Akenaton and Robert Bridge have diagnosed the cause of the problem.

Here in Alberta where it can get very cold and the humidity very low, most people have humidifiers attached to their furnaces. Sweating and ice on the windows is unavoidable if warm moist air is desired in the house, especially if there is a small leak in the double panes.

Our water is high in calcium-magnesium; the filters in the humidifiers have to be either cleaned or replaced often, a nuisance.