Once significant sealing of leaks is achieved, including additional insulation of windows, walls, ceiling, you may find the now accumulated breath of occupants will cause low oxygen, high CO2/humidity, and general staleness.

From reading the "German Passive House" standard, this was dealt with by:

Superinsulate and seal. (done)

Draw in fresh air through a buried earth tube to preheat air to ~50F. (not required at first, but obviously useful, can be added/expanded later)

Slowly pass a small volume per hour of incoming dry cool air through a heat exchanger, to absorb some of the heat of the outgoing moist stale air. (inexpensive and very effective)