What causes condensation on my windows?

In a word, moisture.

But moisture’s good, right? Yes, humidity is good and necessary, but only in the proper amount. Whenever there’s excess moisture, it can cause problems such as condensation on the inside of your windows.

Winter is the time when the problem becomes most visible. Those pools of water on your window sill and those water droplets or frost collecting on your window panes are the visual signal that warm, moist air is contacting a surface that is too cold.

Not only is condensation on windows a nuisance, over the long term it can damage window frames and the walls below.

If there is condensation on only one window, it suggests there is a problem with airflow in that location and it may be possible to correct by keeping the blinds or drapes open or altering the ventilation in that area. However, if there is condensation on most or all of the windows in your home, then the problem is too much humidity.

A humidity level of about 40 percent, and no greater than 45, is about right in the winter time. During very cold temperatures, a much lower level may be needed to prevent window condensation. To gauge your home’s humidity level, you’ll need a hygrometer and if you don’t already own one, you can pick one up at a hardware store or home improvement centre.

So if you are getting a high reading and are experiencing condensation on your windows, you need to take measures to:

  • reduce humidity
  • ensure your home is sufficiently ventilated
  • maintain adequate heating throughout your house; and
  • ensure proper air circulation

More specifically, you should:

  • stop using a humidifier if you’re using one
  • dehumidify the basement during fall, spring and summer
  • make sure your walls and ceilings are adequately insulated
  • heat all indoor areas in your home (blocked off rooms or unheated basements are problematic)
  • ensure your windows aren’t blocked off by blinds or drapes
  • make use of your air exchanger if you have one

There are many possible sources of excess moisture, but here are a few key things to watch out for: don’t hang clothes to dry indoors; make sure your clothes dryer is venting properly to the outside; avoid drying firewood inside your home; avoid steam cleaning your carpets during the winter; fix any leaks in your roof, walls, basement or plumbing; insulate cold water pipes; always use bathroom exhaust fans when bathing or showering and always use the range hood fan when cooking.

If these measures do not correct your condensation problem, a final and sometimes unavoidable solution is to replace your windows with energy-efficient windows. New, high performance windows have better insulating properties due to their special “low-E” coatings and being filled with argon or krypton gas. Although it’s an expensive solution, the energy savings are significant over time.

On a final note, if the condensation is not on the inside of your windows but between double panes of glass, that’s a different kettle of fish; it means the seals are broken and the windows should be repaired or replaced.