Humidity Control

At the height of winter, the formation of frost or ice on the interior of home windows is a common problem for Albertans. What are the causes and consequences of frosty window panes? Should the problem be addressed, and if so how?

Condensation forms when indoor moisture makes contact with the cool glass surface and in sub-zero temperatures ice may appear. Occasional light condensation and frost does not pose a problem, however, excessive amounts persisting over long periods can cause major damage to your window frame and wall below.

The first step in tackling excessive condensation is to control the relative humidity (RH) and moisture sources in your home. RH is the ratio of the amount of moisture in the air at a specific temperature to the maximum capacity of the air at that temperature.

No matter how dry the air feels it still contains some moisture, and the colder it is outside the greater the potential of indoor moisture turning to ice on the window. Here are a few easy ways to control excessive moisture.

• Avoid drying clothes indoors

• Avoid bringing in fresh wood to dry

• Use your kitchen and bathroom vent fans to extract moisture to the outdoors

If you want to measure the relative humidity inside your home use a hygrometer. RH levels vary depending on the window type. The following chart, based on double glazed windows, indicates allowable maximum RH levels at various outdoor temperatures.

A properly functioning furnace humidifier helps control moisture levels inside the home.

If you’ve tried controlling your moisture levels and still experience problems, additional ventilation may be required. One possible solution is the installation of a Heat Recovery

Ventilator (HRV) - a device which exhausts stale, moist air to the outdoors and brings in fresh air to distribute though existing ductwork. Though HRVs are fairly expensive to buy they are very energy efficient to operate. Eighty per cent of the used indoor air is used to pre-heat the cool outside air.

Excessive frosting on windows may also mean it’s time to replace poor-performing windows with warmer energy-efficient ones. Replacing old windows may eliminate the need for controlled ventilation altogether.

If you can not afford better windows, consider placing shrink wrap over your existing windows during the winter months. Installation kits can be purchased at local homebuilding supply stores. Temporary or permanent storm windows are another option.

Make the following inspections if the frosted window is an isolated case.

• Check for air leaks with an incense stick or a thin piece of plastic. Plug leaks with an effective air sealant on the warm side of the window for optimum effectiveness.

• Check to make sure the heat register under the window is not blocked

• Draw your drapes in the evening. This may bring warmth to your window and even help keep valuable indoor heat from escaping.

• Relocate plants situated near the problem window(s) as plants produce quite a bit of moisture

If condensation or frosting problems occur between window glazes, this means the seal is broken and the entire window may need to be replaced.