So keen is this draft, so high this pressure, that some loosely-built houses and rooms, with only a few people in them, will in very cold weather be almost sufficiently ventilated through the natural cracks and leaks without opening a window or a door at all.

And what is of great practical importance, an opening of an inch or two at the top of a window will admit as much fresh air on a cold day as an opening of a foot and a half in spring or summer, so swiftly does cold air pour in.
Bearing this in mind, and also that it is always best to ventilate through as many openings as possible, both to keep drafts of cold air from becoming too intense, and to give as many openings for the escape of the foul air as possible, there will be little difficulty in keeping any room which has proper window arrangements well ventilated in winter.
An opening of an inch at the top of each of three windows is better than a three-inch opening at the top of one.

But you must use your brains about it, watching the direction of the wind, and frequently changing the position of the window sashes to match the changes of heat in the room, or of cold outside.
No arrangement of windows, however perfect, is likely to remain satisfactory for more than an hour at a time, except in warm weather.
This watchfulness and attention takes time, but it is time well spent.
"Eternal vigilance" is the price of good ventilation, as well as of liberty; and you will get far more work done in the course of a morning by interrupting it occasionally to go and raise or lower a window, than you will by sitting still and slaving in a stuffy, ill-smelling room.
Plenty of Heat Needed.

Any method of heating--open fireplace, stove, hot air, furnace, hot water, or steam--which will keep a room _with the windows open_ comfortably warm in cold weather is satisfactory and healthful.

The worst fault, from a sanitary point of view, that a heating system can have is that it does not give enough warmth, so that you are compelled to keep the windows shut.