sealed unit. The glass can be replaced on its own, with the existing frame and hardware retained, which is a considerably smaller job than the one often proposed.
Hinges, handles and alignment
A door that catches on the latch side has usually dropped on its hinges or the frame has moved; a door that catches at the top of the latch side has dropped. Check for a worn hinge, loose screws into soft timber, and whether the frame itself is still square.
Longer screws into solid material fix most hinge sag. On a modern window, the hinges and the locking points are adjustable, and small adjustments there usually restore compression on the seal without any other work.
What to seal and what to leave
Seal the gap between frame and wall, the gaps under a cill and around a letterplate, and the opening edges with proper draught strip. Do not seal trickle vents or airbricks, which are there deliberately, and do not block the drainage slots on the outside of a frame, which let water out.
The outside line between frame and masonry wants a flexible exterior sealant, renewed when it cracks. That single joint is responsible for a large share of both draughts and damp around openings.