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Old 11-19-2008, 07:06 PM   #1
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how to stop condensation on windows?

I have a 4 season sunroom, it has eight large windows, and every morning the windows have moisture on them. We keep it at 50. We keep a small opening in the sliders that go out to it, from the dining room, at night. I am worried about all the moisture out there, and what it is doing to the sills, not to mention possible mold issues. This is my kids play room. I do not know how old the windows are. The house is 34yrs old, and I know the sunroom was built after the house was built..I think it was a porch turned into a sunroom. We are getting an estimate on new windows, but not sure when we will be able to afford to replace them for awhile, or if this is truly the problem. Any ideas, or advise would be greatly appriciated.
Thank you,


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Old 11-20-2008, 09:45 AM   #2
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Have you tried keeping the sliders closed? The warm moist house air is adding to the problem, or maybe is the problem.
You could put a humidity guage out there to monitor how moist it is, with the door open and closed. Check out the internet and look up Dew point and understand how it works. That should help with the issue.


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Old 11-20-2008, 12:18 PM   #3
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Condensation occurs on a window when the temperature of the window/glass goes below the dew point temperature of the air surrounding the window.

In order to stop condensation from forming on the surface of a window, it is necessary to either lower the dew point temperature of the air to a level below the dew point temperature of the window surface, or to warm the window surface to a temperature above the dew point temperature or a combination of both.

Relative humidity is a comparison of actual vapor density versus 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 a certain 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 which will show up as condensation on any surface that is at a temperature below the dew point temperature of the air

There are two ways to lower relative humidity – first, increase the air temperature or second, decrease the moisture content of the air. Lowering the relative humidity, which is the common “remedy” for wet windows, may have absolutely no effect on controlling window condensation or it may completely solve the problem – depending on how the relative humidity is lowered and what affect the “how” has on both the moisture level of the air and the temperature of the window.

Increasing air temperature will lower relative humidity but it will not change the dew point temperature which is based on the amount of water vapor in the air and is not based on the temperature of the air. So while RH is lower with higher air temperature, it may not effect condensation on window surfaces at all – unless the rise in air temperature also results in 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 the air will also lower the dew point temperature as well. If the dew point temperature is lowered sufficiently to drop it below the temperature of the window glass there should have no more condensation issues.

The amount of moisture in the air is typically 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. So in the interest of making this stuff a bit easier to understand for all of us non-metric types, I am going to covert back to Fahrenheit rather than use Celsius temperatures in the calculations

Consider a home at 65 degrees F and with a relative humidity reading of 40%. In this scenario there are 6.25 grams of water per cubic meter of air which 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.

If a home hygrometer measures the relative humidity at 60% while the temperature is 70 degrees, the dew point temperature is just about 51 degrees – meaning that if the temperature of the window surface is below 51 degrees there will likely be condensation on that window surface.

There will be a difference between edge temperature and center-of-glass temperature which can be significant when dealing with condensation issues. Also, 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.

A triple pane with dual LowE coatings and argon or krypton fill (depending on width of the airspace) will have an indoor surface temperature approaching 60 degrees in the zero outside, 70 degree inside scenario. There has to be a lot of moisture inside the home to have condensation on that window.

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

Last edited by oberon; 11-20-2008 at 12:38 PM.
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Old 11-20-2008, 03:32 PM   #4
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Oberon, that is one fine response :-)

I would like to add..

1) Lower the humidity in the house...Bathroom fan, kitchen fan, use the clothes dryer (towels, clothes..), short showers and baths...

2) Raise the heat a bit

3) Install a ceiling fan in the sunroom

4) No shades or blinds!

5) I love using the heat-shrink window film, it will add another layer to your windows and the stuff really helps. (cheap too!)
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Old 11-20-2008, 04:48 PM   #5
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It's not your windows I don't think. I just put new windows in my house and they do it too. My whole house does this upstairs. We have double hung windows. It happens at the seam where the two windows meet and at the bottom of the windows.

I have tried everything, the humidity in the house is normal. And even when I run the dehumid. it does a bit but not a lot. I have tried weather stripping around the seams doesn't work.

When I get up it's there. To get rid of it. I run a ceiling fan and make sure the curtains are or shades are open during the day so the sun gets in. By noon hour the part of the house that gets the sun is clear. I am in the habit now of just wiping a few of them down on the cold side of the house in the morning.
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Old 11-20-2008, 06:25 PM   #6
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All Great Info.!!!!!

Thank you all for your replies, I am sooooo happy this issue is most likely not the windows, just got a quote to replace the 8 windows, over 11,000.00. I will try the sugestions and write back to say what has helped.
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Old 11-21-2008, 09:10 PM   #7
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There could be other problems here to. My parents had a similar problem and they were very concerned because they were new windows. Dad was running the dehumidifier non stop. Turns out that the furnace was vented properly. I also think that the exhaust fan in the bathroom vented up into the attic and not out through the roof.

Best of luck.
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Old 11-22-2008, 12:12 AM   #8
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I think the problem could be solved by simply putting a $12 Wal-Mart fan in that sunroom to circulate the air.

Windows are VERY POOR INSULATORS (see note below). The air near the window will be cooler than that in the rest of the sun room simply because of the high rate of heat loss through the window. So, the inner pane of glass is colder than the rest of the room, and condensation forms on that glass. Also, convection plays a part. Air right in front of a window cools and sinks. That's why the condensation typically forms at the bottom of the window; that's where the air is coldest and can hold the least amount of moisture in suspension. Compare this to an R value of 3.5 per inch for fiberglass batt and 5.0 per inch for extruded polystyrene insulation (like RoofMate).

By putting a fan in the room it circulates the air and washes the inside panes with warm air, keeping the glass warmer and preventing condensation on it. And, even a small fan will break up the very weak convective currents that form in front of windows.

A dehumidifier in the sunroom would be even better than a fan, but even a cheap fan is likely to solve most of the problems.

If you're willing to go to the extent of replacing the windows, consider replacing the entire sunroom. Open your yellow pages phone directory to "Store Fronts". Retail stores most often lease their space for terms of 10, 15 or 20 years. And, a store front that works well for a clothing store or jewellry store may not work at all for a medical clinic or lawyer's office. So, it's common for the new tenant to pay to have the front of the store redone to suit his needs.

By hiring a place that does store fronts, you won't get wood construction. You'll get aluminum construction that is made to be easily erected on site. The advantage is that you'll be paying mostly for materials, not for labour. The insulating value of the windows you get will be as good as those on any store in your area, and retailers are concerned that condensation on windows prevents customers from seeing the merchandise on display in their store front as much as you are concerned about the condensation on your windows.

Also, it doesn't have to be plain metallic aluminum. You can get adonized aluminum in any colour from dark brown to bright red and everything in between.

So, by having a company that erects store fronts redo your sunroom, you'll get a better sunroom cheaper than if you hired a window company to build it out of wood.

At the very least, do yourself a favour and get a quote from a company that specializes in store fronts.

Note below:
Typically, count R= 1 for each pane of glass in your windows. So, a double pane sealed unit will have an R value of about 2 and a triple pane window will be about R3. By definition, the R value of wood is 1 per inch. So, a wood mullion between two windows that is 3 inches thick would have an R value of about 3. If you had triple glazed windows, then there'd be as much chance of the condensation forming on the wood rather than the window glass. With double pane windows, it'll form on the colder glass.


Last edited by Nestor_Kelebay; 11-22-2008 at 12:38 AM.
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