Condensation happens when water vapor in the air turns in to liquid water and settles on a surface. There is always some water vapor in the air of your house due to cooking, washing, bathing and breathing, as well as the water vapor that is present in the air outside.

Warm air can hold more water vapor than cold air. If the air in most of your house is pretty warm in most places, all the water in the air stays as a vapor. But the air that is touching your windows becomes chilled by the cold glass, which causes some of the vapor to settle out on the window pane. As the air in the room circulates over the windows, more and more water vapor condenses out on the window pane, eventually producing drops or puddles.

Anything that increases the amount of water vapor in the air of your house will exacerbate this problem. Adding an occupant adds more water vapor, since breathing is usually the single most important source of water vapor. And additional cooking, cleaning or bathing are all vapor sources as well. And tightening up the house can contribute as well, since it reduces the amount of exterior air (which is relatively dry) that gets in to the house to dilute the humid interior air.

One thing that most people don't think of is the effect of adding a window covering (curtains or blinds or similar). These cause the window pane to be even colder by blocking much of the heat of the room from reaching the window, but because air can still get through to the glass, condensation still occurs, at an accelerated rate due to the colder glass.

Condensation on the windows is indeed bad, as it will eventually destroy them, and may also lead to mold growth. The problem will grow worse the colder it is outside. There is little other seasonal effect.

Running your bathroom fan and your cooking fan is always a good idea, and both can probably be left on longer. The bath fan in particular should run for an hour or so after finishing a bath or shower.

Running a dehumidifier anywhere will help somewhat, but it is treating the symptom, not the cause, and may be expensive.

So what to do?

(1) Provided that condensation is mostly limited to the single pane windows, then the main problem is not excess humidity throughout the house, but rather cold windows that cause the localized condensation you observe. You could consider something like Vesta's SecondPane windows, which are an air-tight secondary window that fits over the interior of your original windows. SecondPane windows reduce condensation problems by preventing humid interior air from reaching the cold glass of the original windows, just like a conventional double pane window. These could be an important part of the solution to the condensation problem, as well as a significant upgrade to the comfort in the living room.

(2) If the condensation regularly extends to the double pane windows, or to other surfaces in the house, then it may be necessary also to reduce the interior moisture somewhat. The dehumidifier is one option, but an expensive one to operate. A new bathroom fan with a continuous low-speed setting is another option. Least expensive of all is a programmable switch that turns the bathroom fan on for a few minutes each hour to help ventilate the whole house. But the best way to reduce interior moisture is with a heat-recovery ventilator, which expels the humid indoor air while pulling in and pre-temping fresh air from outside.