Eliminate Frost Around Window Edges

Moisture and ice at the bottom of windows during winter can be controlled by lowering the indoor humidity.

by Brett Marin

From The Family Handyman

Q  We live in northern Minnesota and it gets pretty cold in the winter (minus 30 degrees F air temperature). Even though we installed low-e double-pane windows and the humidity level in our house is low (35 to 40 percent), we still get ice on all of our windows. The only way I’ve found to defrost my windows is to aim fans at them. What can we do to remedy this situation?

Cheryl Bisping, Virginia, MN

A  First you have to come to terms with what you can realistically expect from your windows. Double-pane windows with a low-e coating have an insulation R-value of about 3. Theoretically, they can sustain an indoor relative humidity of about 70 percent at 30 degrees F and 60 percent at 0 degrees F. With any higher relative humidity, condensation will collect on the glass.

But a window’s performance in the real world is different. The edges of the glass in double-pane windows, the area you see wet and frosted, get considerably colder than the center of the glass. Depending on the sealing method, the edges can only sustain 45 to 55 percent relative humidity at 30 degrees F and 25 to 35 percent at 0 degrees F before condensation begins. If you try to sustain a relative humidity at 40 percent or higher, your windows will be continually wet from condensation.

In your case, the simplest solution for controlling condensation is to reduce moisture sources. That means covering damp crawl spaces, sealing basement walls and floors, covering aquariums, reducing the number of houseplants, running ventilating fans during showers and while cooking and making sure combustion devices are venting properly. Allowing more heat to get to the windows by keeping drapes open and circulating air with room fans (as you already do) will also help. Flushing out moisture by installing an air exchanger (an expensive option) will also lower indoor humidity.

When it comes time to buy new windows, buy a type with a higher performance edge seal or consider triple-pane glass. Either will allow you to sustain a higher, more comfortable relative humidity in zero and subzero temperatures.

From The Family Handyman - February 2006

• • Exterior Projects
• • Home Repair

• Are Triple-glazed Windows Worth the Extra Cost?
• Compare the cost and energy savings for low-e double-glazed windows vs. tripled-glazed windows before buying.
• 
• Solutions To Window Condensation Problems
• Use a hygrometer to diagnose moisture and humidity problems inside the house.
• 
• Insulated Window Glass Fogging
• What is double-pane or Thermopane glass, how does it work and what causes it to fail?
• 

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