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Window Condensation Season Starts Now!

Winter heating bills can be enough of a burden for home owners, but some people struggle with window condensation as well. There seems to be very little understanding of just what causes this condensation and annually there are many conspiracy theories traded around among the neighbors. This time of year can be very unkind to Builders and window Salesmen.

Let us look at the basics. Condensation in its basic form is simply the process of changing water vapor back to its original form of liquid. Whenever water vapor touches a cold enough surface, it can start to condense out and eventually turn to liquid. We see this happen in many places around the home and at all times of the year. We may have a cold-water pipe or a toilet tank sweating in the middle of the summer, or your favorite beverage.


Anytime we cool a surface by removing heat and then expose that now cooler surface to moist air, we can cause moisture in the air to start sweating on that surface. This is just what happens to our can of soda that we put in the refrigerator. The can of soda starts out at room temperature as we bring it home from the store and would not experience any sweating. After a period of time in the fridge the beverage will become colder than the house air. We take it back out, place it on the table and it starts to sweat. It is as simple as that.


Let us compare this soda can to your house windows. To start out with we must remember that it is the house air that tries to keep all the surfaces in your home reasonably warm in the winter. Surfaces lose heat to the outside based on their thermal properties often referred to as R-value. We may have walls with R-values in the range of 19-23, ceilings with R-values in the range of 38-50, but we have windows only in the equivalent R-value range of 3-5. With these much lower thermal values the windows will lose heat faster than any other surface in your home and therefore become the coldest surface your moist house air will come in contact with. Condensation will occur first on the coldest interior surface.


Closing the blinds all but eliminates airflow to the windows.(replacement energy) Turning down the thermostat lowers the temperature of any air that does get to the window, but this is all happening while the sun is going down and the temperatures outside are also dropping. Between this and the already low thermal properties of all brands of windows, it is a sure recipe for some condensation at some point in the heating season.


Windows condensation will typically start at the bottom of any individual window. This could be any style or type of window. Most often we may only have condensation on the bottoms of some of the windows. Why? It is all about replacement energy and airflow. You cannot get airflow to circulate in a square corner such as the bottom of a window set to the outside of the wall. Most folks close their window treatments at night for privacy and turn down the thermostat to save energy. This is typical but most often is the very cause of the window condensation itself.




1. Maintain air temperature in the range of 66-70 degrees while being very careful with nighttime thermostat setbacks and window treatments. Remove all screens from any casement window as they interfere with air movement to the glass.


2. Reduce interior moisture levels according to the outside air temperatures. The colder it gets outside the lower your interior relative humidity (RH) needs to be. You may have to reduce moisture levels to 25-30% RH, with inside air temperatures in the range of 66-70, especially when outside air temperatures get below 30 degrees.


3. Make sure your ventilation system, range hood and bath fans are working so you have the ability to draw down the moisture levels as needed. Have your ventilation equipment tested for function.


4. If your windows are wet in the morning, you must dry them off or the condensation may increase. This is critical. Water and/or ice can further insulate the window from the needed heat in the room. You must dry off the window. Mildew cannot form without a damp surface. Dry off the windows as needed.


Remember, your windows will be the coldest surface in your home, and you will need to be careful. You have to find a balance point which includes your need for privacy, your personal air temperature and moisture settings as well as the moisture necessary to maintain the fine woodwork in your home. These very issues are often in conflict with each other, but all have to be considered. In some cases, condensation cannot be avoided but can be managed.

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For more information contact: Joe Nagan Home Building Technology Services, LLC-Kaukauna 920-766-7578 or

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