I've spent a lot of time in very old houses with limited to no modern heating, and I've never experienced this. The biggest indicator for mold is moisture trapped between walls (including animals getting stuck in walls and dying). But that's a water and humidity thing, not an ambient temperature thing.

I think some very old houses don't have mold issues because they aren't tight enough. You get so much airflow through the walls, they don't really get damp.

Source: lived in a ~100 year old house in the Midwest for 15 years or so. The only place we got mold that I remember was inside the air conditioning unit.

Also survivor bias.

You need a certain amount of wet for mold and mildew. Push the levers too far and you just end up with termites, or a failed foundation, and the whole thing gets replaced with something new.

The houses that have beaten their nemesis, water, are the ones most of us are willing to walk into more than twice.

Reminds me of when my mother had her kitchen renovated. When they removed the lathe & plaster, you could see the road clearly, right through all the slats. From outside they look tight, but they are anything but.

Depends on the construction. I bet Victorian brick and timber houses would be fine, since they're well ventilated throughout the structure, and were not intended to be intensively heated.

I grew up in a house like that and we had an unusable moldy basement because it was supposed to house the kitchen with a range that was burning most of the day. But of course basements are naturally damp, while above ground is not.

Care to elaborate? How does cold cause mold?

Cold spots in a heated house and a lack of airflow seem to cause mold. The temperature difference causing condensation, excess moisture with nowhere to go and then mold.

Interesting whether this would happen in an entirely unheated building.

I suppose it depends if there is some types of poorly placed barriers. In structures that can breath readily and in climates where they can dry it is less of an issue. On other hand those also need to in general be protected from water in structures.

A lot of that is due to structures not having a proper vapor barrier, therefor allowing vapor exchange from the outside world which decreases in-wall condensation.

It does if it experiences any kind of a diurnal heating cycle from the sun.

Moist air from cooking increases the humidity in the house. The walls of the house are cold, the moisture condenses on the cold walls keeping them damp for the entire winter.

If you're thinking, "but I don't cook soup that much", remember the chemical equation for burning methane with oxygen.

Or just humans breathing. And plants. Pets too.

In areas with coastal fog it can be the case that the cooler months with lower evaporation rates are more likely to allow spores a foothold. This is often seen in older spaces designed with passively vented kitchen and baths.

But good building design following current standards isn't going to be affected by this.

Life has had half a billion years to figure out that if you produce offspring at the optimal (min-maxed, really) survival time, you get more offspring.

It's not just that spores work better in certain weather conditions, it's also that there are often more spores in those conditions as well. Mushrooms coming up after a rain, gambling that there will be another one in a couple days, for instance.

You can regulate humidity cheaper than temperature.

Depends if you live in a climate where just regulating humidity is enough to guarantee thermal comfort, something that is dependent on humidity, operative temperature, clothing level, metabolic rate, and air flow rate. And people are general only comfortable in environments that are 30-60% relatively humid so that’s a pretty small window.

Mold growth is actually slowed down by cold. Cold by itself would not grow mold, you'd need relatively high humidity and lack of ventilation. The home construction and regional climate determines how prevalent mold is.