# Condensation on fiberglass window frame



## Ron6519 (Mar 28, 2007)

There is cool outside air getting into the interior. There are probably gaps in the insulation allowing the air into the interior.
What does this mean?
"The weatherstriping they installed is firmly seated against the outside of the window edge,..."
If this is aluminum capping, it's not weatherstripping and does nothing to insulate the window.
There should have been continuous insulating material around the perimeter of each window
Was this a professional installation? New construction or replacement window?
Ron


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## creamaster (May 11, 2008)

I do not think it is cool air getting into this space, I think it is the actual frame is cold by means of conduction and the indoor air is condensing on it. The weatherstripping material I am referring to is attached to the frame on each side of the bottom sashes on the exterior side, it is a foam encased in vinyl material easily seen on the pics , and I checked all the sashes and both sides are firmly seated on every window.

I installed the windows exactlly to the tee per manufactures instructions. I believe this may be a flaw in the design of the window where the frame is not built up enough to be well insulated :confused1:


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## rjordan392 (Apr 28, 2005)

Your humidity is too high. Here is a guide on proper humidity levels for Wintertime.
40 degrees........45% R.H. 30 degrees.......40% R.H. 20 degrees.......35% R.H. 10 degrees.......30% R.H. 0 degrees.....25% RH
-10 degrees......20% R.H. -20 degrees......15% R.H.

But to each his own. I prefer 5% less R.H. then what I posted and its a starting point that may have to be refined by each homeowner.
It appears your house is airtight and excess moisture has nowhere to go except to condense on your windows. Take a look at your habits as a first line of defense. Turn on kitchen exhaust fans when cooking. Put the bathroom fan on a timer, that runs while the bathroom is in use and continues to run for about 10 minutes after use and keep the door closed after use while its running. Look at other sources such as laundry dryers that are being discharged indoors or large fish tanks.

Other then the above, there are two remedies. But get a professional opinion.
1. Either you need a combination fresh air intake and exhaust system added to your furnace or 
2. A whole house dehumidfier.


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## Ron6519 (Mar 28, 2007)

Are all the windows exibiting condensation? Maybe just rooms with a bathroom close by?
What's the humidity level in the house?
These windows have been around for a while. I would think if it were a design issue, there would be many more complaints about them.
Have you contacted Marvin about the problem?
Ron


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## creamaster (May 11, 2008)

I just emailed Marvin about this today so hopefully next week can expect a reply.

All the windows are doing this, living room, bathroom, and bedrooms.

The current outside tempature is 20 F.

The current outdoor humidity is 66%.

The current indoor tempature is 67 F.

The current indoor RH is 47%.

According to your table we are about 12% over where we should be. We vent the dryer outside. No fish tanks. It is myself, my wife and our 5 month old, plus a yellow lab dog . We do not have any exhaust vents in the house. Would a heat recovery ventilator be able to decrease the humidity enough or would we require a household dehumidifier?


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## rjordan392 (Apr 28, 2005)

Would you tell us if these windows are new construction or the replacement type. The replacement type fit into the old window frame and the new construction type, frame and window fit into the opening, brick to brick or other material.
When installing new windows, most homeowners have the replacement type installed. 

Now assuming you have the replacement type, did you apply caulk on both sides of the framing jamb ( the side facing you) and also the top jamb? And did you place a sealing type membrane on the bottom before the window was put in? This is to stop air infiltration.
Before the windows were installed, did you seal up any holes or gaps in the framing with foam or fiberglass insulation? What about the gap on both sides and top of the window, after the replacement window was installed? Cold air can reach the window from inside the wall. Are the walls well insulated? What about the outside; does any area need caulking?

You did not mention if condensation was forming on the glass. A little fog around the edges of the glass is acceptable. If your're getting no fog and just water droplets on the frame only, then I suspect cold air leakage. 

Ask "Marvin" if they will send out an inspector to check your installation. You may have to pay a fee as this is an owner installed job.

I am not a professional but I can recommend this: Try to find a "Condensation Expert" who understands the relationship between temperature, dewpoint and relative humidity and I believe he will be able to tell you whether you need an air exchange system or a whole house dehumidifyer.


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## buletbob (May 9, 2008)

Marven windows are my Number one window of choice. Rjorden has a very good response, I think he hit the nail right on the head. It all depends on your interior habits, Boiling water is the biggest reason other then the bathroom. BOB


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## creamaster (May 11, 2008)

The windows are new construction type, refer to this post for better pictures

http://www.diychatroom.com/showthread.php?t=23577

Also I need to correct my terminlolgy a bit. The condensation is occuring on the side jambs, sorry i used the term frame.

When the temperature started dropping in early fall I noticed alot of condensation on the glass portions. So we moved the dehumidifier from the basement upstairs and manged to bring the RH down. The condensation has all but vanished from the glass but still is condensing on the frames even now with the RH in the low 40's. 

I think you guys are all correct in that these windows are highly rated, im assuming thats why they were so expensive  I will see about having a technician/inspector come out and look at them. I could see a couple windows misbehaving if it was my fault installing them but when they are all doing it, it leads me to belive something else is going on. I will update this thread as I get more info. thanks guys! :thumbup:


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## oberon (Apr 29, 2006)

I posted a shorter version of this reply on a thread a few days older than this one, but here is my rather long-winded explanation of condensation, relative humidity, dew point, and their relationship to your windows. 

You have interior condensation on your windows simply because the surface temperature of the window is below the dew point temperature of the air in your home…that’s it…a very simple explanation.

Unfortunately, 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 a home may have absolutely no effect on controlling window condensation or it may completely solve condensation problems, depending on how the relative humidity was lowered and what affect the “how” has on both the moisture level of the air and the temperature of the windows. All this because there are two ways to lower relative humidity – first you can increase the air temperature or second you can decrease the moisture content of the air – both decrease relative humidity, but they are not the same. 

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 here’s 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 windows…yet 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 – she has a condensation problem.
While your handy humidity gauge reads (correctly) 40% RH – you don’t have a condensation problem…SWEET…well, 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 – 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"…

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!


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## Quonset (Jan 5, 2012)

creamaster said:


> I just installed new Marvin Intergrity double hung fiberglass windows throughout the house this year. As the temperature is getting into the 30's I am now seeing condensation on the inside of the window frame where the sashes click into. Is this normal? The weatherstriping they installed is firmly seated against the outside of the window edge, but its the interior air that is seeping into this space and when it comes into contact witht the colder frame its condensing on the frame. It drips down but is stopped by the gasket they installed between frame and sill. Is this something to worry about? I am trying my best to keep the indoor humidity into the upper 40's but having a heck of a time with a dehumidifier, shy of opening windows, now that the house is more airtight, normal humidity is now 50-55%. Is this still too high, should I try for lower 40's? I hate to think of thousands of dollars and hundreds of hours go down the drain


Hi,

I know this is an old post but I am having exactly the same issue with Marvin Integrity double hung purchased in December 2011. This is a real problem. Marvin has been out 5 times and can not remedy. I agree that the jambs are too cold. I don't buy the moisture excuse; there is moisture in every house. If the temperature is low enough any amount of moisture will condensate. I also installed these windows to the letter and then some. Did you have any luck with Marvin? It is really frustrating because otherwise they are a good window.


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## creamaster (May 11, 2008)

post your indoor temperature and relative humidity. 

To fix this i had to install a Heat Recovery Ventilator and the indoor RH stays about 35-40% with an indoor temp of 67-68 F and no more condensation.


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