You should draught strip them with routered in brush strips to the head rail, meeting rail and the bottom rail, replace the parting beads with a proprietary ones with brush seals already fitted to them. We quite like this from Historic England, please see p 15 in particular.
You should also draught seal round the frame inside and out and check that no draughts are coming through the frame, check under the sill, through the sash box and over the head and past the pulleys etc.