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Trying to understand humidity dynamics.

The gym I go to leaves a large box fan on in the locker room 24 hours, presumably so the air movement will dry surfaces (wet floors). I can't imagine they would think it would help with wet clothing left inside the lockers. Accepting that this is inefficient, is it efficacious?

I also know of an archives building that keeps fans on low all the time, with the intent to keep historic articles—especially ones with fabric—from getting mildew. There is a dehumidifier in the room also that keeps humidity below 60%.

Clearly, hanging clothes to dry in a breeze will help them to dry more quickly.

Question: does induced air movement in a closed environment contribute to the prevention of moisture accumulation in fabrics or on surfaces?

Thanks.

Asked by David McNeely
Posted Aug 12, 2017 10:36 AM ET

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5 Answers

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1.

I just had some wet carpet in the basement. Tried to get it to dry with extra low room humidity and failed. Dried quickly once I also added a fan blowing over it.

I expect than a large ceiling fan would be even more efficient.

Answered by Jon R
Posted Aug 12, 2017 10:57 AM ET

2.

David,
Q. "Does induced air movement in a closed environment contribute to the prevention of moisture accumulation in fabrics or on surfaces?"

A. Indoor air is never at 100% relative humidity. What that means is that indoor air can always accept more water vapor if the water vapor becomes available.

In the case of damp carpeting in a basement, air movement helps dry the carpet faster because mixing the air allows dryer air to come in contact with the damp surface of the carpet, encouraging water molecules in the carpet to vaporize.

In the case of the library with a dehumidifier, the fans help equalize the indoor relative humidity, by mixing the relatively damp air near the bookshelves with the relatively dry air near the dehumidifier. As long as the dehumidifier is running, it makes sense to also run a fan -- to ensure that the dry air moves around and does the work you want it to do.

Answered by Martin Holladay
Posted Aug 13, 2017 5:24 AM ET

3.

Thanks for the responses. Martin, your response is unexpected. At other times you have indicated that humidity is not like temperature, saying that a single small dehumidifier can be effective even if it was not an open floor plan (I am paraphrasing, from memory). From this I interpreted that humidity control did not require air movement.

Clearly fans are helpful in drying significantly wetted materials and surfaces in humid conditions. But please allow me to ask two follow-up questions:

1.) In a large open room with high ceilings, with a single dehumidifier keeping relative humidity at about 50% as measured on the opposite side of the room from the dehumidifier (note the correction in the %), does running ceiling fans at low speed make a measurable difference anywhere in the room?

2.) In an unconditioned building, if doors and windows that are by no means airtight are closed when rain is approaching, will there be any significant difference in absolute humidity between inside and outside? This question is independent of the fan question, but still apropos of the topic question.

Of course you have often said that fans should be left on only when people are in the room, but I understand that this would be in a different context, and would not apply to the topic question.

Thanks for your help. This information will be significant in several different applications that I am currently engaged in.

Answered by David McNeely
Posted Aug 19, 2017 8:04 AM ET

4.

David,
In a perfectly sealed room (i.e., a submarine), without any introduction of air or water, the RH inside the sealed room will eventually stabilize, with few differences from one corner of the room to another.

A library is not a perfectly sealed room, so we can expect that (during the summer) humid exterior air is entering the library through cracks in the thermal envelope, and dry interior air is escaping. Under these circumstances, using a fan to help equalize the indoor RH (from one side of the room to another, assuming the room contains a portable dehumidifier) makes sense.

Q. "In a large open room with high ceilings, with a single dehumidifier keeping relative humidity at about 50% as measured on the opposite side of the room from the dehumidifier (note the correction in the %), does running ceiling fans at low speed make a measurable difference anywhere in the room?"

A. In the long run, probably not. In the short run, definitely.

Q. "In an unconditioned building, if doors and windows that are by no means airtight are closed when rain is approaching, will there be any significant difference in absolute humidity between inside and outside?"

A. Probably. Relative humidity changes outdoors affect indoor relative humidity eventually, but only gradually. Moreover, indoor RH is affected by factors like showers, floor mopping, and houseplants.

Answered by Martin Holladay
Posted Aug 19, 2017 8:15 AM ET
Edited Aug 19, 2017 8:16 AM ET.

5.

Also note that pressure plays a role. If a building is slightly pressurized in the summer, then areas of increased %RH at infiltrating air leaks don't occur. So no need to dry them with increased interior air circulation.

For a piece of dry fabric in the middle of a room - I doubt that it benefits from more than natural room air circulation.

Answered by Jon R
Posted Aug 19, 2017 10:23 AM ET

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