Condensation & Humidity

What Is Condensation & What Causes It?

When water changes from it gas or water vapor form to a liquid form, you get condensation. Condensation occurs when the temperature of a surface is lower than the dew point of the air around it. This often happens when warmer air comes into contact with a cool surface like the side of a cold beverage. Also the natural cooling of the air itself can cause condensation – which is how rain and snow to form.

Exterior Condensation

While energy efficient windows help control temperatures within your home, they can occasionally collect condensation on the exterior glass surface. Exterior condensation happens during periods of warm weather and high humidity near sunrise. Since energy efficient windows reduce the amount of heat that escapes to the outside (beneficial during cold weather), the exterior glass is cool enough to form condensation for a short time.

You can usually see exterior condensation during the following conditions:

• High relative humidity (i.e. outside temperature near the dew point)
• High performance glazing system (e.g. double glazed with Low-E glass or better)
• Early morning
• Unobstructed exposure to a clear sky to the north, west, or south
• No wind

Interior Condensation

Interior condensation happens when the indoor humidity of your home has a higher dew point than the temperature of the glass at the bottom of your window. If the temperature at the bottom of your window is below 0°C then frost forms

How to Reduce or Stop Condensation

There are some simple steps you can take to reduce or even stop condensation from accumulating on your windows.

Condensation Resistance Techniques

A window manufacturer may use one of the following to reduce condensation:

• Inserting gas between glazing
• Adding low-emissivity (Low-E) coatings that increase glass temperatures
• Insulating spacers that reduce heat conduction
• Providing non-conducting sashes and frames

Curtains or Blinds

Keep curtains, blinds, valances, and other interior window accessories from preventing air flow around the surface of the window. Restriction of air movement increases the chance of window condensation.

Hot Air Registers & Heaters

Controlling the Relative Humidity of Indoor Air

• For excessive humidity – increase ventilation in the home
• For humidity issue once or twice during the winter – open two windows on opposite walls or turn on an exhaust fan if the room has one
• For persistent condensation – consider installing a controlled mechanical ventilation system, preferably with a heat recovery unit

Humidity Reduction Tips

• Disconnect humidifiers (fixed or portable) on your heating system
• Ensure clothes dryer is properly vented to the outside
• Turn on bathroom and kitchen ventilation fans when bathing or cooking
• Cover the beaten earth of crawl spaces with 0.15 mm (6mil) polyethylene (if needed, vent crawl space during the summer)
• Keep basement well drained and protected from excess moisture
• Make sure gutters and the slope of the land around your home drain water properly
• Store firewood outside
• Consider installing a heat recovery ventilation system (HRV) if high humidity is a chronic problem
• Check that your mechanical ventilation system works and is turned on