Categories: Windows

More On Winter Window Condensation

I’ve written about window condensation and ice a couple of times, Winter Window Condensation Problems and Winter Window Condensation. In both segments I wrote about how new energy efficient windows are also prone to these problems. I’ve received many questions about window condensation so I thought I’d review the basic problems that cause this winter re-occurrence.

The number one reason that condensation forms on windows (new or old) is excess moisture (humidity) in the home. Condensation and ice form on windows because the window surface is below the dew point for the air near the window, so some of the moisture in the air condenses on the glass. The higher the relative humidity of the air near the window, the higher the temperature of the dew point. Basically there are two ways to prevent this from happening. The first is to reduce the relative humidity in the house and the second is to increase the temperature of the window glass.

In today’s homes we are bathing more, using our big fancy multi-head showers, running loads of laundry, washing dishes and living in homes that are very “tight”. All of this leads to higher humidity levels. New windows only make this problem worse by being so “tight” and sealing out the drafts.

The easiest and most cost effective way of solving this problem is by using sufficient ventilation. The Home Ventilating Institute (HVI) recommends the following Air Changes per Hour (ACH). (See HVI)
I. Bathrooms – 8 ACH
II. Kitchens – 15 ACH
III. Other Rooms – 6 ACH

Using these guidelines you can calculate the cubic feet (length x width x ceiling height) of each room and determine if the exhaust fan you have is sufficient to create the recommended air exchange. If the exhaust fan you have is not large enough, you may be able to set it up with a timer to run while you’re away from the room to get the proper air exchange in a given hour.

If you work on the ventilation and removing the excess moisture you should be able to minimize the window condensation problems.

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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View Comments

Eric says:
September 25, 2009 at 10:05 am
This topic resurfaces every winter. We also run into exterior condensation during the late summer and early fall months. When the night air cools off. Your information regarding the humidity levels are spot on. One point you might mention, is that new windows with low-e coatings are reflective (reflecting heat back to the room in winter or away from the house in the summer). This causes condensation to form sooner, as the glass cools quicker. So while the window industry continues to improve glass technology, ventilation will, and has, become more and more important.