In fact, this tendency of hot air to rise, and of cold air to sink, or rush in and take its place, which is the mainspring of nature's outdoor system of ventilation, is one of our greatest difficulties when we wall in a tiny section of the universe and call it a room.

The difficulty is, of course, greatest in winter time, when the only pure air there is--that out of doors--is usually cold.
This is one of the few points at which our instincts seem to fail us.
For when it comes to a choice between being warm or well ventilated, we are sadly prone to choose the former every time.

Still we would much rather be warm _and_ well ventilated than hot and stuffy, and this is what we should aim for.
The main problem is the cost of the necessary fuel, as it naturally takes more to heat a current of air which is kept moving through the room, no matter how slowly, than it does a room full of air which is boxed in, as it were, and kept from moving on after it has been warmed.
The extra fuel, however, means the difference between comfort and stuffiness, between health and disease.

Fortunately, the very same cold which makes a room harder to heat makes it easier to ventilate.

When air is warmed, it expands and makes a "low pressure," which sucks the surrounding cooler air into it, as in the making of winds; so that the warmer the air inside the room, or the colder the air outside of it, which is practically the same thing, the more eagerly and swiftly will the outdoor air rush into it.