During the winter, I often get high humidity in the home. Especially with multiple people living here, and cooking, or having showers etc.
This leads to condensation on the windows, and moisture on other surfaces, and maybe mould growing.
So how to decrease the humidity? Obvious answer is a dehumidifier, but how much electricity would that use? Or open the windows, but that lets cold air in, and often the outside humidity is nearly as high anyway. Any other options?
The outside relative humidity may be almost as high but the absolute amount of water held in the air is lower. So removing the most humid air is effective. That's the principle behind shower extractor fans and cooker hoods. Replacing a lot of lower humidity air with outside air lets a lot of heat out.
A dehumidifier uses a couple of hundred Watts when it's actually doing something (i.e. when it's making a noise), but this all contributes to heating the house, as does the latent heat recovered from the condensed water. This is a small domestic model that can remove a few litres a day; bigger ones are of course available but they're noisy.
So we use a dehumidifier for slightly more of the year than the heating is on; in summer opening the windows is preferable. In winter ours doesn't run flat out (almost all have humidistats) unless we've got washing drying. Cooking steam and shower steam are at least partly removed by fans as well, running them briefly when most needed.
I agree that at least a couple extractor fans or vents are a good idea, in both the kitchen and bathroom. (And does the clothes dryer vent outside?) Look into Heat Recovery Ventilators (HRV) and Energy Recovery Ventilators (ERV) if you're worried about heat loss. I also agree that just opening the windows once a day would be pretty effective.
It could also be that you really have a condensation problem, more than a humidity problem, from insufficient insulation. Condensation happens on items that are cooler than the room's air. When there's noticeable condensation on your windows, walls, and ceilings, it's worth checking out what your insulation is like, and maybe improving it. When the condensation is on windows, there could be air leaking through, or the window could be not built for winter (purely aluminum frames, for example, conduct heat very well, so in the winter the internal window pane/frame will be much cooler than other things in your room). The windows should be at least double-pane.
If you don't mind burning stuff indoors & have some extra money, a lot of people say wood stoves dehumidify the house.
Your best bet is to thoroughly ventilate the house by opening all windows at the same time. The resulting draft will quickly result in a complete exchange of air within a few minutes, so the "moist" air from your house or apartment will be replaced by the drier outside air (as pointed out by Chris H). The house itself won't lose heat. This is the way we ventilate in winter. The windows are kept closed except for the 5 minutes or so of thorough ventilation. I regard this step as a necessity at least once a day, otherwise the air gets rather pongy.
For a sustainable solution, place dried bamboo in areas with the highest humidity. Bamboo naturally absorbs moisture, odors, bacteria, pollutants and allergens. If you can't grow bamboo in your climate, try Moso Natural for product ideas. And you recharge the bamboo by placing it in the sun for a couple hours.
