Venting significantly reduces the humidity of your air, driving down the heat capacity of it. Meaning you will need less energy to heat up the room.

This thread seems to give a great overview of all interactions: https://physics.stackexchange.com/questions/45349/how-air-hu....

It's actually common part of contracts in Germany that you have to vent at least 10min per day through 2 different rooms etc.

You own link points out that the difference between humid and dry air in terms of heat capacity is a low single digit percent. Unless you can reduce the humidity from "a lot" to "basically nothing" and lose only a couple of degrees in the process (highly unlikely, completely impossible if your method consists of opening windows/doors in the winter) it is going to cost more energy to vent out the humidity and heat dry air than it is to just heat the humid air that's already near temp.

Just because a rental contract includes a clause doesn't make the intentions of that clause consistent with reality.

Furnaces dry out the air. Human activity can contribute moisture to the air. But the net effect is usually to leave the interior air dryer than the exterior air. Depending greatly on your lavatory ventilation.

Where I live, allowing outside air in greatly increases the inside humidity. And I would guess this is true for most people given the widespread complaints of dry winter skin and bloody noses. Which is the product of increased furnace operation.

> Venting significantly reduces the humidity of your air

Even when it's raining outside, etc?

If it's raining, then obviously it's not very cold outside. Unless you're in the habit of cranking your furnace up to tropical temperatures I guess. For me, I keep the temperature above freezing inside but otherwise? I put on a sweater and open the window. It's cheaper, refreshing, and better for the environment. Right now it's 8 C outside; with the window open and a sweater on, I'm as comfy as a clam.

Ok but you must that this is unusual and most people keep their thermostat around 21 C.

I don't know, it's typical for my family. It seems inappropriate to me for people to set their furnace so high just so they can dress like it's summer in the winter.