Condensation
http://www.replacement-windows.com/windowbb/viewtopic.php?t=88
Posted: Mon Oct 18, 2004 1:15 pm Post subject: Condensation
Relative humidity is the ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor the air can hold at that particular temperature (and pressure).
In simplest terms, it is a ratio of the air's water vapor content to its capacity:
Relative humidity is given as a percent.
So, air with a 50% relative humidity actually contains one-half the amount of water vapor it could hold.
Air with 25% relative humidity contains one quarter of the amount of water vapor it could hold.
Air with 100% relative humidity is said to be saturated because it is filled to capacity with vapor.
Increasing or decreasing the amount of water vapor in the air will cause the relative humidity to go up or down as a consequence.
Dew point temperature is the temperature the air must be cooled to for saturation (or dew) to occur.
In a nutshell, the surface temperature of your windows is below the dew point or saturation capacity of the air in your home. When water vapor in the air comes into contact with a surface whose temperature is lower than the dew point the water vapor will condense and moisture is the result.
The difference between air temperature and dew point can indicate whether the relative humidity is low or high, for example when the air temperature and dew point are far apart, the relative humidity is low; when they are close to the same value, the relative humidity is high. When the air temperature and dew point are equal, the relative humidity is 100%. ..makes sense?
You mentioned that your home has a 48% relative humidity, which is rather high for the inside of a home. I am curious how you know that it is 48%?
There is a standard measurement of the ability of a windows resistance to condensation. It is called CRF, which means Condensation Resistance Factor. Without going into formulas and all, windows are tested to condensation resistance with an internal air temperature of 70F and an external air temperature of 0F.
A number from 30 to 80 is then assigned to the window, with the higher the number the greater the ability to avoid condensation. Some manufacturers include this number on their NFRC labels and some do not.
A CFR rating of 35 is generally considered to be the minimum acceptable rating.
All this stuff isn’t really getting to the root of the problem, which is, how to control the moisture on the surface of your windows.
Two ways to do it. First, lower the indoor relative humidity, which will raise the dew point.
The second option is to warm the surface of the glass to a temperature above the dew point for the temperature and relative humidity in your home…which is the reason that you bought these windows for in the first place.
ALL windows will “sweat” if the indoor air has enough moisture and the window glass is cool enough – which is subject to the outdoor temperature and the make up and quality of the window.
Your installer has suggested that this problem may be seasonal. He may be correct. It is not uncommon for windows to sweat a little in the change of the season from summer to winter…and the problem that you report is also not unknown among window manufacturers.
It really is entirely possible that this problem will correct itself as the air becomes drier and winter sets in.
But, I would suggest that if you could lower your indoor relative humidity to 30%-35% that it would really help as well.How can I find out more information about my options?
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