Okay, this is a really long answer. I have posted this same answer to this same question previously - but it is a really good question and it becomes a common question this time of year...

The reason that you have moisture on the inside surface of your windows is because the temperature of the glass is below the dew point temperature of the air in your home.

In order to stop moisture from forming on the surface of your windows, you either have to raise the temperature of the glass to a level above the dew point temperature of the air in your home, or else you have to lower the dew point temperature of the air to a level below the temperature of the window.

Unfortunately, suggesting those two solutions is often much easier than actually being able to accomplish either one.

I am going to include a post about window condensation that I wrote and posted on a different forum in THS. It is really long and even has a little math in it...but I think it does an okay job of explaining some real basic information about window condensation.

-------------Window condensation --------------

The reason why there is condensation on the interior of your windows has a really simple explanation the surface temperature of the window is below the dew point temperature of the air in your homeÂthatÂs itÂa very simple explanation.

Unfortunately, the reason that the window surface temperature is below the dew point temperature can potentially become somewhat more complex, but I am going to offer a few thoughts and 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. Basically, 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 condensationÂand I bet that that statement is a bit of a surprise to some folksÂit is true however.

There are two ways to lower relative humidity  increase air temperature or decrease moisture content. If you increase the air temperature 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.

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.

Although a single pane with a good and tight storm window can help the interior lite to avoid condensation (when compared with a single lite and no storm), the storm window itself will 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 unlikely that there will be condensation problem on those particular windows despite the relatively high relative humidity in the home.

But what happens 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!

Somewhere back in this post I mentioned that lowering the relative humidity in your home may not help control condensationÂthat is still trueÂIF the relative humidity is lowered because of an increase in temperature. But, lowering the relative humidity by removing water is a different story because in that case you will also be lowering the dew point as you lower the relative humidity and that WILL help to control condensation on your windows.

I hope that made sense...please feel free to question anything that I didn't explain well...

In 1987 I bought my home. It had cheap windows (wood lock) out of canada. The double hung bottom sash tilted in but the top was stationary. I never checked that before I bought home., Well, I noticed on really cold nights I had condensation inside. My friend said put on storm windows which I did. The moisture went away since I created another air space. Now in 2008 I had all new Andersen tilt wash windows put in. I asked the builder if I would have moisture like I did before and he said NO YOU WILL NOT. Well the first winter I had loads of moisture. These past 7 years you got to get up wipe it all with a towel. The windows are wood inside and they started to stain with the moisture. When it was in the teens, I get ice can you imagine Ice inside. Just today, I went to the local lumber yard where I spoke to a gentlemen who in fact went thru the same thing and he has the same Andersen window I have. Well, I am doing what he did He bought the Andersen combination storm panel that snaps right into the slots where the exterior screen fits. It has a screen and two glass panels. His problem corrected itself immediately. My house is not over damp inside. All the people tried to say it was my fault. As I wait for the 12 new panels, my friend had two from his home and they actually fit into my bathroom windows. I cannot believe that the noise from outside is also much lower. I cant wait to get all my panels in in the coming weeks. These snap right in and have an optional set of clips that will allow the panel to stay on permanent. Some people take them off and put up full screen in the spring. The only complaint is the glass up top is stationary. I always open all nine upstairs windows all summer with the top sash open and what a difference when you clear out the hot summer air from the ceiling. I get so much air transfer by opening the top sash, well the issue is now the new panels have glass on top all the time. I really do not want to have to remove these panels each spring, I was hoping to leave them up. Maybe I can leave the top sash open half way like I do, and the screen on the bottom of the panel will allow enough air in that will come in and go over the two double hung sashes and still make it in Any way atleast there is a fix for the moisture

We have a new house full of Anderson 100 series windows and a 400 series slider and on a 400 next to it to match the wood on the inside of the house..

When the house was new the 100 series windows were always wet and the 400 were fine and now after 3 years pretty much the same in the cold months of the year.

Talking about this with our builder he always said we had too much moisture so we did what we could even having a fire in our wood stove and that helped after time but we still had wet windows and with the blinds down they would freeze shut at night.

Today the 400 windows work perfect and the 100 well to me are unsafe junk...Do Not Buy The 100 series windows if u live in a cold area....Just my 2

Do you have any relative humidity numbers for inside the home?

I'm no apologist for the 100 series window, but please read through Oberon's post above if you have a few minutes to spare. The condensation is going to have an environmental cause in 99.9999% of cases, and there are a million and one reasons why the 100's might be seeing the issue and the 400's are not. The most obvious of which would be the location (facing direction, room, etc) , as I'm thinking that you probably don't have two different window series right next to each other. Here's a nice chart for reference:

You can also use a calculator based on the conditions in your home here: www.dpcalc.org