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Understanding Window Condensation

posted by Ann on December 5, 2014


New homes are more energy efficient than older homes.  Why? Because they are built with a greater amount of insulation, more vapor barriers, caulking, tighter windows and the building practices used cut down on air infiltration.


In the first year of a new home humidity levels in the house can be higher than normal.  Why? Because many of the materials used to construction a new home contain high amounts of water.  For instance the cement, the drywall compound, the mortar used to set tile stone & brick, grout, the paint etc. are all made with a high amount of water.  In addition, your home was NOT built in a controlled manufacturing plant.  It was built outside in the elements.  So many of the materials that went into the home were exposed to the weather and may also have absorbed moisture during the building process.


That is why when we were building your house we may have continuously ran an industrial sized dehumidifier in order to pull as much moisture back out of the building products as possible.


 In addition, now that you are living in your new home you are producing a normal amount of humidity by cooking, breathing, showering, doing laundry, etc.  It should also be pointed out that household size and lifestyle will affect the amount of humidity present in your home.


What does all of this have to do with condensation on your windows?  Moisture condenses on the windows since it is the coldest object in any given room with the glass having a much higher rate of heat loss and hence being the colder surface during the normal heating season.  Since you have a new more energy efficient house that is very tight, high humidity levels will cause condensation and frost to appear on any colder surface like windows as well as around outlets, recessed lights, exterior doors and even on drywall.


Here’s something important to know… WINDOWS DO NOT CAUSE CONDENSATION!  THERE IS NOTHING WRONG WITH YOUR WINDOWS! Phew! That’s a relief!


You will never completely eliminate window condensation, especially when the temperatures outside drop quickly like they do in Wisconsin!  However, there are a number of things you can do to help the situation:


  1. It is very important to exhaust the humidity from your home.  This can be done by running bath fans and cooking exhaust fans that are vented to the outside when necessary.  These fans should be run for at least 20 minutes after a shower or a cooking session.  If you have ceiling fans it is very helpful to run these in the winter time as well to move air and reduce moisture.
  2. You should run the furnace fan constantly—do not put it on auto.  Set the furnace fan to on and this will help to move air as well as decrease moisture/condensation in your home.
  3. You can crack a window to permit excess moisture to escape.
  4. Buy a humidistat from the hardware store.  You may want to purchase one for each level of your house if you have more than one level.  These are very inexpensive!  Use this humidistat to monitor the humidity level in your home.  As the outside temperatures drop, the indoor relative humidity level of your home should be decreased in order to reduce condensation appearing on the windows and other surfaces.  The following table will help you determine what your humidity level in the house should be based on the outside temperature:


Outside Air Temperature             Inside Relative Humidity

(for 70 Degrees F Indoor Air Temp)

-20 degrees F                                     15 to 20 percent

-10 degrees F                                     20 to 25 percent

0 degrees F                                         25 to 20 percent

+10 degrees F                                    30 to 35 percent

+20 degrees F                                    35 to 40 percent


  1. If you cannot maintain proper levels and the problem persists it may be necessary to purchase a de-humidification system or even an air to air exchanger.  Please note:  just as too much moisture causes problems as described above, insufficient humidity or excessive dryness can cause other serious problems.
  2. If you have a humidification system installed on your furnace system, excessive moisture on the windows is an indication that your humidifying equipment is set too high and producing too much moisture.
  3. Window treatments can reduce the temperature on the glass and also cause condensation issues.  It is recommended that interior screens be removed from casement windows during the heating season and that raising blinds or opening draperies will help to minimize the situation.


We hope that this helps you better understand the natural process of condensation and humidification and how to better manage it in your home! Andersen Windows has a great short video describing this process above so be sure to check it out!



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