You should consider adding an active ventilation system, which turns on periodically. This can be as simple as a bath fan on a timer with a make-up air opening

You should consider adding an active ventilation system, which turns on periodically. This can be as simple as a bath fan on a timer with a make-up air opening elsewhere in the house. If you are concerned about the inherent heat loss that comes with ventilation, consider installing a heat (or energy) recovery ventilator, which uses a heat exchanger to recycle a large portion of the heat while also introducing fresh air into the house. If you are in the UK or the US Northeast you probably experience worse stuffiness because the dominant method of heating is hydronic which doesn't circulate or filter air.