There are whole-house dehumidiers that operate separately than your heating/cooling system. Set a relative humidity (RH) range and let it work; e.g.:

* https://www.santa-fe-products.com/product-category/dehumidif...

The air movement problem in a tightly sealed house is dealt with an HRV (or ERV) which brings in filtered fresh air, but tempers it with the exhausted stale air:

* https://en.wikipedia.org/wiki/Heat_recovery_ventilation

* https://en.wikipedia.org/wiki/Energy_recovery_ventilation

These are solved problems, it's 'just' a matter of budget and mechanical constraints in fitting the equipment (existing ducting can be re-used).

In some regions with strict insulation building codes, balanced ventilation with a heat exchanger for recovery is mandatory. Well, almost mandatory, it can get complicated.

Even old draughty homes suffer really badly from damp and mold due to condensation once you get down below a certain temperature for sustained periods. I grew up without central heating (Im 40 and this isn't that unusual for a lot of the UK) and later lived in older unheated houses a couple of times and damp was always a huge issue despite their draughtiness. 13° sounds about right for average indoor temp in unheated parts of the home in winter in the house I grew up in, but it's worth noting that a) you spend most of your time in the bit of your home that you do heat, be that the kitchen or the living room, and b) when you wake up in an unheated bedroom in the middle of winter your walls are damp and if it's got below zero you will have ice on the inside of the windows because the temperature will have dropped a lot lower than 13°. This is, not to put too fine a point on it, miserable and if you want to control the damp you then have to open the windows to air out your room, so it never gets warm. Having said that 18° which is the temperature I keep my house at in winter is perfectly pleasant as long as you wear a jumper and is warm enough that I've never had issues with damp or mold.

Old houses tend to pull moisture into and out of the insulation between summer and winter, which can become a problem above and beyond the high heat bills that they create. Matt Risinger had a great video where they were rebuilding a 1930s house and you could see where all the moisture had built up and wept back out of the insulation, staining the wood behind the plaster.

My understanding is the problem with making old buildings airtight is the lack of proper vapor barriers under the house, especially if it was built on a slab. For homes built on crawlspaces, you can "encapsulate" them, which reduces energy losses and improves air quality. Typically encapsulated crawlspaces are ventilated and/or dehumidified, which is supposed to stop moisture ingress into the living space.

Love to see a Matt Risinger comment here!

It doesn't need to be at the dew point inside, if it's a few degrees lower we already win a ton. The 21°C is really just comfort, buildings will be fine at 15°C just as well.

We recently got a dehumidifier for a room that we aren't using most of the time. Better put some greenwashed kWh into taking moisture out of there than burning gas with the conventional heating system to keep the room hot (and it would still be too wet).

We've done exactly the same. I find the air 'feels cleaner' as well.