Humidity and Window Condensation in Your Home

Humidity in your home is caused by human factors such as cooking and bathing. When humid air meets a cold surface like an interior window, the moist air may condense on that surface. This condensation can damage painted surfaces over time and can also lead to the formation of mold in your home.

Condensation formed on a window.

What is humidity and where does it come from?

Humidity is vaporized water in the air in your home. Any human activity that involves water will cause humidity levels too rise. We generate moisture when we breath. A home with many occupants can cause a significant increase in humidity levels in a home.

Why should I be concerned with humidity in my home?

We need some humidity in the air in our homes to maintain proper air quality and to ensure that we don’t get dry, itchy skin. Too much humidity is not desirable either. Overly humid air in the winter will condense on your windows and other cold surfaces and that could lead to damage to the window or to the development of mold.

How do I maintain proper humidity levels in my home?

  • Make sure all your vents (basement, attic, crawl space) are open, adequately sized and cross-ventilated.
  • Turn your exhaust fans on in the kitchen laundry room and bathroom when you are using them.
  • Be careful to follow your manufacturer’s direction for properly venting gas appliances.
  • Turn down, and consider stopping the use of a humidifier in the winter months
  • Ensure you have fresh air in your home by opening a window for a few minutes each day
  • Make sure that your exhaust fans vent directly outside and not into attics or crawl spaces. Exhaust fans over ovens are especially important to vent in the winter months.
  • Window coverings should be open during the day – like blinds or drapes – to ensure proper airflow over your windows.
  • Check your humidity levels on the thermostat on your furnace.
  • When you cook, use covers on pans and pots to minimize moisture into the home air.
  • If possible, water your plants less. They release water vapour into the home air.
  • Consider installing an air-to-air heat exchanger if you have a high efficiency, tightly insulated home.
  • Ensure that your furnace is working properly and that it’s serviced regularly.
  • Check to make sure that your fresh air intake is not blocked by snow or other obstructions.
  • Use the bathroom exhaust when showering or bathing and take shorter showers.
  • Remove the interior screens on windows during cold months
  • If you’re having challenges lowering humidity levels you may want to consider using a dehumidifier in your home.
  • Check to make sure that the cold air returns in your home are not blocked or covered to maintain proper air circulation in your home
  • Basements walls can be a source of humidity. Using a local dehumidifier in the basement may reduce humidity levels in your home.

What are the ideal humidity levels I should consider in my home?

Building and government housing agencies recommend that homeowners keep humidity levels between 20% and 50%. The warmer the air in your home the more water it can hold. Relative humidity is a term that describes how much humidity is present in the air, compared to how much there could be. The amount of humidity air can hold is relative to the air temperature.

The following table shows the recommended indoor humidity levels in relation to the outdoor temperature:

Outside Temperature Recommended Relative Humidity
+20° and above 35% to 40%
+10° 30%
-10° 20%
-20° 15%

Source: Moisture and Air : Householder's Guide - Problems and Remedies

When interior humidity levels are too high in comparison to the cooler outdoor temperatures, condensation can form on the coldest surface in a room – like the glass in a window or door. As a result, homeowners should lower the relative humidity levels in your home during the cold winter months.

What causes humidity?

We are the source of high humidity levels in our homes.

  • One person’s breathing releases about half a cup of water into the air per hour.
  • Showering puts a cup of moisture into the air, bathing about half a cup
  • Cooking for a family of 4 produces 10 cups of water into your home over a 24 hour period

Adding only eight to 12 cups of water into the air raises the relative humidity of a 1,000 sq. ft. home from 15% to 60%, assuming that the temperature remains constant.

Is there anything else I can do to reduce humidity levels in my home?

Installing new windows that are more energy efficient than your current windows can provide better thermal insulation and, as a result, increase the temperature of the inside glass of your window. This will result in less condensation on your windows.

To ensure you maximize the benefit of higher insulation values you need to make sure that the windows are properly installed. This includes properly caulking and insulating the new windows. This will help reduce cold air infiltration due to poor window installation.

The quality of your window installation is just as important as the quality of your windows. Some questions to consider:

  • Does the company that made your window also install them?
  • Do they guarantee the installation in exactly the same way that they guarantee the window itself?

Can new, energy efficient windows, that are properly installed actually can cause more condensation on my windows?

Yes, because new properly insulated windows reduce the amount of air leakage in your home. Older windows will generally allow more airflow to take place between the inside and outside of the home. Old window usually are poorly sealed and caulked and the older windows themselves can be quite porous due to wear and tear and natural breakdown of the windows.

High efficiency window that are properly installed will be much more air-tight. The moist air in your home will stay in your home. That means you need to be much more diligent about controlling the humidity levels in your home.

What causes condensation on the outside of my windows?

Homes with high-efficiency windows may experience condensation forming on the outside of their windows under certain weather conditions. This condition usually occurs in late summer and early fall when the night time temperature drops several degrees – much like dew forming early in the morning.

Early in the morning, dew forms as droplets of water that appears on thin, exposed objects in the morning or evening due to condensation. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.

Moisture on the outside of the window is condensation. It will evaporate when the outside temperature rises. As we know, condensation happens on cold surfaces. During the night, if the outside temperature drops below the dew point then condensation forms. The fact that the condensation is forming on the windows means that very little heat from inside the house is escaping through the windows. That’s a good thing and proof that you have energy-efficient windows.

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