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Under the stairs that take to my bedroom there is a tiny room, as big as a cupboard. I can barely stand in it. I smell mould in it, I tried keeping its small door opened and I think situation vastly improved. With door closed I get 80% / 90% humidity and with door open around 50% / 60%.
For this reason I would like to improve its ventilation and I thought about cutting the door to put a vent with or without a fan or just shorten the door.

Am I going in the right direction?

Is there a safe and easy way to calculate the fan / vent I need?

Here is a picture of the entrance: enter image description here

And a picture of what could be the source of humidity, a non complete part of the floor where I see dirt (but I am not sure) on its top you see the beginning of the stairs: enter image description here

Thanks for your help!

Edit after comments: In the room one wall is communicating with a tiny toilet (closed, no windows) another one is shared with my neighbor (I guess, I don't know if there is any space between us), one is the one with the small door you see and then there is the one where the stairs are.

I also attach a couple of pictures that are often worth much more than words

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  • Are you keeping damp stuff in there? Is it against a cold outside wall? Ventilation is all very well, but you need to deal with the source of excess humidity as well – Chris H Apr 1 '20 at 9:58
  • It will take more than just a single vent as you need both an inlet and an outlet for air circulation. Is there a heating/cooling duct nearby that you can tap into and get a source of fresh air? – jwh20 Apr 1 '20 at 10:48
  • ...and you might want to directly attack the mold - ventilation might keep it from coming back - it won't make it go away. – Ecnerwal Apr 1 '20 at 11:33
  • Very good points ChrisH , @jwh20 and Ecnerwal ... I've attached pictures to clarify. – Pitto Apr 1 '20 at 15:14
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I believe I see concrete, if a ground floor with concrete this may be your issue to know for sure tape a piece of plastic to the floor for 24 hours . When you pull the plastic up if it is wet you have found the source of the moisture.

I have had good results using 2 part epoxy paint on concrete sealing damp concrete. I usually wait until summer when the moisture is lower. I prep the floor with a muriatic acid & water etch after that is done and the concrete has dried (a fan blowing over it helps to dry faster) I coat the area with the 2 part concrete floor epoxy this has eliminated several homes problems with basements and daylight basements that had the same issue.

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  • This is simply great suggestion! What do you suggest about the very big hole in the concrete? It seems to contain sand / dirt... Should I just throw more concrete on it? – Pitto Apr 2 '20 at 20:11
  • Ps Can you please also explain what do you mean by piece of plastic? – Pitto Apr 2 '20 at 20:12
  • I think this is the procedure you suggest: youtube.com/watch?v=wBeUWXvK0nU – Pitto Apr 2 '20 at 20:16
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    I did not realize that was just dirt, I would get a bag or two of premixed or sakrete to fill any void that is open to dirt it can be mixed in a 5 gallon bucket if you don’t have a wheelbarrow. Put it in the void and troll it smooth. When I was talking about plastic a plastic trash bag would work , but knowing that the floor is open I would say it is the source. Coating the floor will need to wait ~3 weeks prior to coating with epoxy, with fresh concrete acid etching is not needed as much as old. But it helps. I would first fill the area with concrete to seal it that may be all you need – Ed Beal Apr 2 '20 at 21:23
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    I would guess they ran out and left the hole in a place that did not matter. I always order extra but sometimes when the truck starts rattling it’s close so they may have underestimated and left it that way (I would have used my portable mixer to fill the hole). – Ed Beal Apr 3 '20 at 15:19
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If there is an outside wall inside this space, I would insulate that wall with styrofoam sheets glued to that wall. Then I would go to Lowes, Home Depot , or a large hardware store and ask to see a "through the wall" fan. Install the fan where convenient and install a small heating system grill on an opposite wall. The fan will ventilate the room and add some heat from the heated area of the house.

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  • Thanks for your input, I've added more data and pictures to the post, I hope it can be helpful to better understand situation. Maybe I could put a grill / fan on the 1st step of the stairs and one on the door? What do you think about the dirt? Should I just throw concrete on it? – Pitto Apr 1 '20 at 15:16

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