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netminder 12-12-2010 10:48 AM

condensation on double pane windows
Hello I am hoping some one can help me with this problem I am having any suggestion, ideas is greatly appreciated.

I have:

-HI-efficiency furnace with its own intake & exhaust
-3" flex duct connected to the cold air return about 10' away from the furnace
-Conventional water heater which vents up the chimney
-Cold air return registry installed in the basement.
-Home is 59 years old and the windows are 20 year old but vinyle and double pane.

The humidity is 29% according to my humidity reader, dehumidifer would not come on because it is below 35%. The ouside temperature is -20 deg C and my house temperature is set at 66 deg F.

I have lots of condensation and frost on all of the upstairs windows which is double pane and no condensation in the basement. Can some one explain why I am having this problem the humidity is low enough I cannot go any lower because it will dry out my house.

I taught it was back draft from the water heater but I don't know can anyone think of any thing.



yuri 12-12-2010 11:05 AM

-29C where I am now. Hot air rises and creates convection currents so it takes the moisture with it. Moisture condenses on the coldest surface and that is at the bottom of the windows. I have low E argon 3 panes and it still condenses on the bottom of mine for an inch or 2. The only solution is to keep lots of air moving over the windows so they stay warmer and don't get cold enough to hit the dewpoint where it condenses. Practically impossible. I would rather wipe mine once in the morning and keep the humidity above 30% RH so it is more comfortable than try eliminate a bit of sweating. Try keep your house at 70 may help but it does cost a bit more. Not a perfect world we live in. I keep mine at 70. I want heat in my house and a full belly and I don't skimp $$ on both.:no:

Better quality 3 pane low E argons are more comfortable and sweat less. The best window is only a R4 so they all will sweat in our climate.

netminder 12-12-2010 11:22 AM

Thank you yuri, I am going to adjust my heat to 70 and keep it like that lets see it it gets any better. Do you think it could be backdraft from the water heater? I don't think it is but could that contribute to the condensation on the return in the basement?

yuri 12-12-2010 11:58 AM

You probably have what we call a fresh air pipe from the outside to bring in dry air and fresh air to the house thru the return duct. It will help you with your moisture problem and it is normal to sweat. Better insulation on it helps.

beenthere 12-12-2010 12:11 PM

At 66F inside temp, and 29%RH. The dew point is 33F. And at -20CC(-4F), your windows are too cold for that high of a humidity.

So as Yuri said. Either keep a constant warm air moving over them, wipe them off, or just have lower humidity.

netminder 12-12-2010 04:20 PM

Hello can you all help me out by telling me the following:

-What is the current humidity in your home
-What is the temperature outside your home
-What is the temperature inside you home

I just would like to know what every one else humidty level is in comparison to minds.

thanks very much

yuri 12-12-2010 05:20 PM

32% RH , -23C outside, 70 deg inside

beenthere 12-12-2010 05:37 PM

42F outside, 71 in, 35%RH. Of course, it is raining outside too.

netminder 12-12-2010 06:04 PM

-18 deg C (feels like -25 outside), 68 deg F (inside), 28% RH. (I am about the same as every one yet I still have ice on my windows, hmm. Maybe time I look at newer windows I guess)

Scuba_Dave 12-12-2010 06:21 PM

54f outside, 66-74 inside, 50% humidity
It was down to 30% & I started running a humidifier
Today its raining out so RH rose, still running the humidifier
I'm heating with wood right 74+ in main room
66-74 in surrounding rooms depending upon air flow
Andersen 400 LOWE windows

yuri 12-12-2010 06:28 PM

I was reading that the average 2 pane window is a R2, not so good. Window technology is still not so great if the best ones are only R4.:wink:

netminder 12-12-2010 06:39 PM

Hello here is the pictures of my setup as I said earlier, this shows the cold air return vent, location for cold air return in regards to the water heater and furnace as well as the fresh air intake.
Let me know what you all think. Right now it is -19 deg C (outside), 68 deg F (inside) and 28% humidity but still frost on windows, maybe the windows are just gone and need replacing could be a posibility.
Anyways that vent I show in the picture I currently have a card board over to restrict the return air flow in the basement. I am thinking that the return vent in the basement is pulling to much cold air in comparison to the ones upstairs I am trying different things to see if it will work.

yuri 12-12-2010 07:05 PM

I am not a believer of taking ANY return from the basement. Robbing Peter to pay Paul as less is taken from upstairs. I recommend electric baseboard heaters for the basement. In the Spring/Fall your furnace won't run too much and it will be cold in the basement so the electric baseboard heaters solve that problem. Better to take all the return air from upstairs IMO.

Probably your windows are shot as they lose their seal between the panes but that is supposed to cause fogging between the panes I am told. My low e argon 3 panes are so effective you can stand 2" away from them and feel no cold air or draft. INCREDIBLY comfortable they are. House is very well insulated with 2x6 walls. Other people tell me the same story with their argon 3 panes.

Marty S. 12-12-2010 07:13 PM

10F outside,35% RH and 73 inside. Four year old double pane windows with film and additional storm windows. No moisture on the glass.

oberon 12-15-2010 08:01 PM

In the interests of confusing the discussion a little, I am going to add my own two cents to the thread.

Dew point is defined as saturation vapor density or 100% relative humidity.

Dew point is the transition temperature between evaporation and condensation. Despite popular usage it is not “the amount of moisture that the air can hold at a given temperature”.

Warm air does not “hold” more moisture than cold air – that is a myth. However, it is not really a bad myth because it does help to explain the concept RH relatively easy to follow…. even if it isn't technically correct.

Condensation occurs on windows because the temperature of the glass is below the dew point temperature of the air resulting in moisture on the glass surface – it occurs on (or in some cases in) everything else for exactly the same reason.

Condensation may occur on the interior or exterior surfaces of the window or between the lites of a dual pane window if you have seal failure. Seal failure only affects condensation between the lites of an IG unit. If you have condensation that you can touch, it has nothing to do with seal failure.

In order to stop condensation from forming on the surface of a window you have to:

(a) lower the dew point temperature of the air to a level below the temperature of the window surface, or

(b) you have to warm up the window surface to a temperature above the dew point temperature of the air, or

(c) a combination of both.

Because there are two ways to lower relative humidity – either increase the air temperature or decrease the moisture content of the air – simply lowering the relative humidity of the air may have absolutely no effect on controlling interior surface condensation, or it may completely solve the problem.

How the relative humidity is lowered, and the affect that the "how" has on both the moisture level of the air and the resulting temperature of the glass, will determine whether or not a condensation problem can be controlled or eliminated.

Because there are two ways to lower relative humidity – either increase the air temperature or decrease the moisture content of the air – simply lowering the relative humidity of the air may have absolutely no effect on controlling interior surface condensation, or it may completely solve the problem.

Increasing air temperature will lower relative humidity but it will not affect 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 the RH is lower with higher air temperature, it may not effect condensation on window surfaces – unless the rise in air temperature also results in a corresponding rise in glass temperature to a level above the dew point.

Lowering the amount of water vapor or moisture in the air will lower the dew point temperature and if the dew point temperature drops sufficiently – to below the surface temperature of the glass – then it will affect condensation formation on the surface of the glass.

The amount of moisture in the air is most easily measured in grams per cubic meter (versus fluid ounces per cubic foot), which is kind of nice for metric folks but not so nice for 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 for a couple of quick example calculations.

So consider a home at 65F with a relative humidity reading of 40%.

- In this scenario there are 6.25 grams of water in a cubic meter of air in the home which equates to a dew point temperature of 38F.

- 38F is 100% relative humidity or saturation vapor density or the temperature where condensation occurs.

- Consider a second home at 75F but also with 6.25 g/m of water in the air. This second home has a relative humidity of 29% - versus 40% in the first home - but, and here’s the kicker, the dew point temperature hasn’t hanged. In the second home it is still 38F.

So, while the relative humidity in the second home is much lower than is the relative humidity in the first; if the surface temperature of the glass in either home is 35 or lower those windows are likely to have surface condensation regardless of the substantially lower RH reading in the second home. But if the glass surface temperature of the windows is 40 – only five degrees warmer – it becomes much less likely to find condensation on the windows.

A very quick rule of thumb on window performance:

Single Pane – R1 or U1
Single Pane and storm window – R2 or U.5

Dual Pane, clear glass – R2 or U.5
Dual Pane, LowE coated – R3 or U.33
Dual Pane, LowE coated, Argon fill – R-3.5 or U.29

Triple Pane or dual pane with storm, all clear glass – R3 or U.33
Triple Pane, one lite LowE coated – R4 or U.25
Triple Pane, two lites LowE coated – R5 or U.20
Triple Pane, two lites LowE coated, argon fill – R6 or U.17
Triple Pane, two lites LowE coated, krypton fill – R7 or U.14
Keeping in mind that these are glass-only numbers that will vary depending on factors such as airspace width, type of LowE coating used, and other specific construction details.

Whole window performance will vary even more depending on style (casement versus double hung for example), sash and frame material (wood versus vinyl versus fiberglass versus aluminum), etc etc etc

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