@chdot "amount of natural draught" rather a lot I'm afraid, that's why the figure for efficiency gains I was quoted was so low. For reference a neighbour's teenager decided to seal up their window with blu tac one winter to keep more heat in and despite their house being otherwise typical that trapped enough humidity in the room to cause black mould to grow behind the kid's curtains before the parents realised what was happening. In my flat I have my desk into a bay window and after they did draught proofing on the frames of the traditional wood windows years ago I complained I could still feel a breeze across my legs, their reply was "that's the minimum amount of airflow for the building sorry".
If you stop the place up tight to prevent draughts, you should either be airing the whole flat out with open windows every few days according to the info I was given by a tenement window specialist, or getting mechanical ventilation - at which point you're spending so much you might as well go whole hog and aim for EnerPHit(the issue being with subsidies being what they are ie practically nonexistent plus the fact you have to pay VAT on renovation work, only people who A; have a fair whack of cash and B; aren't planning to move any time soon will do that work, since it won't add enough value to the property to cover the costs completely).
There's a reason you have to get an official blower test of air changes per hour to achieve Passive certification.
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