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Reducing Humidity from Basement Floor

D107 Member Posts: 1,832
Back in the mid-50s in this 1924 house when sewer lines were put in they buried the stack in the basement floor, ran it to a cleanout and then to the house trap/cleanout. The attached photo shows the dug-up and then sloppily re-cemented floor area where the waste line from the slop sink ran to meet the stack. (mostly under the work bench, maybe 30 sq ft, then another twenty from the first cleanout to the house trap.) That twenty is now hidden under the stairway closet. For years we've had to have a de-humidifier on 24-7 in summer which costs alot in electric charges. I always thought, because of some wall effluorescence that that north wall was the source. But I recently noticed how that floor cement never quite gets dry in summer, and wondered if I should find some way t seal this with paint, etc. If I have to re-cement with better, more waterproof material, it might be wise to re-do the underground slop sink waste line in case it's leaking. Any quick fixes for this? Note I don't see this problem in any other basement wall or floor area.


    HVACNUT Member Posts: 5,034
    CI main waste? Maybe there's a section not fully seated in the hub and it leaking. If that's the only area its damp, there's someone, I mean something down there causing it.
  • D107
    D107 Member Posts: 1,832
    From another site:

    "The source of the moisture on the concrete in summer is most likely to be coming from the ventilation air, not the ground. Air sealing the basement from outdoor ventilation air will do more for this problem than any sealer. (It doesn't hurt to apply a sealer though.)

    A skim coat of Xypex® or a silane/siloxane based concrete sealer would reduce capillary draw from any ground moisture, but won't do anything for adsorbed moisture from the air."

    Then I answered:
    "Thanks there are gaps in the rim joist in this mechanical room; I could fill with home depot foam but I heard that termites like to nest in stuff. I guess I had always assumed water was seeping in through the walls, hence the efflorescence before I had the wall re-pointed some years back."

    "There may indeed be moisture seeping through the walls, but that's a year-round phenomenon. If the concrete only looks actively wet and the dehumidifier duty cycle is only high in summer, it's almost certainly the high humidity of summertime air.

    If you don't want to seal the gaps in the band joists or between the sill and foundation with can-foam, use polyurethane caulk (up to about a half-inch). ALL air leaks into the basement matter, not just the ones in the mechanical room. The single largest air leak in most previously untreated homes is the band joist & foundation sill leakage, which is usually several times that of all the window & door crackage in the house."

  • hot_rod
    hot_rod Member Posts: 19,287
    If it stays wet in just a small area, I'd suspect a leak or water table at that level.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    JUGHNE Member Posts: 10,581
    If your worst damp floor is localized in that area, I would tend to thing UG drain leaking.
    For $350 or so you can buy a FLIR handheld camera/viewer. I have one and have found it one of the most useful tools there is, almost outdoing the one-handed 18 volt Hacksall .
    It works better than the "X-ray" glasses I always wanted to order from those boys magazines >:)

    IIWM, I would run as much super hot water down the slop sink as possible, with the FLIR gun you could probably see the floor temp rising. If you had a large glowing circle that is probably where the drain is leaking. That 350 investment is probably less than a sewer camera scope job. Also I read that you have HW heat and there are a lot of things you can sort out with that FLIR.
    Anyone that run that concrete probably only poured 2" or so.

    Even breaking it open to see where the dirt was the most wet would gain you something. You could patch it better than it is now.
  • TheKeymaster
    TheKeymaster Member Posts: 36
    I agree you could just bust that up to see. If you just wanted to cut a small section to break up I've done this. I don't have a concrete cut saw but I'll keep my eye open at garage sales and buy a cheap curricular saw and put a masonry blade on it. It pretty much kills the saw but if I paid 5 bucks for it then it's cheaper than renting one. I do the same with drills for mixing cement and things. I buy cheap used ones cause they just burn up and I don't do it enough to buy a real mixer.

    You could always rent a saw and cut it all out in straight lines and repair it right. If it's wet under the new patch that was put in will break up like peanut brittle. On a crappy job like that they never fill up all the space from washed out soil. You could probably hit it with a framing hammer and it would shatter.

    I have a scope that hooks up to my phone that I got off I think amazon. It was like 30 bucks. It's 10 meters long and has adjustable LEDs on it. It's definitely not the same quality as what a master plumber has but the picture is pretty damn good for 30 bucks. I've used that to see where gutters downspouts go, to see whats in a wall with only making a small hole or sticking in a duct. I got a lot of use out of it for the price. Plus it's cool to look in your mouth and nose with. haha
  • Zman
    Zman Member Posts: 7,447
    Try taping a 2'x2' square of clear poly to the floor. If moisture forms on the bottom, it is coming from from the ground.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • unclejohn
    unclejohn Member Posts: 1,830
    edited June 2018
    Zman always comes up with amazingly simple solutions. Hence his tag line.
  • Leonard
    Leonard Member Posts: 903
    edited July 2018
    Had to smash spot in cement floor to put a steel pipe handrail in once. As I hammered thru floor I notice cement was funny soft. Guessing water table was high and it never cured right in the first place.

    In another location in same house we just punched a hole in sewer pipe so underground water would drain. City sewer, so unlimited ability to carry away water
  • D107
    D107 Member Posts: 1,832
    Thanks for the suggestions; once I put up a sheetrock wall on the other side of that boiler room I'll move away the workbench and get a close look at that area and see what can be done. Meanwhile I've added a few fans to see if they can keep the humidity level under 60% instead of using the dehumidifier--uses more KWH--which keeps it 45-50%.
  • FranklinD
    FranklinD Member Posts: 399
    In my neck of the woods, every (old) house has a dehumidifier running 24/7 in the basement. Just a fact of life around here. Right now on the first floor it is 74% RH at around 80°...it’s a tad “moist”. The basement, though, is at a constant 65° in the summer and without the dehumidifier running it would be insufferably damp - and yes, my slab will “sweat” as well. We just live with the electric costs in exchange for a drier basement.
    Ford Master Technician, "Tinkerer of Terror"
    Police & Fire Equipment Lead Mechanic, NW WI
    Lover of Old Homes & Gravity Hot Water Systems
  • D107
    D107 Member Posts: 1,832
    Right now with two fans going I'm at 55% RH 79ºF. We'll see what it's like tomorrow when it'll be 100º outside.
  • jumper
    jumper Member Posts: 2,068
    FranklinD said:

    In my neck of the woods, every (old) house has a dehumidifier running 24/7 in the basement. Just a fact of life around here. Right now on the first floor it is 74% RH at around 80°...it’s a tad “moist”. The basement, though, is at a constant 65° in the summer and without the dehumidifier running it would be insufferably damp - and yes, my slab will “sweat” as well. We just live with the electric costs in exchange for a drier basement.

    Smaller more energy efficient dehumidifier?

  • BillW
    BillW Member Posts: 198
    I agree with the sheet of poly film on the floor. Do you have "floating slab"; that is one with a 2" or so gap between it and the basement wall? If so, that is a prime source of moisture infiltration from the soil. Have you recently cleaned your gutters and downspouts and do the discharges from the downspouts put the rainwater at least 6 feet from the foundation, and does the rainwater drain away from your foundation? I have seen many basement water issues caused by clogged gutters. I also have had decent luck with UGL Dri-Lok paint in stopping wet spots. Try drilling a hole in the lowest course of the wet cinder blocks, on either side of the center. If you get water flowing out, you need to improve your drainage. You also may need to call a pro to fix the water issue.
  • D107
    D107 Member Posts: 1,832
    @BillW polyfilm--yes (I've been told if it's wet on the down side that's water seepage and if wet on side facing inside, it's high humidity in air.) No floating slab as far as I can see. Gutters clean, downspout discharges probably 4 ft from foundation; had it further away before didn't notice difference. Note the only place it sometimes looks a little moist is on the floor re: the attached photo where they'd done a poor job of re-cementing years ago.

    No water coming in. Some efflorescence on a few spots on the wall--could be old not new. Once I clear all the stuff around there away I'll see better what's what, maybe have to repoint some of the brickwork. Without dehumidifier, on muggy days I'm sure RH level would be at least 65% in basement. I'm pretty lucky here it's not that severe but if I can lower that by 15-20% I can save alot on electricity. As it is I'm on the lowest speed of a three-speed dehumidifier, and I have that on a timer so it's not on all the time. keeps RH 45-50. Air sealing also on my list, though leery of spray foam due to termites being attracted to it.