My guess is that you get plenty of fresh air change during cold weather and a little wind. Mainly the stack effect makes the house fresh air infiltration increase. Calm wind and mild temperatures slows natural air change.
Keep us posted.
Regards Teddy Bear
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"