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My Aunt has a seasonal cabin in NH. At the end of the fall we shut the water off and close down the cabin. Last year we had to gut and re-do the entire cabin due to a mold problem. Part of the problem had to do with water getting in the half basement and then mold forming, but even before that problem, it always had a mold issue. It's my understanding that it's probably the late fall early spring that causes the issue, but that is just speculation. I am interested in putting a de-humidifier in the cabin, to ensure that it stays fairly dry, but I'm concerned about the fact that it will soon get below freezing. Perhaps there is on that has a frost shutoff? or perhaps I can rig something that will turn it off when the temperature goes below a certain value. I'm looking for recommendations on how to tack this problem.

  • Why do you think you need a freeze shutoff? If you have a draining humidifier, no water accumulates. And when the temperature drops, the relative humidity also drops, so it probably won't cycle on much anyway. – bib Oct 22 '15 at 15:01
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    @bin it's actually the opposite, given it's a closed space ( moisture content stays constant ) Relative humidity increases as the temperature decreases ( cold air holds less water vapor ) .... unless I'm miss understading what your saying – user379468 Oct 22 '15 at 15:47
  • I may be wrong about the latter point, but where would the water freeze? – bib Oct 22 '15 at 16:22
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You are correct that dehumidifiers typically do not operate below freezing temperatures. In fact, many do not operate well if the temperature is below 65F. Certain high performance units are rated to operate down to temperatures a little above freezing.

If the goal is to suppress mold growth by maintaining indoor relative humidity below 55% during cold temperatures, then the obvious answer is that both a high performance dehumidifier and a heater are needed. Looking at the psychrometric chart for moist air, 56F air at 55% humidity has a dew point of 38F. With a high performance dehumidifier and a heater set to maintain an air temperature of at least 56F degrees, the RH of the air in the cabin could be maintained at or below 55%.

  • Hi Chris, I don't understand where the 38F dew point comes into play? – bitsmack Oct 22 '15 at 23:32
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    The coldest the air can get on a good evaporator coil is roughly 5F warmer than the boiling refrigerant in the coil. The boiling refrigerant must be above 32F or the coil collects ice. 33+5 = 38F. That is about the coldest the discharge air from the evaporator coil can be during continuous running without freezing. Consequently, the dehumidifier cannot lower the dew point below that.. At that dew point, the air must be heated to at least 56F to achieve the desired low humidity. – user39367 Oct 23 '15 at 1:56
  • How much do you think it'll cost to heat a cabin all through a New Hampshire winter? It's probably safe to assume that it's not even insulated. I don't think that's a practical suggestion. – Mike Baranczak Oct 23 '15 at 4:22
  • I imagine it would be costly. Other than what you tell me, I know nothing about the details of your particular situation. Good luck with whatever you decide. – user39367 Oct 24 '15 at 2:59
  • @MikeB: My answer is not really a suggestion. It is more of an answer to the OP's question. If you have a practical suggestion for the OP, please post it. – user39367 Oct 25 '15 at 1:36
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To dehumidify in the late fall/winter/early spring you don't need a dehumidifier at all. Simply adding heat will automatically lower the relative humidity. Of course, if you did not heavily insulate when rebuilding the cabin, this will be expensive to do.

You could get cute and use a humidistat to control the heater rather than a thermostat, but you can also simply run a small heater full time.

  • You are correct, of course. I see the average climate in the fall for Manchester, NH is temperature around 60F with RH around 75%. Under those conditions heating the building to about 70F would achieve 55% RH indoors without a dehumidifier. Perhaps mold resistant building materials would be better than either one. – user39367 Oct 24 '15 at 4:21
  • Great idea to use a humidistat to control the heater, BTW. – user39367 Oct 25 '15 at 1:51

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