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I have some expensive antique furniture stored in my cellar. The cellar came with an air con in it (believe it or not). We added a fan that will blow air OUTSIDE, and a dehumidifier. This is to make sure that there is ventilation, so that the furniture doesn't rot.

The aircon is always on, so that the temperature is always 24°C. The problem is that the dehumidifier needs to run 24/7 and it's pulling 40 litres of water a day!

The cellar is made with two layers of bricks with a cavity, and is surrounded by earth. There are gaps in bricks. The flooring is concrete.

Questions:

  1. Where the heck is the water coming from?

  2. Is there any way I can fix it so that the cellar itself doesn't need so much de-humidifying? I am ready and willing to build a polystyrene box around it

  3. I read that running a dehumidifier is not a solution, because wherever the water is coming from, it will pull more water in. Is that true?

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  • Related: diy.stackexchange.com/questions/14284/… May 18, 2022 at 15:15
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    What humidity level do you need to maintain? The usual <60%RH for mold prevention, or something drier than that? What is the current humidity you have there? What climate are you in? (I suppose those 24 degrees are Celsius, which feels quite warm for a cellar. Is it hot outside all year, or is the aircon heating the cellar?) It could be that your fan is pulling in a whole lot of somewhat moist air all day and the dehumidifier is thus effectively drying the atmosphere, but it's impossible to know without all these details.
    – TooTea
    May 18, 2022 at 15:58
  • The polystyrene box idea is just like encapsulating a crawlspace. 6 mil plastic on the floors and walls (to prevent moisture from migrating in); maybe a dehumidifier and, blowing conditioned air from your living space into the crawlspace (or basement, in your case).
    – SteveSh
    May 18, 2022 at 16:43
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    I think you were misled on the dehumidifier. By that argument, if you turned the dehumidifier off, the moisture inside and outside the basement would somehow equalize so that no more moisture/water made it's way into the basement.
    – SteveSh
    May 18, 2022 at 16:46
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    What is the purpose of the fan blowing air out? All you're doing with that is sucking the cooled air out of the basement, making the AC work even harder. Note: an air-con (AC) unit will actually help dry the air (assuming the hot air is properly vented). Blowing that drier air out is counter productive.
    – FreeMan
    May 19, 2022 at 14:19

2 Answers 2

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Having a high humidity in a brick and concrete basement is not unusual, I have had great luck with 2 part floor paint (epoxy paint).

you can prove it to yourself that the moisture is coming through the slab and walls by tapping a plastic bag down and waiting 24 hours the next day the area under the plastic will be damp.

As far as a dehumidifier causing more moisture I don’t think it would but as you pull moisture out it is replaced, the best method for walls is excavation sealing and backfilling with gravel so moisture won’t be sitting on the brick work. But sealing with a quality epoxy paint may work it works well on slabs.

So start out by sealing from the inside, this method won’t stop high water but it may stop the vapor (it has for me on several homes.

If internal sealing is not working (significantly reducing the collected water) then exterior excavation, sealing and backfilling with gravel is about the only way to solve the high humidity in your basement.

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You haven't said what the climate is like in your locality in terms of normal temperature range and humidity. In particular, how do the conditions you wish to create in the cellar compare to those found in the rest of your building and outside?

It may well be that the fan you have added is making matters worse. If you are blowing air out of the cellar, you must be drawing it in from somewhere else to keep the pressure balanced. Can you figure out where the air gets in? If that air is normally warmer and/or moister than how you are trying to keep your cellar, then that will add to the dehumidifier's work. The air might also be being drawn in past/through the earth surrounding the cellar, and picking up moisture on the way. I would certainly try switching off the fan for a while to see what effect that has on the amount of water produced by the dehumidifier.

I would also look for other potential sources of moisture. Do you have any other equipment in the cellar that could raise the humidity? Cooking, showering and drying clothes are typical culprits in domestic buildings - but a constant 40L/day is a lot of water, so it's more likely to be something that has gone wrong. Are there any signs of leaking water pipes? The air-con is worth investigating too. If it is operating in cooling mode, it will also produce condensation, like the dehumidifier. Is it draining properly? There's an outside chance it might even have a humidifier mode, and be fighting against the dehumidifier.

More likely though, I would guess that the moisture is probably coming from the soil around the cellar, through the walls and floor. What to do depends on how wet it is, and why. If it is not very wet, fixing the airflow might be enough to help. You might also be able to help matters by improving rainwater drainage around the building, or fixing a leaking pipe that is saturating the soil. If you can monitory the humidity or soil moisture, you may find you are gradually drying out the cellar's surroundings. But if you have a high water table in your area or lots of rain, you are probably going to need to try to make the cellar watertight. Without doing this, moisture is always going to diffuse from the damp surfaces into the relatively drier air produced by the air-con and dehumidifier.

Even if the dehumidifier is pulling in water from the surrounding soil, it may still be a viable solution if it is powerful enough to maintain the humidity you want. It depends whether you can identify and cut off the water source or not, and the balance of the cost and hassle of doing so versus keeping the dehumidifier running.

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  • The AC should drain its condensate into a drain or be pumped outside. There are models that blow the condensate out through the condenser, it helps the efficiency. You are getting 40L a day how did you determine that.
    – Gil
    May 19, 2022 at 15:14
  • I guess the OP got 40L/day from how many times they had to empty the dehumidifier tank - assuming it's not a continuous drainage model.
    – JRI
    May 19, 2022 at 17:40
  • If the water is coming from the floor area consider putting in a sump pump. If you have one be sure it is functioning. It is also possible your footer tile outlet is blocked. Posting some pictures would help a lot.
    – Gil
    May 20, 2022 at 3:20

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