Quite simply. You house is light years from being air tight. It leaks more than enough air, every minute, of every hour, of every day.

We tend to air out individual rooms by opening windows, roughly once a day depending on usage/personal tastes, kitchen more frequently (as much because it gets warmer and most usage when everyone around). Mostly we don't leave open for long periods, just a quick airing out. The house gets enough air circulation without much effort, so we're not fanatics either way.

In summer months, windows may stay open for much longer - we mostly don't need A/C.

Basement is a bit of a challenge in humid summer; windows mostly kept closed and dehumidifier running frequently.

Exceptions/differences of opinion? Teenage boys want maximal privacy, rooms get stuffy, then want to leave their windows and doors open when they emerge from their caverns, resulting in drafts throughout the house (rather than just airing out their own rooms). Older generation tends to forget to close windows even when cold.

But mostly comes down to personal preferences. Biggest problem is if someone in neighourhood has a fire - smokey scent can linger.

You mean windows actually open???

Ooh, I've lived in a place with cotton wood fluff. Awful stuff.

^^what Sammy said. Between allergies in the spring/fall and central air for the summer, I flip my lid if anyone opens a window/sliding door.

I totally get where the Chicagoland members are coming from. In cold weather, we don't crack a window and leave it open - open it (even wide), close the door to the room for 5-10 minutes, come back and close the window. House doesn't appreciably cool down, the goal is to replace the air in that room, not regulate the temperature (rare exceptions); for that we turn down the heat. Note I'm not advocating for the need to do this (most houses, probably not) - we just like it.

Months at a time.

14+ years thus far.

I don't know if the OP is ever returning, but it seems to me that s/he does not live in a cold climate to even ask this question. Where I live, the windows are shut tight end of Sept through end of April. Then when spring comes, we can't wait to open them, even if it is just a bit too cold to do so, we are so eager to open them we might just open an inch or two just cause it's so great to finally do so. During the summer, it's intermittent opened depending on how hot, but most of the time it's not that hot here.

Fresh indoor air is needed for the health of the structure and the people in it. Opening a door to enter or exit doesn't do anything. Keeping windows closed for even days on end is a bad idea.

Those living in areas with extreme temperatures (either hot or cold) can install devices called ERV (energy recovery ventilation) or HRV (heat recovery ventillation) to provide an important and healthful flow of air while minimizing the intrusion of outside air temperatures. They work by exhausting inside air and using its heat (or coolness) to temper the incoming replacement air from the outside. They're not that expensive and provide a lot of benefit to the well being of the people inside.

I have returned lol

Fresh outside air is critical to good health. Unfortunately we don't know what levels of what contaminants are unhealthy. Is 1000ppm CO2 OK? Too high? And on and on.

On the upper end we only close windows if the outside humidity is above about 65-70% or temp above about 80-85°f. Or to keep rain or very strong winds from blowing in.

On the low end we won't necessarily keep a lot of windows open when it's below about 45°f but will often have one or two open a few inches either through the night or for a few hours. I doubt we ever go more than maybe a week without opening some windows unless high temps are below 0°f.

It's not unusual when we've had a cold spell (highs of -15°f are no unheard of) for a bit to see lots of people in our neighborhood open windows as soon as it's above 0°f one day.