Mold is due to inadequate ventilation, not low temperatures.

It's both. Humidity will more readily condense on lower temperature walls and windows resulting in mould. You need a combination of ventilation to lower the dewpoint and warmer walls so as they don't meet the dewpoint.

Your choice of spelling of "mold" is consistent with your apparent lack of experience living in high humidity parts of the world :-)

Are you saying that the US is low humidity? Visit Florida.

In most buildings, ventilating a room while preserving temp in winter involves heating the incoming air. So there's a sweet spot between heating and ventilating when it comes to avoiding humidity.

that's why you don't ventilate by opening windows wide open but with purpose built fans with a heat recovery device.

That's the sense of the "most buildings" caveat: double flow ventilation systems are still uncommon in Europe, especially for housing.

If you keep your place at 15C and have a hot shower, you're going to need a hell of a lot of ventilation to keep the mold down.

Having a hot shower would be rather inconsistent with keeping a place at 15C, at least when the purpose is to save energy.

But you cannot ventilate everything and mold comes with moisture.

If you can't crack the windows open a few times a day for a few minutes, then you got bigger problems that require proper solutions. In this case a proper dehumidifier is much more energy-efficient than keeping the heaters on.

If you have a old house, with a basement and lots of stuff, than it is quite hard, that the ventilation reaches every corner.

And those corners will get moldy.

Oh and I don't have a house, nor a mold problem, but know enough people who do have, even though they do ventilate reasonable. Houses were build with the assumption, that they are heated.

Now you can build new houses that are better designed - but you cannot magically adjust all the existing houses.

>Houses were build with the assumption, that they are heated.

Yes and no.

It depends on the country and areas within it and it applies mainly to houses built in the last 60-70 years or so, maybe a few more years back in cities (for heating with radiators) before that time many, many houses were (poorly) heated by fireplaces and/or stoves.

Besides, the permeability of the building materials was traditionally much better, and windows/doors were leaky, guaranteeing a much better ventilation.

At least here in Italy the heating was a luxury until well after World War II, particularly in the country and in small vilages, anecdotally my family used to have a house in a small village in central Italy where the heating until the '70's was based on the devices that can be seen here (Italian):

https://www.muet.it/storie/tutte-le-storie/item/il-riscaldam...

Particularly the kitchen and dining room was heated by the "stufa economica" that was also used to cook meals, the sitting room had a fireplace that heated also (a little) the bedroom behind it, and there was another large bedroom heated by a "stufa in terracotta", having a "prete" for each bed was needed.

The house was anyway very, very cold, particularly at night/early morning.

"It depends on the country and areas within it"

Well, obviously it does, but I would think most houses in europe do have to be heated and were build in the last 60-70 years and those I was refering to.

And before that, yes it was something like 2 bugs cancel each other out. Leaky windows prevents mold.

(And in general only rich people could afford to heat the whole house)

Dehumidifiers are actually a pretty good way to heat your house if you have a humidity problem. You recover significant amounts of latent heat of vaporisation when running it.

I keep my house at 18C. It feels ok with a jacket or a blanket. There's no mold in sight anywhere. I guess you would need to have a humidity problem in your house to be affected by mold (my house has a central ventilation system which also keeps the humidity at bay - never above 40%).

A jacket at 18?!

If you are sitting all day in front of a computer, you need something. If you walk around, you'll be fine.

I reduced the indoor temperature this year to see how it was, and kept ~18'C. I think I'll keep it anyway - less energy used, and it's totally doable. When I have someone over for dinner, I bring it back to 20-21'C.

When I was sitting at my computer for long periods I'd just make a warm bottle with hot water to put my feet on and also drink hot tea. That was enough to be comfortable even at around 16°C.

Maybe we mean something different with "jacket". Don't people wear sweaters and thermal underwear at home? And with those you would be conformable at 15 to 17.

English is not my first language. I suppose you would say "sweater" (but I said jacket because I prefer those garments with a zipper at the front: which I thought was always called "jacket" regardless of whether it's very light or thick).

You're not wrong. What people think will depend on context, and probably on region. In US English, jacket is a wide ranging term that could easily include a zip up sweater. If you wanted to be extra clear, you could saw "sweater jacket" or "fleece jacket" (if it's not woven).

It is very difficult to compare self-reported temperatures! Really if you get into it, there are many crappy sensors, crappy thermostats, thermostats mounted near single pane glass, etc... I spend some time complaining about my Nest V3, many people did, but it's just too difficult to get a reliable measurement. Btw, in the end my conclusion was that the Nest was doing quite well. Just the Shelly and the Hue sensors were easily 1-2 degrees off.

Btw, what's up with Nest? I want another V3 in my new house, but they are not available anymore??

> There's no mold in sight anywhere.

Not yet anyway. It usually starts in e.g. bathrooms.

I've been keeping it cold for at least 7 years. Maybe because my bathrooms have heated floors, so there's never any water after a few hours you've showered, there just isn't any mold at all, it's perfectly clean.

I suggest buying a hygrometer so you don't have to guess. In most places you'll quickly find out that you can bring humidity down to acceptable levels with a few minutes of "shock" ventilation. You can of course turn on the heating for a few minutes to bring humidity down a few percent-points more (entirely optional), but it absolutely doesn't have to be turned on all day long to have acceptable levels.