If you open up both sides you’ll pick up even slight breezes. It’s relatively still today, but I have to keep both doors blocked open because of the cross-breezes.
My other half tended to go with the closing up the house approach too, but I convinced up to take the approach from the tropics when we tested temp differences with both ways of doing it.
The closing up approach works better if the RH is below 45%, or if you’re blocked in by so many tall buildings you’re effectively cut off from any breeze. Although I’m inclined to think there’s a psychosomatic element too - if you grew up in hot countries, dark rooms are associated with hotter conditions, but here I gather it’s more likely to be associated with coolness.