Not heating a house leads to mould. It’s cheaper to keep the house adequately warm than get rid of mould.
https://www.youtube.com/watch?v=IEMZk5Kjqkw
Freezing pipes is another issue, but that can be remedied by conditioning to 40 or 50 degrees (freedom degrees™) but I suppose you could build a building with anti-freeze dedicated to the piping itself.
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...
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.
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++
Long story short; if you still heat your home, but some degrees lower than you used to do, you are probably fine regarding mould.
[0] https://www.omnicalculator.com/physics/relative-humidity
As @bombcar pointed out, preventing airflow to retain heat can cause mold.
* climate (ambient humidity and temperature)
* ventilation (both HVAC systems and habits of leaving windows open etc., but also draftiness in older houses)
Old houses can be more affected due to the addition of inappropriate modern materials, or for example lacking a bathroom extractor fan.
How do you cheaply create airflow in winter? Open windows 24/7 and turn on a fan? Thus increasing the need for body-heating and reducing house temperature even further?