Your missing the fact that you open doors to go into and out of the house. Depending on how old the house is and how energy efficient it is, you are losing air exchanges through your walls. If the is a fairly new house and very efficient, the builder would have to have installed a make-up air unit. Additionally, every time you turn on a toilet exhaust and range hood you loose air. Hope that helps.

"Turns out that the air in my 16,000 cubic foot house (2000 square feet x 8 foot tall) at 77F and 50% humidity, contains 1.3 gallons of water. "

That seems on the low side. How was this calculated.

You can look up calculations of water capacity of air, and they're all the same - at 77F 100% relative humidity, there are 22 grams of water in 1 cubic meter of air.

I don't open doors much, or for long. True, vent fans will exchange air, but a standard bathroom/stove fan is about 50 cubic feet/minute, so I'd have to leave it on for five hours to displace the volume of the house once. I might have them on for an hour or so in any given day. They aren't going to make much of a difference.

No, I KNOW from my condensate calcs that I'm cycling the air in my house every 8 hours (and yes, probably a lot is infiltration through walls). What I'd like to know is where authorities get away saying that the air MUST be exchanged every three hours or less. If it's 100F outside, I don't want to cool a houseful of that temperature air every three hours.

I'll take your word for it but seems on the low side. Your EPA link also says a minimum of 15 cfm per person, so a bathroom fan, other than the very smallest sizes (which you could replace) and a cracked window would be more than enough to run for a few hours a day. Better than nothing.

Fresh air is a health matter. I've visited places where AC can be a 24 x 7 matter in the summertime and the stuffiness of the interiors is very apparent and uncomfortable to me. You may be used to it.

If you opened windows in the morning for an hour or two (yeah, I know it's humid) or if your house is older and not tight, you're probably getting enough. But "no fresh air" isn't the right answer. You could get one of those ventilation devices (ERV) that tempers incoming air with outgoing air if you're concerned about the energy consequences. They're not all that expensive to buy or operate and will supply your home with enough fresh air. .

One example. There are loads.

https://home.howstuffworks.com/humidifier1.htm

"If the temperature is 25 degrees C and a cubic meter of air contains 22 grams of water, then the relative humidity is 100 percent."

There are two people living in my 2000 square foot house. In the winter, we keep things open. In the Texas summer, no way! With 8 hour exchange rate everything is fine. No smells, no stuffiness, no lack of oxygen. 3 hour cycle time just doesn't make a lot of sense, unless there is some specific rationale behind it. If you have a house full of smelly people, cooking up a storm, I guess I can see some value. As far as I know, all my neighbors do the same. No evident discomfort around.

I am not looking for reasons to ventilate my house more. I'm asking why people say that I should.

Here, AC *is* a 24/7 matter in the summertime. Temps 75F-95F, and dewpoint permanently over 70F. Welcome to the south.

Why you should is that stale air is unhealthy and fresh air is important. You can look up why.

You sound like an engineer. Look up Dr. Joseph Lstiburek (the L is silent) and any of the many available papers and lecture videos on YouTube from him or colleagues in the company he founded. He's been a leader, innovator and game-changer in the building science field for decades. I'm sure you'll find plenty from him about the need for ventilation and also other innovations and changes he recommends for long practiced and mind-numbing mistakes that are common with buildings and also HVAC systems in the hot and humid South. It's been a special interest of his.

I agree that stale air is unhealthy. I guess what I'd like to know is what makes 4 hour old air stale.