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Old 12-19-2009, 10:52 AM   #31
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Moisture on windows


Got the wrong end of the stick again MJW...no-one is disputing that windows can be placed incorrectly - so no need to supply pics of some windows somewhere that supposedly are.

Personally, I think the probabilities are low that these new windows were installed improperly, but eitehr way, there's no refuting what oberon said about humidity...

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Old 12-19-2009, 06:02 PM   #32
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Moisture on windows


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It sounds to me like someone read a book that a liberal wrote.

If it gets cold enough here again, I'll bring you a pic and show you that brand new windows can fail and have condensation without high humidity in the home.
Yep, I have read many books written by liberals. And I have read many books written by conservatives. And likely I have read many books written by people of indeterminate political ideals. None of which changes the fact that the cause of window condensation (or any other condensation for that matter) is that the temperature of the material is below the dew point temperature of the surrounding air. If there is condensation then there is excessive moisture for the temperature of the glass. Physics, plain and simple.

I have seen many windows with condensation, so pictures aren't necessary.

Outside temperature and humidity level means absolutely nothing when dealing with interior condensation EXCEPT that the exterior air temperature will affect the interior temperature of the glass and that can bring the glass temperature below the dew point.

I am curious what you consider to be window failure mode that results in condensation in a home with low humidity. And also what do you consider low humidity?

Last edited by oberon; 12-19-2009 at 06:05 PM.
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Old 12-19-2009, 07:16 PM   #33
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Moisture on windows


If the window leaks cold air, it will have condensation on it, regardless of the inside humidity level. It's the sudden temp change which creates the moisture just like you said. It's no different than having a single pane window condensate even when using dry heat, like a wood stove.

I'm not saying excessive humidity won't cause the same problem, I'm just saying I doubt that is the case here.
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Old 12-19-2009, 07:20 PM   #34
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Moisture on windows


I went through the same thing as you and just had my home energy audit done last week.

Buy a humidty guage from your local hardware store and start checking your humidity. If it's over 50 then you are too high and your windows are going to get moisture.

I just installed an HRV this week and it took the humidity in my house down from 65% to 30%-40%. Guess what? No more moisture on the windows.

That being said. Before I installed it I bought a few of those plastic window cover kits, the ones that you use the hairdryer on to tighten them up. I did four windows in the house that I knew were leaky. Guess what? After I bagged them. No more moisture on the windows. The hot moist air in the house was not meeting the cold glass.

I still have a window that has a small leak in the corner. I found it when I got my blower test done. The window has no moisture on it. Because I took the humidity out of the air.

It's all about the moisture in the air hitting that cold window.
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Old 12-19-2009, 07:27 PM   #35
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Moisture on windows


The pic I was going to show was in my own home. I have 2 windows (all brand new) that have leaking weather stripping on the top of the bottom double hung. The humidity in the house is 30%. The upper window had moisture on it when it got below 0 degrees F outside.

Sure a dehumifier will suck the moisture out of the air and clear up your windows, but why is there moisture in the first place? Because cold air is meeting hot air and hitting the dewpoint as oberon said. It's just a cover up for leaky windows.
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Old 12-20-2009, 05:25 AM   #36
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Moisture on windows


BTW, per BSC, a winter relative humidify 25% at 70F is appropriate for Minneapolis .
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Old 12-20-2009, 07:18 AM   #37
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Moisture on windows


Well then, with 'leaky' windows that create a draft by letting in cold air, all that is being done is that you are lowering the temperature of the window quickly and effectively, around where the leak comes in - thereby emphasizing the moisture condensation problem. But that problem may be there even if you windows weren't drafty, so it is wrong to say that "leaky windows are the cause of condensation" even though that is what you observe.

It helps; but that's about it. What I was trying to say was that it is the high humidity in the air hitting a cold surface that causes condensation - and that can happen regardless of the drafty windows.
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Old 12-20-2009, 08:08 AM   #38
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Moisture on windows


I hear ya..........I don't think the OP really cares much because he hasn't been back yet.

If what you are saying is true, then in Woody's case, he should have had moisture on the plastic on his windows also. I think he would have noted that in his post.

I really don't see how you guys think moisture is in the air, inside a home, in the winter, with the furnace on. Right now it's 80% humidity outside and 33% inside and my windows are perfectly clear...........
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Old 12-20-2009, 04:08 PM   #39
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Moisture on windows


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That being said. Before I installed it I bought a few of those plastic window cover kits, the ones that you use the hairdryer on to tighten them up. I did four windows in the house that I knew were leaky. Guess what? After I bagged them. No more moisture on the windows. The hot moist air in the house was not meeting the cold glass.


It's all about the moisture in the air hitting that cold window.
In a small sense, by covering the window with the plastic you have recreated the scenario that I mentioned earlier.

Obviously, the plastic keeps moisture from the cold window, but when you originally covered the window you had moisture in the space already, so why no condensation?

The warm moist air in the space migrated out thru the original window and it was replaced by cooler drier air from outside. The air (in the newly created airspace) in contact with the plastic is going to be warmer than the air in contact with the window surface which sets up a convection current within the airspace as the warmer air rises and the cooler air drops. Cool stuff!

Are the original windows (the ones covered) single pane, single pane with storm, or dual pane? Curious...
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Old 12-20-2009, 04:56 PM   #40
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Moisture on windows


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I hear ya..........I don't think the OP really cares much because he hasn't been back yet.

If what you are saying is true, then in Woody's case, he should have had moisture on the plastic on his windows also. I think he would have noted that in his post.

I really don't see how you guys think moisture is in the air, inside a home, in the winter, with the furnace on. Right now it's 80% humidity outside and 33% inside and my windows are perfectly clear...........

A shame that the op went away because this has been a really intersting and fun discussion.

You make a really good point about exterior and interior humidity levels - but, you have to keep in mind that relative humidity is relative. As I mentioned way back in my first post "Relative humidity is a comparison of the actual vapor density versus the saturation vapor density at a particular temperature". There is no such thing as an absolute humidity level (at least not in the sense that we are talking about here). So comparing relative humidity outdoors to relative humidity indoors does not apply because the numbers relate to different levels of moisture at different temperatures.

Example - you mention 33% RH in your home and 80% outside. What does that mean?

Assuming 65F in your home and 33% RH, you have about 5.15 gm/m of moisture in your air and a dew point of 33 (the 33% RH and 33 dew point in this example is coincidence - nothing special aboyut the 33).

Raising your home temperature to 70F without changing the moisture level in the air will lower the RH in your home to 28%, but it will not change the dew point temperature which is still 33 and is based on the amount of moisture in the air and not on the temperature of the air.

Conversely if you lower the temp in your home to 60 without changing the level of moisture, the RH will rise to 39%, but again, dew point temperature remains unchanged.

As you say, and I agree completely, that if you have a cold air leak the cold air on the glass can lower the glass temperature to a level that will produce condensation - but it is still dependent on getting the glass below the dew point temperature which is based on the moisture level of the air and not on the air temperature.

Okay, the outside RH is 80% versus the indoor 33%. Again, keep in mind that it is much colder and drier outside - despite the RH reading.

Figure outdoor at 10 and 80% RH - that means that the outside air has about 1.38 gm/m of moisture in the air and a dew point of 12F.

Raise the temp to 20 but keep the same 80% RH (rather than keeping the same moisture level) and now the outside air has 2.5 gm/m of moisture in and a dew point of 15.6. Sill half the moisture content of the air inside, and a dew point temperature half the indoor dew point temperature, but 2 1/2 times higher RH level versus indoor RH.

Higher air temperature and more moisture content in the inside air = lower RH reading but higher dew point inside. Lower air temperature and less moisture content in the outside air = higher RH rewading but lower dew point temperature outside.

So, depite the fact that the outdoor RH is much higher than the indoor RH, the indoor air has a higher moisture content than does the outdoor air.

And keep in mind that you don't change moisture level by adding heat, even "dry heat". You will change the RH and that changes our perception of the level of humidity in the air (it may feel "dry" to us), but it does not change the actual moisture level nor does it change the dew point.
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Old 12-20-2009, 04:59 PM   #41
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Moisture on windows


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People also forget about 'air movement'...moving the air either near the windows or generally throughout the house then do that...It dilutes the high humidity-laden air near the windows and spreads it around.

If your heating system allows you to run the fan for all or part of the time great, otherwise a table fan - just something to move the air. I know it may feel cooler on the skin (nothing a sweater wouldn't cure) but the benefit is less mould and degradation around the windows.

As I mentioned earlier, it is now 0 deg F outside - we're in an artic air cold snap; our forced air furnace fan is on constantly, it filters the air too, and there's not a drop of condensation on the inner of two panes of glass...and the temp is 60 deg F inside, the way we like it.
Absolutely true. Air movement against the inside glass is a huge plus when it comes to preventing window condensation. Blocking warm air to windows is one of the leading causes of interior window condensation.

And thanks for the coments about my really longgggg posts.
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Old 12-20-2009, 05:13 PM   #42
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Moisture on windows


I appreciate the time you took to explain your facts. It does not change the problem. A leaky window will condensate. A window that doesn't leak air will not, unless you hit the dewpoint, but then you are at 100% humidity (raining).

Take a look at the posts in the thread. Everyone has the same scenario. A leaky window letting cool air in that creates humidity and condensates on the glass.

A fan, plastic, or dehumidifier only partially cures the problem. It masks the problem really. The real problem is leaky windows, like I posted in the first place.

The only reason cold air feels dry is because it is below freezing. Once that leaky air heats up, it is moist. I'm not sure where the assumption that warm air is always humid.

It's a simple test in your own home. Next time it gets below zero F, open your window and see it fog up. It's no different than a leaky window.
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Old 12-20-2009, 05:45 PM   #43
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Moisture on windows


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As you say, and I agree completely, that if you have a cold air leak the cold air on the glass can lower the glass temperature to a level that will produce condensation - but it is still dependent on getting the glass below the dew point temperature which is based on the moisture level of the air and not on the air temperature.
I agree. You have opened my eyes to a few things, and like I said, I appreciate your time to make the posts.

Removing the moisture from the air doesn't fix the real problem though.
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Old 12-20-2009, 07:33 PM   #44
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Moisture on windows


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I appreciate the time you took to explain your facts.
You are welcome, again this has been a fun thread and I enjoy the exchange of ideas.


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It does not change the problem. A leaky window will condensate. A window that doesn't leak air will not, unless you hit the dewpoint, but then you are at 100% humidity (raining)..
The problem is that the glass is below the dew point. It is entirely possible for the glass in a window to go below the dew point without leaking air via conductive heat transfer thru the spacer or the sash interface. Remember that heat transfers in three ways, conduction, convection, and radiation, and conduction heat loss at the edge of the IGU is a much more common mechanism for interior condensation than is leaking air.

A window can have condensation with absolutely no leaking if it is cold enough outside. The edge of the glass where it meets the sash is going to colder than it is in the middle of the glass because heat will be conducted thru the unit via conduction loss - no need for an air leak.

And you are 100% correct when you say that "unless you hit the dewpoint, but then you are at 100% humidity (raining)..." which is exactly what is happening on the window surface. The glass has water on it because it has reached 100% RH - which is based on a combination of window temperature and moisture content of the air - leaking air could be the reason that the glass temp is below the dew point, but in most cases it is not.


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Take a look at the posts in the thread. Everyone has the same scenario. A leaky window letting cool air in that creates humidity and condensates on the glass.
In some cases, sure. But that doesn't mean that the original question follows that reasoning. Again, windows condensate because of conductive heat transfer as well as leaking air.

You don't "create" humidity. Humidity is is nothing more than moisture in the air. Relative humidity is a comparison of the amount of moisture in the air versus the amount of moisture it takes to reach saturation - 100% RH.

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A fan, plastic, or dehumidifier only partially cures the problem. It masks the problem really. The real problem is leaky windows, like I posted in the first place.
None of those fixes "cures" the problem. There may not even be a problem in the sense of anything flawed in the window. What they do in each case is either limit the amount of moisture in contact with the window or they raise the glass temperature enough to take it above the dew point temperature. You go above the dew point temperature and there is no more window condensation.

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The only reason cold air feels dry is because it is below freezing. Once that leaky air heats up, it is moist. I'm not sure where the assumption that warm air is always humid.
No, cold air feels dry because it is dry. It isn't a coincidence that Antarctica is the driest place on earth. Cold air has a lower dew point than does warm air, therefore cold air gives up its moisture at a much lower temperature than does warm air.

Heating air does not make it moist. If a given volume of air has 5 grams per cubic meter of moisture content then it has 5 grams per cubic meter - no matter the temperature - unless you go below the dew point or above the boiling point. Air with 5 grams of moisture is going to feel very damp at 32F because at that temperature the air is at 100% RH.

But, that same air with the same moisture level is going to be feeling really dry (almost middle of the desert dry) at 90F because the RH will be only 17%. Changing temperature changes the RH - it does NOT change the level of moisture in the air and it does NOT affect the dew point temperature.


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It's a simple test in your own home. Next time it gets below zero F, open your window and see it fog up. It's no different than a leaky window.
Opening a window dumps warm air and moisture outside - IF the outside temperature and moisture level are below the indoor temp and moisture level. The window condensates in that scenario because it is cold and it has gone below the dew point of the air INSIDE the home - it has nothing to do with the air outside the home other than lower temperature.

This really isn't something that I need to experiment at home - this is what I do in my day job and I get paid to work with window companies on these sorts of (and other) issues.

Last edited by oberon; 12-20-2009 at 07:41 PM.
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Old 12-20-2009, 08:44 PM   #45
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Moisture on windows


You know what's wrong with this...........#1. lab results vary greatly from real world #2. You keep explaining plain laws over and over again proving me right, but won't give me credit for it. #3. I do this for a living also, and I don't need a paragraph explaining how moisture forms. #4. It comes down to two words....crumby windows

You create a scenario, which fits what you are saying, but it doesn't have real world effect. If it did we'd have a few more posts about moisture on windows.

Why does this persons home have the moisture, condensation.......that's the question. I think we all know how it forms now, but why? What causes it?

I think we know that humidity will cause moisture to form on cold surfaces now, especially if it's very humid, but why is it humid?

This is the difference between a Contractor and an engineer. The engineer just explains how to do things out of a book and put it on paper. The Contractor actually has to make it work or fix the situation.

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