Questions about humidity control in unconditioned space
Two locations: Zone 3A, coastal N.C. (fishing cabins converted to a non-profit retreat facility—insulated and heated, but no AC); and Deccan Plateau in central India —wet, continuously humid 3-month monsoon and scorching hot & dry 4-month summer (HVAC nonexistent).
Primary question: how can mold and mildew issues be prevented and/or managed in an unconditioned space?
During warm & humid weather, rooms are left open to the exterior as much as possible, with fans going almost continually for both human comfort and to reduce mildew and sodden fabrics. During winter in NC, the cabins are closed and heated, but the mold becomes insufferable. I suspect mold is on every surface of nearly every building element, despite insulation.
Is there any remedy? For example, if rooms were made weather-tight and dehumidified, and then opened in the cooler evening, how quickly would the space come to humidity equilibrium with the exterior? Would the benefit of keeping fabrics drier be defeated entirely?
Another example, if a 200 sq. ft. room were conditioned with a mini-split, how much would the temperature conditioning suffer from leaving a door open (for public access to a small museum)? How much would the humidity conditioning suffer?
Thanks for your help in understanding humidity better.
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Replies
David,
The brief answer: If you have electricity, and you can afford to do things the modern way, the right way to control high indoor humidity is with an air conditioner and/or a dehumidifier. For more information on this topic, see All About Dehumidifiers.
If the cost of purchasing or operating a dehumidifier is daunting, you should be able to control indoor humidity during the heating season (any time it is cold enough to operate a heating system) by ventilating the space -- either with a continuously operating bathroom exhaust fan or with a small supply fan. This works because outdoor air can't hold much moisture when the temperature is low. During the winter, outdoor air is dry.
During hot, humid weather, a fan won't do much except keep you more comfortable. In the old days, people accepted high indoor humidity because they had no choice. Clothes with mildew were laundered. Surfaces with mold were cleaned. In North Carolina, things dry out somewhat during the winter.
And in India, most people live outdoors as much as possible. Wealthy people install air conditioners.
Concerning the small museum with an open door for public access: I suggest you install spring hinges on the door to keep it closed, and put a sign on the door that says, "Welcome! The door is unlocked."
If the door needs to stay open, your cooling bills will be high. But you can still operate an air conditioner. In Hong Kong, lots of stores leave their doors open all day, allowing the cool air-conditioned air to spill out onto the hot sidewalk, to attract customers. The only drawback to this approach is signing the checks to the utility company.
A 200 square foot space (other than a greenhouse) doesn't have much cooling load with the doors closed, but with the door open during monsoon type weather both the sensible and latent cooling loads go up quite a bit. Even a loose flap type of door would makes a huge difference.
Unoccupied heated cabins in NC should not have mold issues in winter unless there is a moisture source, such as ground water or roof / wall / window leaks. The wintertime outdoor air dew point averages in NC are well below mold-inducing levels. Using Wilmington as a representative climate, the average dew point if below 55F from half-past October through the beginning of May, which is below 65% RH @ 68F. (The dew point graph is at the bottom of the page: https://weatherspark.com/averages/30590/Wilmington-North-Carolina-United-States ). Ventilation could work, but could also become a significant heating expense.
If treating it at the source isn't possible, portable room dehumidifiers set up to drain to either a plumbing drain (preferred) or to the outdoors (has freeze-up potential during cold snaps) works. The heat of vaporization recovered from the condensed moisture with a dehumidifier ends up in the room, not outdoors, raising the room temperature, and thus offsetting the sensible heating load. The efficiency as a space heater is high compared to a resistance type electric space heater- it's a heat pump. A small Energy Star labeled unit runs about 1.85 liters per kwh. A kwh of power used indoors adds 3412 BTU of heat into the room, but heat of fusion of 1.85 liters of water condensed is worth another 1665 BTUs, so it's roughly a COP of 1.5, from a space heating perspective. (It'll actually be higher than that at very high humidity, but shoot for 60%, not lower, since it's COP will drop at lower humidity levels.)
As long as the room's relative humidity is held below 65%, the mold potential is quite low.
A daily drying should prevent mold growth at a lower cost than 24 hours/day of drying with a dehumidifier.