Air tightness is important to achieve projected performance.
Leak detection (using smoke pencil or similar) allows uncontrolled air inlets to be found and sealed. This is particularly important around upper floor ceilings, service penetrations, skirtings and floor junctions.
Floorboards can be sealed using strips of rubber, wood or mastic.
Air tightness tapes are very effective and can be used to seal draughty areas around windows - particularly old sash windows may have a substantial gap around them if you take off the window surround.
Doors can also be examined for air leaks, with the possibility of adding a draught proof layer to the back of old doors that are going to be retained.
Further information is available in the Retrofit Pattern Book