Vinyl windows by Unlimited, Inc.

Guide to Condensation on Windows: Know the Facts

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Why do I have moisture on my windows?
What is raising the humidity level in my home?
My old windows didn’t have condensation, but my new windows do.  Why?
Can I buy a window that will prevent/eliminate condensation?
What should the humidity level in my home be?
How can I test the humidity level in my home?
What can I do to help prevent the moisture on my windows?
We had an extremely cold night and now I have condensation/ice on my windows?
Can high humidity levels cause problems other than condensation on my windows?

Why do I have moisture on my windows?
The number one reason that condensation forms on windows is excess moisture (humidity) in the home.  Condensation usually occurs first on windows because they have the lowest temperature of any of the interior surfaces in the house.  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 as that air cools.  In practice, condensation will occur first over the meeting rail and lower part of the window because the glass surface temperatures are not uniform, being cooler at the bottom of the window than at the top.
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What is raising the humidity level in my home?
There are many activities happening inside your home that raise the level of humidity.  They include but are not limited to:
           
Showers
           
Dishwashers/Clothes Washers/Dryers
           
Dripping Faucets
           
Open Fish Tanks
           
Cooking with Gas – Moisture is a product of gas combustion
           
Overuse of humidifiers
           
Excessive plant containers that require watering
           
Damp basement/Crawlspace
           
Simple breathing – A family of four generates 12 pints of water vapor a day
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My old windows didn’t have condensation, but my new windows do.  Why?
One of the reasons you had new windows installed was because you wanted windows that were more efficient than your old windows.  Chances are that your older windows were drafty and leaking air.  That air leakage keeps your home “breathing” and allows excess moisture to leave the home before it reaches the dew point and collects on your window.  So the reason your old windows did not accumulate moisture is because they were doing very little to keep the heat (moist air) inside of your home.  The condensation on your windows is an indicator that your new, energy efficient window is working by locking in the warmer, moister air inside your home, not allowing it to leak out like it did with your old windows.
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Can I buy a window that will prevent/eliminate condensation?
It is not possible to buy a window that will prevent moisture.  Buying energy efficient windows with Low-E glass, argon gas or triple glazed units will help reduce moisture build up, but if the humidity level is high in your home, it cannot prevent it.
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What should the humidity level in my home be?
It should be at a level that reduces/eliminates the condensation that has been appearing on the windows.  When objectionable condensation is spotted, steps should be taken to reduce the humidity level by controlling the moisture sources or by increasing ventilation.  The following shows the maximum humidity that can be tolerated to avoid condensation in cold weather:         

Outside Temp (F)

Inside Humidity (%)

-20

20

-10

25

0

30

10

35

20

40

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How can I test the humidity level in my home?
One way to determine the approximate indoor humidity level is by observation; is there condensation on the windows?  Condensation on double glazed windows that persists when outside temperatures are zero F or above and inside temperatures are approximately 70 degrees F indicates humidity levels above 40%.  You can also purchase a hygrometer which is used to measure humidity levels.  You can purchase a mechanical hygrometer which normally also comes equipped with a thermometer or you can purchase an electronic hygrometer.
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What can I do to help prevent the moisture on my windows?
There are two ways to prevent condensation on your windows:  Reduce the relative humidity in your home or increase the temperature of the window glass.  Reducing the humidity is the most effective way to reduce window condensation.  Make sure that exhaust fans are vented properly and in good working condition.  Bathrooms, dryer vents and kitchen vents are the biggest culprits for increased humidity.  Add a dehumidifier to your home to remove excess moisture.  You might also need to reduce the number of indoor house plants.  Increasing the temperature of the window is a bit more challenging.  Make sure window curtains and blinds are opened during the day to let sun in and also to allow air movement to help dry things out.  Opening curtains and blinds also allows the warmer air from the room to warm up the window.
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We had an extremely cold night and now I have condensation/ice on my windows?
If you use the chart on the previous page for recommended humidity levels based on outside temperature you can see that the colder it is outside, the lower the humidity level should be inside the home to prevent condensation on your windows and other objects in your home.  When there is an extreme dip in outside temperature, the humidity level inside of your home does not automatically go down because the outside temperature goes down.  If you have a humidity level of 35% which is recommended at 10 degrees F, and the temperature drops down to below zero and into the negative digits, your humidity is now too high for those low temperatures.  Until you reduce the humidity level in your home or the outside temperature goes back up you will see condensation on your windows and other objects in your home.
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Can high humidity levels cause problems other than condensation on my windows?
Yes.  If you are seeing condensation on your windows the humidity level inside of your home/room is too high.  Seeing the condensation on your windows gives you an opportunity to manage that humidity level before damage can be done.  With the high humidity, the condensation will happen on the coldest surfaces of the room or where air movement is the least.  This includes but is not limited to the corners of rooms, north facing walls, behind furniture or near and on windows.  The moisture that forms on these surfaces can blister paint, peel wallpaper and spur the growth of mold.  These damages are not just confined to areas that are visible.  You could have damage to the interior of your walls and damage to the interior of your attic as well if the humidity level is not managed properly.
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