Ventilation controls the mold in places you can see, like showers, but has little effect on mold growth in concealed places like sealed wall cavities. That's the far more dangerous form of mold, because you can't clean it.

Many old uninsulated wood houses, despite their reputations for being preserved primarily due to their mold resistant old growth structural lumber, were preserved in major part by the fact that they were constantly being heated all winter by high capacity heating equipment like wood or coal stoves that had to put out a ton of heat (at very low efficiency) to overcome the lack of heat distribution, resulting in significant vapor drive [1] wherein radiant heat and air pressure pushes water vapor out of warm spaces through permeable walls, therefore keeping them dry.

Vapor drive and the location of the dewpoint boundary are important principles in designing a house's envelope, and maintaining permanently enclosed mold sensitive materials above the dewpoint is needed to prevent mold in walls and other areas.

This is especially critical in houses with vapor barriers (usually plastic film in the wall), which essentially trap vapor on either side of the barrier, so the dewpoint boundary (the line in the wall assembly maintained at a temperature where vapor condenses) should always be in a place that can easily dry out the condensate that develops there. Otherwise, mold will likely grow.

1. https://www.finehomebuilding.com/project-guides/insulation/v...

That's the main point - much of the mold problem can be attributed to vapor barriers; if you have them, you have to maintain the thermal envelope and the moisture content; if you do not have them, you only really have to worry about it in perpetually wet areas (bathrooms, kitchens) as the others will let the air and vapor through.

This is why abandoned houses often don't have much mold to speak of unless you're in a very year-round damp climate.

So if we wanted to build mold-resistant houses that were designed to idle at 50 or 60 degrees, we certainly could.

Most houses should probably have more humidifiers/dehumidifiers than they do; much of what we do for "climate control" is very ancient technology.

> That's the main point - much of the mold problem can be attributed to vapor barriers;

Indeed, if I lived in a cold climate, I'd much rather have a vapor permeable but continuously sealed and insulated wall assembly than a vapor barrier.

> So if we wanted to build mold-resistant houses that were designed to idle at 50 or 60 degrees, we certainly could

Totally. Wood framing, plywood sheathing, and drywall's paper backing are prone to mold when exposed to condensation, but those are the most economical materials we have for building. Keeping condensation off those materials at low temps is challenging, and most easily accomplished by heating them along with the space.

OTOH concrete (i.e ICF) is pretty impervious to condensation, although it has other major issues with that make it not ideal for most house walls.

> much of what we do for "climate control" is very ancient technology

Totally++

At least codes are starting to get better - requiring results rather than methods.