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Community and Q&A

When to vent vs dehumidify basement

stanfloyd | Posted in Energy Efficiency and Durability on

My sister in east central Wisconsin has been trying to keep her unfinished basement humidity at around 50% RH. Dehumidification is costing her around $50 per month. I would like to determine under what exterior ambient conditions would it be more cost effective for her to turn off the dehumidifiers and vent the basement with external air.  I am an engineer and could pull out my dusty psychrometric chart and run some calculations; but I know there are more capable people on this forum who might have already addressed this question.   

Here are the details: The house (2 stories plus basement) was extensively remodeled to current standards 2 years ago.  80% of the 1000sf unfinished basement was from the original homestead house that was built in the 1800’s.  Those old walls (see photo) are 7ft high and built of dry laid stone with a layer of mortar on the inside. The newer part of the basement (see photo) is poured concrete with exterior insulation.  The entire floor is cement slab.  There is never any moisture on the walls or the floor so I suspect that most of the humidity is coming as vapor through the old stone walls. The house is heated with hydronic propane-based heat.  There is a single radiator in the basement and the room is maintained at about 65 degrees.  There is no door between the basement and the middle floor.  There is only one (rarely opened) window in the basement.  Propane currently costs about $2.25/gal and electricity costs $.15/kwh in that area.         

I am pretty sure that my sister should be venting on warm dry days and I suspect she should also be venting (and heating the exchange air) on some winter days.  I would like to give her some advice on how to pick those days when she should be venting (e.g. at certain dewpoint?) and what the cost and potential savings might be. 

Thanks in advance to any responders

Stan Floyd

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  1. Expert Member
    DCcontrarian | | #1

    I made up a spreadsheet where you can plug in some numbers, it's at:

    Go to the tab named "Venting," under "entered values" put in your indoor and outdoor temperature and relative humidity and fan CFM.

    If it is colder outside than in, you will have to heat the air that is brought in. This heating load is shown under "sensible cooling." In heating season, the question is whether this load exceeds the energy saved by not running the dehumidifier.

    When the dewpoint outside is higher than the dewpoint inside -- which you can see under "Intermediate Values" -- venting does you no good, it brings in more moisture than it exhausts. When it's cold venting isn't efficient because the heat lost takes more energy than running the dehumidifier. I think what you'll find is there's a narrow band of moderate weather where venting is a net positive.

    Two observations:

    The energy used to run the dehumidifier isn't lost during heating season. A dehumidifier is a very efficient space heater, when it's wringing water out of the air it's also wringing heat out, it produces about three times as much heat as the electricity consumed. The heat that it extracts is being put into the building and helping to heat the house.

    Second, the biggest thing you can do to reduce the need for the dehumidifier is to seal the walls and floor.

  2. stanfloyd | | #2

    Thank you very much for the spreadsheet and quick response. How many air changes per hour would you recommend for humidity control - assuming of course that the outside air dewpoint is lower than the basement air dewpoint

    1. Expert Member
      DCcontrarian | | #3

      There's really no way of knowing how much moisture is coming through the walls. It also depends on the outside dew point.

      Does the dehumidifier keep humidity under control? If you know it's rating in pints per day, 24 pints per day equals 1,000 BTU/hr so you can figure out its equivalent in BTU/hr. In the spreadsheet see what you have to put in for CFM to get that many BTU/hr.

  3. stanfloyd | | #4

    She seems to be keeping up with two 45 pint per day dehumidifiers - one running continuously and the other running for 10 hours overnight.

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