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Old 12-17-2009, 05:47 AM   #16
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Moisture on windows


The really long version if you want to do some early morning light reading...

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 problem 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 not the same way.

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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Old 12-17-2009, 06:21 AM   #17
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Moisture on windows


Wow....I, for one enjoyed reading that, oberon, thank you. I applaud your patience in writing that too...it's more than I could have done, by far.

I like to think we know a few things about indoor air quality - and we do - but it's always nice to get a refresher course from another party to smooth out the rough edges.

Tricky subject - but excellent post, oberon!
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Old 12-17-2009, 06:59 AM   #18
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Moisture on windows


Quote:
Originally Posted by baum View Post
so what can i check as a homeowner to see if they were installed correctly? or to see if its a window problem?
Pull your blinds all the way open. Check your humidity level, and please let us know the results. Leave the curtains open for at least a few hours, if not all day or night. See if it does anything. This will give us an idea.

What is your heat source?
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Old 12-17-2009, 11:26 AM   #19
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Moisture on windows


Also, keep in mind the condensation you can't see - the condensation within walls when warmer (and usually moister) air moving toward the exterior hits the dew point.
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Old 12-17-2009, 02:43 PM   #20
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Moisture on windows


I also live in Minnesota and have had the same issue. I have reduced the moisture in my house and raised the temp a little during the evening and this has helped. In all of my bathrooms I installed this timer switch which seems to help immensely. Like the previous posts have mentioned running the fans for quite some time will help. We run it for 60 minutes anytime someone takes a shower. I also made sure my windows had proper protection on them for the moisture. If they are stained, make sure and get a few good coats of helmsman spar urethane on them. Best of luck.
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Old 12-18-2009, 07:51 AM   #21
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Moisture on windows


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.
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Old 12-18-2009, 10:13 AM   #22
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Moisture on windows


Like I said, I doubt it is a humidity problem. It's winter and it's dry. The problem is because the windows are no good and should be replaced.
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Old 12-18-2009, 10:44 AM   #23
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Moisture on windows


I can't agree completely with that statement MJW.

Yes the windows probably don't help the problem, but it is the humidity in the home was lower, the condensation issue would be less severe.

I suggest the OP first look to stop as many drafts as possible around the windows. Open the curtains as you suggested to increase air flow around the windows. So far his cost is minimal.

If the problem still exists do you recommend him spend $12,000 on new windows or to try a $250 de humidifier first?
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Old 12-18-2009, 10:55 AM   #24
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Moisture on windows


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I can't agree completely with that statement MJW.

Yes the windows probably don't help the problem, but it is the humidity in the home was lower, the condensation issue would be less severe.

I suggest the OP first look to stop as many drafts as possible around the windows. Open the curtains as you suggested to increase air flow around the windows. So far his cost is minimal.

If the problem still exists do you recommend him spend $12,000 on new windows or to try a $250 de humidifier first?

To be cost effective, I agree. There may be a warranty though that can help.
To stop the problem, you have to go to the source which is the windows.
I have never heard of anyone using a dehumidifier in the winter. Usually it's dry in the home and you are using a humidifier.
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Old 12-18-2009, 12:38 PM   #25
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Moisture on windows


Replace the windows after 2 years??? in a townhouse? Find me the sucker who approves that solution to a humidity problem in winter so I can buy him a beer.

Have lots of customers for him...
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Old 12-18-2009, 01:02 PM   #26
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Moisture on windows


Windows can be faulty right from the factory. They can also fail very soon. We have replaced windows under warranty within the first year.

If it is in fact a window problem, it IS NOT a humidity problem. Like I said, in winter, humidity is not a problem without an underlying problem.
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Old 12-19-2009, 09:10 AM   #27
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Moisture on windows


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Windows can be faulty right from the factory. They can also fail very soon. We have replaced windows under warranty within the first year.

If it is in fact a window problem, it IS NOT a humidity problem. Like I said, in winter, humidity is not a problem without an underlying problem.
I have to disagree. Excess moisture in tight homes in winter is a very real problem and in some areas it is pandemic where it is resulting in mold and other moisture related problems.

In the "good old days", houses were a long way from being air tight. They breathed....often thru the windows and doors. The windows and doors were simply not very good at keeping inside air in and outside air out. This had certain advantages versus today's tight houses, but one really big diasdvatge as well - it wasn't very energy efficient. Not a problem when energy was cheap - big problem when energy is expensive.

Thermodynamics 101 - heat goes to cold and wet goes to dry. Nature is a socialist, nature wants everything to be the same. Both heat and moisture want to negotiate with cold and dry until everybody is equal. Heat and wet want to balance with their respective opposite.

Nature does not like things that are not in balance, and since the air inside of a home in winter in most situations is going to be both warmer and wetter than is the air outside, nature does not like this situation and tries very hard to fix it.

In the old days this was very simple - the house wasn’t tight (especially around the windows) and all that heat and moist air simply went thru the windows in an attempt to equalize the cold, dry air outside. Since this wasn’t even remotely physically possible, it became a continuous cycle…warm up the air inside the home and out it goes trying very hard to get the outside warm. As warm, wet air escapes, it is replaced by cool, dry air from outside. Ignoring the cycle meant that the air inside would eventually be exactly the same as the air outside.

The people living in the house had other ideas about that situation however and they pumped loads of heat into that house in an attempt to stay warm. This worked, within reason, but the homes were then very dry. People generally attributed this to the warm air from the furnace “drying” out the air in the house when it was actually because moisture from inside the house was migrating to outside the house along with the warm air - back to nature's attempt to equalize the inside and outside temperatures and moisture levels as well.

As air inside warms up it does feel dryer than it does when it cooler since the relative humidity goes down as the temperature goes up. The warmth isn’t actually drying the air, it is simply changing our perception of it. Hopefully, I was able to make that point in my earlier post also.

When the air became very dry, it was not very comfortable, so people began to add moisture to their air in an attempt to raise the relative humidity to more comfortable levels. As long as the house remained relatively loose, this “new” warm and moist air migrated to the outside and was replaced inside and the cycle continued.

But newer houses are tighter. Newer windows are tighter. We have caused a rift between what Mother Nature intended and what we allow her to do. We are trying very hard to keep the heat inside our homes and avoiding it going outside. This has the added “benefit” of also keeping moisture inside as well….often much more than we would like to keep inside for both comfort and eventually health and structural reasons.

Houses today are often too wet, rather than too dry. In the winter furnace humidifiers are often overkill for the needs of today’s homes.

Obviously, it would be foolish to suggest that a home will never need additional humidification, but in today’s world it is also not correct to say that all homes will need humidification because of dryer winter air either.

The need for humidification or dehumidification is very dependent on both the construction of the home and the lifestyle of the people living in it. And obviously, there are many, many houses that are still leaking air and moisture to the outside in the winter…as well as those that are very tight.
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Old 12-19-2009, 09:23 AM   #28
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Moisture on windows


-46 celsius gonna quit moaning about 2' snow in virginia today
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Old 12-19-2009, 09:34 AM   #29
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Moisture on windows


Always nice - and refreshing - to read something well thought-out, correct and well written!
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Old 12-19-2009, 10:06 AM   #30
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Moisture on windows


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.

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