GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Damp cellar

Matttheram | Posted in General Questions on

Hi I will be embarking on tackling my small cellar next year. We have lived at the property for 4 yrs now and have come a long way in terms of reducing damp and humidity levels in the house (1750s coach house) 
One remaining problem area is the cellar. It is damp down there. Signs of damp such as cardboard boxes soften fairly quickly and previous wood rot from many yrs ago. I plan to take the walls back to brick next Yr. Re point in lime and lime wash the compartment. 
I do need some advice on ventilation though. I am aware this is key yo solving and damp/condensation issues as it is all about air changes over time.
My thoughts are to set up a negative pressure system using a tube fan and vent it directly to the outside atmosphere. My hope is this will create a negative pressure thus drawing dryer and fresher air from the house into the cellar. Also this will hopefully create a poultice for the cellar walls to draw out the moisture within and dry them.
The cellar is only small and will only be used for storage not a living space. Dimensions approx 5 x 2 metres. Any advice appreciated 🙏

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    No, actually. Ventilation is nowhere near the top of the list of possible remedies for your problem.

    For better approaches, see this article: "Fixing a Wet Basement."

  2. Expert Member
    NICK KEENAN | | #2

    Ventilation is only effective when the dew point outside is lower than inside. In my climate that isn't true roughly half the year.

    I know there is a product that is marketed as a miracle cure for damp basements, I forget the name. But it's a humidistat controlled vent fan. It's very expensive and would be expensive to operate.

    1. Matttheram | | #3

      Hi can you explain why this is? Surely dew point internal/external is irrelevant if you are using a 1 way negative pressure fan?
      I understand that theory if you are bringing in the air from outside in but I plan to bring air in from the internal envelope of the house?

  3. walta100 | | #4

    The walls are trying to be 55° inside like the ground on the outside of the walls. Many days the air in the living space will have a higher dew point than the basement walls temp. Bringing in more air into the basement could make things wetter depending on your walls and weather.

    I think your problem is you have an 18th century basement and 21st century expiation of that basement. Back then people expected basements to be cool, damp and musty, today we aspect the basement to be like the rest of the house.

    My wild guess is your basement is damp because liquid water is leaking thru the wall and evaporating once inside. If you really want\need a 21st century basement you will need to being your walls up to 21st century standards you need exterior water proofing and an exterior drainage system .


    1. Matttheram | | #5

      Cheers for the reply Walta. No I definitely don't want to introduce any modern waterproofing system on my cellar. I would just like to control the amount of moisture (which isn't terrible) so that it prolongs the life of timbers etc down there. I accept it will never be a liveable space but if I can control the environment through ventilation and lime based (breathable/capillary active) materials to stop the trapping of moisture within the building fabric that would suffice. I really just need to know whether a positive or negative fan system would be best for the situation

  4. Expert Member
    NICK KEENAN | | #6

    The problem is in the summer your interior air isn't dry enough to dry out the crawlspace, even if it's conditioned. (If it's not conditioned forget about it completely). If your interior air is at 75F, 50%RH -- which would be typical for summer, even maybe a bit dry -- it has a dew point of 56F. At 62F it would have RH of 80%, which is really the upper limit for humidity for wood to be safe.

    A typical consumer dehumidifier might do 72 pints a day, or 3 pints per hour. Let's say your crawlspace is at 62F and 100% RH, your interior air is at 75F and 50% and you're content with getting your crawlspace to 80% RH. To remove the same three pints per hour you'd have to remove 1250 pounds of air per hour which is about 300 CFM. You're moving a substantial quantity of air, which needs to be replaced in the conditioned space, and you're not even getting the crawl space that dry.

    If you put in a dehumidifer you can set it for 40% or 50% RH. But as Walta points out the best solution is keeping the moisture out in the first place.

  5. walta100 | | #7

    I get nervous when well meaning people want to “improve” old buildings the old building survived because they found a balance the water in and out. The old walls leaked lots of air and heat. the heat evaporated the water and the air carried the water away. Well meaning people seal up the air leaks and insulate the keep the heat in now the water is stuck in the wall and things start to rot.

    I am not saying you can improve your house but I am saying you need to be very careful and find ways to keep the water out first and find a new balance point.

    A good first step is to check/improve gutters, down spouts and grading to get as much water away as possible.


  6. the74impala | | #8

    You say the air will come from above. How do you know where it will come from? How do you know you aren't just pulling in air from outdoors?

    1. Matttheram | | #9

      My cellar is pretty well sealed which is also part of the problem (no air movement) you enter via an oak door which is inside the house (with air gaps) down a small steep brick stair and into the area (fairly small) the cellar is brick construction walls and ceiling. So the only real air flow path is from the entrance door. Therefore my thoughts are if I used a negative pressure fan venting to the outside of the house it can create a low pressure (only slight but enough) and so air will be drawn into the cellar (air that is dryer than existing within there)

  7. user-1072251 | | #10

    I'v been in my house for fifty years. For much of that, the basement was either very damp or wet, with a dirt floor; basically unusable, until I added on a section with a lower floor (for better headroom); then, for several weeks a year, the lower part was under water. (The house is on a slope) I finally figured out what I needed to do: I excavated the lower section several inches, installed washed stone, and installed a daylighting drain through the bottom of the wall to daylight. It then stayed dry. Then I leveled out the dirt on the rest, installed 2" of polystyrene foam on the floor and extended it up a few inches at the edges to isolate the slab, covered the foam with a layer of heavy sign plastic (about 20 mil) with the seams taped. Then we added 2-3" of concrete, 2-4" of polysiocyanurate to the flat walls, spray foamed the antique stone walls and started inhabiting the space. We now have dry storage, and a wood shop. Tools do not rust. After coming close to completing the DER to my home, I removed the oil furnace and propane boiler (we now use minispilts). We have no heat source in the basement, but it has never gotten below 52 in midwinter, and is useable and comfortable year round. Right now, in Dec., it's 550 with a humidity level of 46%. It is doable, and very much worth it! We use essentially the same methods in our NZR homes, and the basements are dry and stay comfortable. A few months ago I (almost) completed my HRV installation (one more return to snake) and have a supply and return in the basement, so the air is clean also!

    1. Expert Member
      NICK KEENAN | | #11

      What you did sounds similar to what Joe Lstiburek advocates in this article:

      He also advocates inserting a capillary break between the sills and the top of the foundation. In my (limited) experience that isn't necessary if the foundation is stone, unlike concrete stone is not porous enough to move significant amounts of moisture.

  8. user-1072251 | | #12

    Yes; there is a good reason it sounds like Joe Lstiburek's advice. The only issue I have with the capillary break is that it involves jacking the house to create a space, and while I have done that in other cases, it's a lot of work which I was not ready to take on here. Eventually that part of the house will need to be jacked up and replaced by a new foundation and new sills, but that was not on the agenda at that time.

  9. Matttheram | | #13

    Thank you all for your replies. However I now understand there is a vast difference between the ideology of the US and UK.
    My house is over 250 yrs old and there is no way I am touching it will spray foam, plastic sheeting and nasty chemicals. That isn't the way it was built. I am trying to let nature take its cause and allow the walls to breathe and not add barriers that will speed up damage to the fabric. I am a member of English heritage and they try and use traditional methods when carrying out repairs. So I want to keep the fabric as is but add something that will improve the air changes within to reduce moisture and damp. Thanks again for your comments

    1. GBA Editor
      Martin Holladay | | #14

      Many of GBA's suggested measures are entirely consistent with historical preservation. Provide drainage, if possible, using perforated pipe (or clay pipe if you are a traditionalist) that leads to daylight. The drainage pipes should be surrounded by small stones. The drainage can be on the exterior or the interior, as you prefer.

      If your drain pipes can't lead to daylight, they should lead to a dry well.

      Adjust the exterior grade so that the grade slopes away from the building in all directions.

      These simple techniques -- drainage and grade adjustment -- are centuries old. The techniques I describe were well understood during the Roman Empire. They can make a huge difference.

      1. Matttheram | | #15

        Yes I agree with that part of your advice. Good advice. It's the other bits that's started my heart beating fast!!! Use of spray foams etc 🤣

  10. the74impala | | #16

    It sure seems like it would be easier to just set up an oscillating fan and a dehumidifier to run down there.

    1. Expert Member
      NICK KEENAN | | #17

      That's kind of what I was getting at in post #6.

  11. Expert Member
    NICK KEENAN | | #18

    I think it's helpful to think about where the moisture comes from.

    One source is liquid water. Usually rain, although it could be snow melt. It lands on the ground and works its way down the walls and into the cellar. In post #14 Martin lays out techniques for handling it, which mostly involve diverting it somewhere else. It's always a good idea to keep the liquid water away from your structure.

    Another source is capillary action, the tendency of liquid in a porous material to flow from areas of greater concentration to those of lesser concentration. Unlike liquid water, which follows gravity, capillary action moves in all directions; sometimes it's called "rising damp."

    A third source is ventilation. If the air being brought in has a higher dew point than the air in the cellar, bringing in fresh air means bringing in more moisture. In my climate -- northeastern US -- exterior air will have a dew point in the 60's or 70's for half the year, and interior conditioned air will normally have a dew point in the 50's.

    There are two approaches to reducing humidity. One is to remove as much moisture as you can, the other is to keep out as much moisture as you can. For removing moisture, a dehumidifier generally works better than ventilation, because the effectiveness of ventilation is limited by the moisture content of the incoming air, which is often too high to be useful. If you're going to dehumidify you want to seal off air leakage as much as possible, because that exterior air often has more moisture than your dehumidified air.

    But against capillary action even a dehumidifier has limited effectiveness. To stop the capillary action you'd have to dry out all of the soil around your house, which is like warming all the air around your house with a space heater, for practical purposes you can view the soil around your house as an infinite reservoir of dampness. In fact, the more you dry the interior of the cellar, the stronger the capillary action becomes.

    Just as it's more effective to insulate the shell of a house than it is to try and warm the air around it, it's more effective to put a vapor barrier between the ground and a house to block capillary action than to try and dry out the ground. Foam insulation and plastic sheeting are commonly used, but they're not the only choices. If you have a concrete floor painting it is effective. If your foundation is stone the stone itself is pretty impervious; pointing up the mortar and then painting the mortar with a vapor barrier would probably be effective.

    1. Matttheram | | #19

      Thankyou for providing a detailed response 👍 I will add a couple more pics. Apologies for the quality I have isolated my lighting temporarily whilst I decide on next action

  12. Matttheram | | #20

    Pic 2

  13. Matttheram | | #21

    Thankyou for providing a detailed response 👍 I will add a couple more pics. Apologies for the quality I have isolated my lighting temporarily whilst I decide on next action

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |