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Old 12-16-2010, 09:15 PM   #1
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Thoughts on house moisture situation


This is going to be a little long. Please bear with me as I want to give as many details as possible.

  • We live in Minnesota and its been consistently below freezing since about Thanksgiving.
  • Our house is 35 years old
  • We had a energy inspector here late last winter and he had no significant heating leaks findings.
  • Our heating system is five years old.
  • Our furnace humidifier was replaced late last winter after we realized it wasn't working and perhaps hadn't worked since the new heating system was put in place.
  • This summer we put in cellular shades on 80% of our windows
  • We have a hot tub in a three season porch (unheated)
  • On Thanksgiving, we had a house full of relatives, the door to the three season porch was open and the children used the hot tub. The windows in the house really steamed up.
Since Thanksgiving our windows have been really, really fogged up, especially in the morning after we raise the cellular shades. Indeed, the moisture was so bad that the sliders frozen shut last week. This is a fire/safety issue.

We thought the problem had to be the new blinds since the humidifier was turned down as low as it would go. We talked to someone who suggested we have an air exchanger installed and were ready to go down that expensive road.

However, this morning, completely unexpectedly, the windows were clear and dry in the morning and stayed that way all day. There was a small temperature change outside in that it got somewhat warmer -- from hovering around zero F, to about 17F but that is all that has changed.

Any thoughts about what might be going on and what we should do about it?

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Old 12-17-2010, 09:22 AM   #2
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Thoughts on house moisture situation


When you had the spa open and the door to the porch open you dumped a massive amount or humidity (moisture) into your house from the steam rising off the spa water. The humidity near the windows froze because that is an area of greater heat loss. I would buy a digital indoor/outdoor thermometer with humidity and monitor that to keep indoor humidity between 30% and 40%. Also when you close your blinds at night it keeps heat away from the windows allowing them to freeze easier. When humidity is high and temp low outside you might leave them partially open. I would not leave the door to the porch open when using the spa. Maybe use a small heater to warm the room beforehand. Too much moisture in the house=mold and other problems.

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Old 12-17-2010, 07:49 PM   #3
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Thoughts on house moisture situation


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Originally Posted by Superrick View Post
When you had the spa open and the door to the porch open you dumped a massive amount or humidity (moisture) into your house from the steam rising off the spa water. The humidity near the windows froze because that is an area of greater heat loss. I would buy a digital indoor/outdoor thermometer with humidity and monitor that to keep indoor humidity between 30% and 40%. Also when you close your blinds at night it keeps heat away from the windows allowing them to freeze easier. When humidity is high and temp low outside you might leave them partially open. I would not leave the door to the porch open when using the spa. Maybe use a small heater to warm the room beforehand. Too much moisture in the house=mold and other problems.
We considered this possibility, but Thanksgiving was on the 25th of November and the problem cleared up on the 13th of December. Does that make sense?

By the way, DH actually turned the humidifier up today because it was getting to dry in the house. Literally, in one day we went from way to wet to dry.

I will take your advice about getting something to measure humidity. Thanks for your thoughts!
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Old 12-17-2010, 07:52 PM   #4
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Thoughts on house moisture situation


Also, to note: After the doors froze, we did bring up a fan and a little space heater to move air around the two patio doors. After the second full day of this is when things got very much drier. They were turned off at that point.
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Old 12-20-2010, 08:17 AM   #5
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Thoughts on house moisture situation


So what part of Minnesota do you call home? I was born and raised in St Paul.

Here is a not-so-short blurb on window condensation that I seem to rewrite and repost any number of times this time of year and it may or may not help, but here goes anyway...

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 65°F 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 38°F.

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

- Consider a second home at 75°F but also with 6.25 g/m³ 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 38°F.

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.
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