Is the condensation on the interior of the glass - where you can touch it?

If so, then what has happened is that the surface temperature of the window glass has gone below the dew point temperature of the air in your home. A not uncommon occurance this winter when large parts of the country were quite cold.

I am including a rather long explanation of what is actually happening with your windows - I have posted this one before on THS (and elsewhere)...

The reason why there is condensation or frost or ice on the interior of your interior window or storm window is because the surface temperature of the window is below the dew point temperature of the air in your homethatÂs itÂa very simple explanation.

Unfortunately, the explanation as to why the surface temperature of your window (glass) is below the dew point temperature of the air in your home may be a bit more complex  so I am going to offer a few thoughts and maybe even throw in a few numbers that I hope might help your situation.

In the summer, when you pull something cold and refreshing out of the refrigerator, and the air is warm and humid, that cold and refreshing beverage container suddenly and quite magically becomes instantly wet  just as soon as it is exposed to the air. What has happened is that the temperature of the container fresh from the refrigerator is below the dew point temperature of the air  which has caused condensation on the outside of that container.

What happens to your windows in the fall and winter is that the surface of the glass is below the dew point temperature of the air in your home  which is causing condensation on the surface of that glass.

Dew point is defined as saturation vapor density...or put in simpler terms, when the air reaches 100% relative humidity and can hold no more moisture.

Relative humidity is, well, relative.

Relative humidity is a comparison of the actual vapor density versus the saturation vapor density at a particular temperature. Put a bit more simply, dew point is 100% relative humidity or the point where the air - at that temperature - is no longer able to hold any more moisture. If the air has reached vapor saturation (100% relative humidity), then the air will release moisture...be it on the outside of that cold beverage container in the summer time, or be it on the interior glass surface of your windows in the winter time, it makes no difference. If the surface temperature happens to be below freezing, then that moisture becomes frost or even ice.

In order to stop condensation from forming on the surface of a window, you either have to lower the dew point temperature of the air in your home to a level below the dew point temperature of the window surface, or you have to warm up the window surface to a temperature above the dew point temperature of your home, or a combination of both.

Lowering the relative humidity of the air in your home may have absolutely no effect on controlling window condensation or it may completely solve your problem  depends on how you lower the relative humidity and what affect the "how" has on both the moisture level of your air and the temperature of your windows. All this because there are two ways to lower relative humidity  first, you can increase the air temperature in your home or second, you can decrease the moisture content of the air in your home.

By increasing the air temperature in your home you will lower the relative humidity but you will not change the dew point  which is based on the amount of water vapor in the air and is not based on the temperature of the air. So, while the RH is lower with higher air temperature, it may not effect condensation on window surfaces at all  unless the rise in air temperature also caused a corresponding rise in window glass temperature to a level above the dew point temperature.

But, lowering the amount of water vapor or moisture in your air will lower the dew point temperature as well. And if it lowers the dew point temperature sufficiently to drop it below the temperature of your window glass  no more condensation issues.

The amount of moisture in the air is measured in grams per cubic meter, which is kind of nice for our metric folks but not so nice for our non-metric folks; but the metric version is much easier on the calculator than the English version. However, in the interest of making this stuff easier to understand for all of us non-metric types, I am going to use Fahrenheit rather than Celsius temperatures in the calculations.

Okay  consider your home at 65 degrees F and with a relative humidity reading of 40%. There are 6.25 grams of water in a cubic meter of air in your home in that particular scenario - which then equates to a dew point temperature of 38 degrees F. So at 38 degrees the air will be at 100% relative humidity or at saturation vapor density.

Now, if your neighbor keeps her house at 75 degrees, but she also has 6.25 grams of water per cubic meter in her air, then the relative humidity in her home is 29% - versus your 40%. But, and hereÂs the kicker, the dew point temperature in her home is still 38 degrees.

While the relative humidity in her home is much lower than is the relative humidity in yours; if the surface temperature of the windows in her home is 35 degrees she will have condensation on those windowsÂyet if the surface temperature of your windows is 40 degrees  only five degrees warmer  you will not have condensation on your windows.

So, while her handy humidity gauge reads (correctly) only 29% RH Â she has a condensation problem.
While your handy humidity gauge reads (correctly) 40% RH Â you donÂt have a condensation problemÂSWEETÂwell, for you anyway, not her.

If your home hygrometer measures the relative humidity in your home at 60% while the temperature of your home is 70 degrees, you will have a dew point temperature of about 51 degrees  meaning that if the temperature of the window surface is below 51 degrees then you will have condensation - so now we talk a little more specifically about windows.

The interior surface temperature of a single lite of glass, when the temperature outside is 0 degrees F and the inside air temperature is 70 degrees, will be about 16 degrees.

Add a storm window on the outside and the surface temperature of the inside lite jumps up to about 43 degrees  a huge improvement.

But these are center-of-glass readings and not the temperature readings at the edge of the window where condensation usually forms. A typical clear glass dual pane window is going to have center-of-glass temperature reading pretty much the same as a single pane with a storm  something that is often claimed (correctly) by folks who advocate refurbishing windows rather than replacing (something that I am not going into here  I am NOT advocating either replacement or restoration in this post. It is long enough and detailed enough already without opening that particular can-of-worms!)Â
However, if that dual pane has a LowE coating and an argon gas infill then the center-of-glass temperature will be about 57 degrees  a 14 degree improvement over a clear glass dual pane or a single pane with storm window  but again, and more importantly, there will be a comparable edge of glass improvement as well, particularly if the IGU (Insulating Glass Unit) was manufactured using a warm edge spacer system. Also, the dual pane is going to have desiccant between the glass layers. Desiccant absorbs moisture keeping the inside of the dual pane system very dry.

The advantage? If it gets cold enough outside, the temperature in the airspace between the lites can get very low. By keeping that space dry, it helps to keep the dew point temperature very low as well; something not always possible when using a single pane and storm window.

Oddly enough, a single pane with a good and tight frame and sash assembly may be more prone to condensation than will a less tight single pane window simply because air (and moisture) will leak out of the looser window while the tighter window may be more likely to trap the moisture inside the home. And, while a tight storm window can help the interior lite to avoid condensation (when compared with a single lite and no storm), the storm window itself may frost up when the temperature is low enough  at a temperature usually well above the temperature that will cause the dual pane to ice up. It is unavoidable given the right circumstances

So what does a window temperature of 57 degrees mean? Well, as I mentioned earlier a home kept at 70 degrees with a 60% relative humidity has a dew point temperature of 51 degrees so it is much less likely that there will be condensation problem on those particular windows than there would be with a less energy efficient window - despite the relatively high relative humidity in the home.

But, there is always a "but"Â

Again, that 57 degree glass temperature is still a center-of-glass reading and the edge of glass temperature will be lower - actual temperature is dependent on both the spacer system used in the IG unit construction and on the material used to construct the sash. So even with a "57 degree" center-of-glass temperature it is still possible to get window condensation if there is enough moisture in the air.

And consider that the interior glass temperatures are based on the fact that moving, warmer, indoor air is actually in contact with the glass at a given time. Curtains, shades, other obstructions can cause problems by blocking airflow across the glass  airflow that can have a huge effect on the condition of the window relating to condensation. Also, bay and bow windows can be more prone to condensation  again because of the possibility of decreased airflow over the glass.

And finally, what can happen to the dew point if you keep your home at 70 degrees and you have a 65% relative humidity? Well, for one thing the dew point has jumped up to 57 degrees which we have already noted is the same as the window temperature. For another thing, anyone with 65% relative humidity in a home at 70 degrees has way too much moisture in their air and they are in serious need of some sort of ventilation system  or at least several good exhaust fans!

No, it is in between the panes of glass.

catherinet:
condensation forms between the panes following the same principle described by oberon, namely the temperature between the panes has reached the "dew point" for the level of moisture in the space. Moisture can/will increase because the seal is failing. Unfortuntely after any condensation forms it often leaves some deposits on the glass when the weather warms up and the condesation evaporates. There is no way to clean the glass in the cavity space and the insulated glass unit (IGU) is considered "failed".

Thanks sdello,
I had the feeling that it meant the beginning of something not-so-good!
Oberon......I do appreciate all the work you put into explaining the other condensation, and I appreciate that too!

geeze oberon..is this chapter 2 of your book??
(you know the one you should be writing!)
good explanation..ok great explanation.

Chapter three actually ;-)

Anyway, I probably should have waited for the answer to the first question - but I assumed (and we all know what that means!) that it would be inside condensation because of the cold.

But, all is not lost. As sdello said, while the between-the-lites condensation means that there is moisture in there, it doesn't necessarily mean that the windows (spelled IG or dual pane glass) is failing.

When a manufacturer makes an IG, they put a desiccant material between the lites as well. This desiccant material is designed to suck up excess moisture. This was especially important a few years back when the products and techniques may have left a bit of moisture inside the IG when it was sealed.

Well, as I mentioned somewhere in chapter 3, as temperature falls so does dew point. It doesn't matter if it is inside the dual glazing or outside. The desiccant that is between the lites helps keep the dew point temp very low by limiting the amount of moisture that is in there. But if it gets cold enough then it is possible for the very small amount of residual moisture to be relesed by the air and result in condensation between the lites.

Again, it is possible to have between-the-lites condensation - if it gets cold enough - without having a seal failure.

But, what if turns out that the interior condensation is actually from a seal failure? Well, if the seal breach is small enough then the desiccant will still maintain a very low dew point in the airspace. This will mean that although there is some air transfer between the airspace and the world outside the airspace it is very possible that condensation between the lites won't happen.

Eventually, depending on location (and a few other factors), it is also possible that if there is a breach in the seal that the desiccant will absorb enough moisture to eventually saturate - and then there will be condensation issues between the lites.

Condensation between the lites is a cosmetic issue. It does not affect IG energy performance of a clear glass IGU, but it can certainly be more than a little annoying and it is ultimately unacceptable. If there is a LowE coating inside the IGU then the moisture will eventually corrode that coating. Obviously if the unit had argon (or any gas fill), then the argon is gone and that does affect the performance of the unit.

I wouldn't worry about the windows unnecessarily. If they go bad, then they go bad, and it may be necessary to do something about it. But if you are only seeing condensation at really cold temperatures then it is entirely possible that this was a fleeting problem - certainly no promises, but at this point it is wait and see.

Thanks oberon!

oberon,
You obviously know windows! Like catherinet, I am experiencing seal failures and have significant condensation forming between the paines. I have a 7 yr. old home with 37 double hung/foyer windows from American Weather Seal and the seals fail at a rate of about 2/year. (A month ago I submitted warranty claims for 4 more. AWS is refusing to honor the 10 yr. warranty stating that the orig. mfg which they acuired no longer exist and AWS is not responsible.)
What is the best way to get these 4 fixed and is there anything I can do to prevent the failures of the others???

Thanks in advance for any thoughts.

You can't prevent any future failures. The ones that have failed can be replaced. Just call a company listed in the phonebook that does insulated glass.
Ron