2

The optimum zone of the relative humidity of one's home at any given time is between 40% and 60%. This is the range to ensure that the humidity does not damage one's health or one's furnishings, in either direction.

My hygrometer notified me that the indoor RH was only 20%. This is the lowest figure that the hygrometer is capable of outputting, so the actual RH could be even lower than that. (I have not found an inexpensive hygrometer that is capable of displaying RH levels below 20%.) Naturally, this reading spurred me to turn on the humidifier.

I have an Aprilaire central humidifier next to my furnace. It is an old unit (around 25 years old), but it still functions. The humidifier control panel has a knob with a range of 15% to 45%. Because of the guideline mentioned in the first paragraph, I set the humidifier to the maximum setting of 45%. My thermostat is set to 72° F.

Here is info I just collected with the inexpensive hygrometer device (currently at 11 PM at night):

  • Current indoor temperature: 70° F

  • Current indoor humidity: 51%

Here is the current info from Weather.com:

  • Current outdoor temperature: 9° F

  • Current outdoor humidity: 81%

Now, on to the issue...

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Most of my double-pane windows currently have significant condensation on them. The condensation is on the inside of the windows, not between the panes. Some windows are practically coated edge-to-edge in water droplets, requiring several paper towels to absorb it all. I wipe it off, and the condensation fully returns within an hour or two.

Gravity makes the condensation drip down the length of the window, which gets absorbed by the surrounding wooden window trim. This has resulted in trim that is pitch black in some areas, an indicator of dry rot, I assume. I touch the wood and I can feel that it is very wet.

Wanting to prevent dry rot from occurring and further destroying my trim, I tried to find a fix online. I found this article from Star Tribune, the largest newspaper in Minnesota. Excerpt:

The following list, supplied by the Minnesota Department of Public Service, is based on a double-glazed window and an indoor temperature of 70 degrees. You will notice that the lower the outdoor temperature, the lower the indoor humidity should be.

• If outside temperature is 20 to 40 degrees, humidity indoors should not be more than 40 percent.

• If outside temperature is 10 to 20 degrees, humidity indoors should not be more than 35 percent.

• If outside temperature is 0 to 10 degrees, humidity indoors should not be more than 30 percent.

• If outside temperature is 10-below to 0, humidity indoors should not be more than 25 percent.

• If outside temperature is 20-below to 10-below, humidity indoors should not be more than 20 percent.

• If outdoor temperature is lower than 20-below, inside humidity should not be more than 15 percent.

(The article did not specify, but I assume that the "outdoor temperature" is the outdoor temperature without including the windchill factor.)

Huh? I thought that an RH below 40% was too dry for one's body. But, according to the above prescription, based on the outdoor weather conditions, my current indoor RH should not exceed 30%. In some extreme cases, it should be as low as 15%.

My questions:

  1. Why does condensation form on the inside of my windows in the winter?

  2. Why does the amount of condensation seem to significantly increase at night? My thermostat and humidifier settings are constant.

  3. The recommended RH range for one's health or one's furnishings contradicts the recommended RH range to prevent condensation on windows. How do you choose which guideline to abide by? In an indoor environment with an RH of 30%, I would be breathing in dry air and drying out wood furnishings. Shouldn't one's health take precedence over the development of window condensation?

  4. Is there anything I can do to stop the formation of condensation on windows, besides decreasing the indoor RH level? (I suppose that I could periodically walk from window to window, wiping off the condensation with a towel, but I'd have to do this so regularly that it would nearly be a part-time job.)

4

(1) Condensation forms on the inside of the cold windows in the winter because warm air in the house can hold more water vapor than cool air. Once warm air in contact with the cool window is cooled below the dew point, water falls out of the air and onto the surface of the glass. An ice cold drink left out on the counter sweats for exactly the same reason.

(2) All else being equal, condensation forms on the inside of the windows more rapidly at night because the temperature of the indoor window surface is colder at night. The colder the windows are, the more water will fall out of the air onto the surface of the glass. If you want to know the exact temperature that condensation will start to form, it can be looked up on a psychrometric chart like this one:

enter image description here

Find the intersection of dry bulb temperature and relative humidity, then follow the dashed diagonal enthalpy line intersecting that point left and up to the edge of the curved area on the chart (the line marked “saturation temp”). That is the temperature at which your air will start to deposit condensate on surfaces. As you can see on the chart, cold air with a high RH carries less water in it than warm air with a moderate RH and cold air requires even colder surfaces before the remaining water vapor will condense.

(3) Set the humidity in the house low enough that there is little condensation on the windows. Very low humidity (< 20% RH) can be hard on one’s skin and one’s wood furniture, but 30% RH is not that bad. When you witness large amounts of condensation on the windows there may also be condensation in cold locations concealed within the walls and roof, and that moisture can lead to water damage or mold growth. Water damage and mold growth are harder on one’s health and pocket book than dry air.

(4) (a) Check that the supply registers are not aimed directly at the windows. With a central humidifier running, the air from the registers can be much more humid than the room average. There will be less condensation on the windows if the registers are aimed to mix with room air before coming in direct contact with the glass. If the registers cannot be adjusted to mix supply air into the room without directly hitting the glass, then consider adding directional covers to the registers or replacing the registers. (b) Decrease the room temperature. At any given relative humidity, cool air carries less water than warm air, leading to less condensation on the glass.

3

First, it's worth noting that "wind chill temperature" isn't an actual temperature. It's a way of describing the rate of heat loss as compared to a no-wind scenario. For purposes of this discussion, it's irrelevant.

Why does condensation form on the inside of my windows in the winter?

If the temperature of the glass is below the dew point inside your home (which is dependent on temperature and absolute humidity, condensation forms. It's for this reason that you must manage RH carefully during winter. You may wish to be more comfortable, but your home will rot around you if you don't.

Why does the amount of condensation seem to significantly increase at night?

Outside temperatures are lower at night, and air movement due to human activity is less at night, so moisture has more chance to accumulate on windows.

Shouldn't one's health take precedence over the development of window condensation?

It is a delicate balance, but if you wish to avoid significant damage to your home, moisture must be managed with that goal as a priority. In my experience, skin and respiratory irritation aren't too much of a problem if the guidelines you cited are followed. However, it's easy to see indoor RH drop below those levels with certain HVAC systems and where human activity is low (single-occupancy homes, for example).

Is there anything I can do to stop the formation of condensation on windows?

Not really. You'd have to wipe your windows daily or risk damage.

My wife and I used to run a humidifier all winter to prevent itchy skin and sore sinuses here in Minnesota. Now that we have two preteen children showering and breathing and cooking alongside us, winter indoor humidity maintains itself fairly well. In fact, we almost have the opposite problem, especially early in the season before things dry out. Our home has substantial woodwork damage due to the way the previous owners managed (or didn't manage) the home's moisture level.

2

This related question answers 1 and 2: condensation forms when moist air meets a cold surface (your windows). Those windows get colder at night, since the outside air temperature drops as well. Not a whole lot you can do about that, except get better windows or decrease the humidity.

You've stumbled upon a problem in the winter months - what's best for your body may not be best for your windows. Decrease the humidity and your windows stay dry, but your lips crack, hands dry out, and nose bleeds. Increase the humidity and your nose stops bleeding but your windows condense the water vapor on them.

The newspaper article is likely trying to balance those two opposing recommendations.

Also note that the link you included giving the optimum humidity levels is from a NZ HVAC company. Homes in NZ, being on an island, tend to have a high humidity, so this company is offering a product to bring the RH down closer to the 50% level. What's good practice in NZ is not necessarily good practice in Minnesota.

  • "Condensation forms when moist air meets a cold surface." This is what I don't understand; if this is true, shouldn't every surface outdoors be covered in condensation? The outdoor temperature of 9° F is very cold (making all outdoor surfaces very cold) and the outdoor humidity of 81% is very moist. E.g., shouldn't there be significant condensation on the outside of my home's windows, and my mailbox, and my door, etc.? – Fil Jan 9 '17 at 21:31
  • 1
    By morning, a lot of outdoor surfaces are covered in condensation - that's what dew is. You'll notice your car windows are covered with moisture (ice if it's cold enough outside), the grass, metal fences, etc. The key is the surface has to be cold enough for the water vapor to condense. As @isherwood said in his Answer, it's really the dew points which matter, not the the RHs. Once the temperature drops to or below the dew point you can start having condensation form. – mmathis Jan 9 '17 at 21:46
  • We get frost regularly overnight, and sometimes it's downright stunning. The thing to remember is that very cold air, say that below 0F, has very little water in it. There's just not much to freeze out. Relative humidity is misleading because it's relative to the amount of water the air can "hold". That changes markedly with temperature. – isherwood Jan 10 '17 at 2:59

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