While the above facts about air-leakage and lack of insulation are true, the single biggest cause of lower RH during the winter months is central heating.
Heated air is very dry and sucks the humidity out of indoor air.
Not quite true, although people often think of it that way. Heating, in and of itself, does not take any moisture out of the air. But the relative humidity goes down, because the warmer air is capable of holding more water vapour. The biggest cause of low humidity is winter air plus excessive air exchange. The outdoor air in winter is naturally "dry" compared to summer air, and when warmed up to room temperature it has a very low relative humidity. If you had no air exchange in a house though, the humidity would gradually buildup up from cooking, bathing, breathing, etc. In a tightly sealed house in our climate you actually need to add ventilation to control excess humidity (and also to control air quality). But we tend to associate low humidity with central heating because, before the advent of sealed combustion furnaces, the central heating system was forcibly introducing more air exchange in our houses than was really necessary.
If you leave your hood exhaust fan off (not normally a good idea, especially if you cook with gas), the steam will raise humidity throughout much of your house.
