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My basement is generally very dry but I want to know how much moisture is too much, to better prioritize when I need to reduce it.

The basement is mostly concrete floors and walls with exposed wood joists and sheathing. It is in three small parts, with one of them well ventilated to the outdoors (for better and worse - needs better insulation); one with a painted concrete floor, drywall, a drop ceiling and a couple of windows; and one utility room with an exposed drain pipe in the floor and one window venting appliances.

Most of the time each space varies between 15 and 35% relative humidity* this time of year. The house overall is extremely dry this time of year, with all areas under 50% humidity that I've measured. Surprisingly even in cold temperatures (the room at 55dF) clothes dry relatively fast (1-3 days) because the air is so dry. During times of heavy snowmelt, a small area of the basement (less than 1 square meter total) does get dampness seeping in from the ground near the edge of the house.

At what point do I need to act to reduce moisture in the basement? Obviously if it's flooded, items inundated with water can be damaged and all the moisture can lead to other indoor environmental problems (like mold). But are small spills of water - e.g. from seasonal seepage, or from a pinhole leak in a pipe - alright on concrete? Roughly how much water can be there and for how long before it must be dried up to avoid further repair needs / permanent damage?

Some people tell me hanging clothes to dry in a basement is too much moisture, and the previous owner actually ran a dehumidifier in the utility room in the past. To me, <40% humidity* seems like there's no need for a dehumidifier, and even if clothes are sopping wet out of a wash for some reason (not usual), the shallow splashes/puddles they leave temporarily don't seem like they'd cause any problems in the long run. Neither does routine hanging clothes to dry ~1/week.


*Note, my humidity measurements might be wrong because I'm using a cheap electronic device. I did check my humidity gauge against a humidifier's and they matched so that's reassuring, but I wouldn't be surprised if mine isn't very accurate.

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    follow your nose. if pooling pudles stick around long enough to stink, it's too humid. If they dry overnight or whatever, it's fine. it sounds more than dry enough to me... – dandavis Jan 19 '18 at 19:58
  • @dandavis That's what I was thinking. Puddles from seepage, wet clothes, accidental spills from a slop sink, or whatever have all dried up in a day or two and not gotten to the point of smelling or staining the ground aside from collecting some dust & debris that's on the concrete. Seems OK to my intuition. – cr0 Jan 19 '18 at 20:01
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    if i were going on vacation in the spring or summer, i might invest in a damp-rid snake, or even a beach towel, to intercept the puddle, just in case an expected tsunami comes through. – dandavis Jan 19 '18 at 20:16
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I have worked on homes that had a drain trench around the entire basement that drained into a sump and was pumped out, the walls were always wet and a dehumidifier was needed in the winter. With the levels you have listed I would not worry at all. If the 1 location that gets wet gets worse or larger some exterior work may be needed to get the water away from the wall. The dampness will not affect the concrete, think about all the bridge pilings that are in & under water. If you don't have a mold or mildew problem I don't see anything wrong with hanging clothes to dry. We have a large kettle on our wood stove to add moisture to the home probably not much different to the amount the clothes provide plus it saves energy.

  • The classic kettle on the wood stove was something I thought of too. I know many extremely handy farmers who have a big open pot of water or tea continuously simmering on their wood stove, to humidify the air and offer a warm drink. If you can boil water all season in a wood room (never heard any of them worry about excess-moisture problems), probably fine to have an occasional puddle or damp set of clothes in a basement. – cr0 Jan 19 '18 at 20:04

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