[memetherapy]

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.