4. Treat windows by type, not with one product
Window fixes split cleanly by window type, and buying the wrong product is the most common mistake here.
Casement windows that open outward on hinges usually leak at the frame seal. Peel out the old perished gasket, clean the channel and press in new self-adhesive P-profile rubber. P-profile handles gaps of 2 to 5 mm; E-profile handles 1 to 3 mm. Squeeze the old gap with a folded piece of card to judge which you need, and err toward the smaller profile, because oversized seal stops the window closing and people then leave it unlatched all winter.
Sash windows leak at the meeting rail where the two panes cross, and no amount of tape around the outer frame fixes that. Fit a brush pile strip in the staff bead, and add a simple sash stop or a strip of secondary brush at the meeting rail.
Single-glazed windows of any type respond best to shrink film. A kit for five windows costs around $15, CA$22 or £12. Tape the double-sided strip to the frame, stretch the film across, then heat it with a hairdryer until it goes drum-tight and invisible. It adds a trapped air layer worth roughly the difference between single and basic double glazing on that pane, and it peels off in spring. The catch is that you cannot open the window until you remove it, so skip bedrooms where you sleep with a window cracked.
Heavy curtains that touch the sill help more than thin ones that stop halfway, but keep them clear of radiators — a curtain hanging over a radiator sends most of its output straight up behind the fabric and out the glass.