> AC still needs to run to drive out building moisture so that they don't develop mold.

Serious question: How was this handled in pre-AC times?

Is it just an issue because buildings are much better sealed than they used to be, or is there more to it than that?

Depends, some climates buildings were moldy.

But old buildings generally aren't even close to air tight. Under those conditions as long as they are heated even a little the humidity is mostly lower than outside. Mean the amount of water in the air as a percent is the same inside and outside. But since it's hotter inside the house the relative humidity is lower. In temperate climates you don't get mold.

It's actually modern air tight houses that have terrible problems with mold. Because they don't breath water vapor builds up inside.

Older, less insulated buildings, breathe better. The more your home "breathes" the more it transfers heat and humidity to the outside, which defeats most insulation, unless the breathing is done though a heat exchange.

As an update, as I'm getting a bunch of down votes for some reason (please comment!).

Newer homes and buildings have a vapor barrier beneath the outside cladding. This article on the Perfect Wall [0], explains the benefits and options.

Older houses, like the Victorian that I live in, didn't have modern insulation, which can deteriorate if it gets wet, so they were made to breathe, so that any moisture that got into the wall could be removed through air movement. Not to mention that fireplaces required a constant stream of new air to replace that which is pulled up through the chimney.

[0][https://www.buildingscience.com/documents/insights/bsi-001-t...]

Building design has changed tremendously since the introduction of AC, the number of usable windows and ceiling overhead has drastically decreased in those regions traditionally needing them.