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Buy a dehumidifier or create a register in the return and supply plenums in a damp basement?

agurkas | Posted in General Questions on

I have a 1200 sq ft basement. 1/3 of it is crawlspace under an addition to the house with two former basement windows connecting it to the rest of the full height basement. I have insulated the walls in the basement and crawlspace is covered all the way to rim joist with 20mil reinforced poly to reduce radon.

Yesterday I was inspecting the crawlspace and found dew on poly in the lowest parts of the crawlspace. I do monitor moisture in both crawlspace and basement and we are lately at 78%.

I wonder if I should create a register in the return and supply plenums. Or do I just pay up for a 50-pint dehumidifier?

P.S. I am in Northeast

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  1. GBA Editor
    Martin Holladay | | #1

    A dehumidifier will do a better job of dehumidifying the air than simply installing a supply register and a return-air grille.

    But a dehumidifier should be your last resort. Here is a link to an article that may help you: Fixing a Wet Basement.

  2. agurkas | | #2


    I have read and re-read that article on numerous occasions. Actually in the last year I pretty much did everything mentioned there. I suspect my source of moisture is about 500 or so sq. ft. of basement that has old 1950s concrete floor, which may not look cracked, but it does seem to be darker gray now than it was in the winter. I assume it is were a lot of moisture comes in via evaporation. The other 700 sq. ft or so of the basement is 70s concrete floor with paint on it, but I don't think it is epoxy, so likely some moisture evaporates in through there.

  3. charlie_sullivan | | #3

    A damp concrete floor can be either because of moisture wicking up through it or moisture condensing out of the air onto it. If humidity is 78% and the floor is colder than the air, that makes condensation sound likely. To confirm, tape a square foot piece of plastic on the concrete, and see which side gets wet.

    Actually, I think you already did that on a larger scale with the plastic you mentioned--which side of that was wet?

    If moisture is condensing on the floor, it's coming in somewhere else, most likely in humid air from outside or from the house. I'm not sure where you are in the northeast or whether you are running air conditioning. But in a hot humid climate, even if you have perfect air sealing and no moisture coming through the slab, you'll get high basement humidity just because the low temperature increases the relative humidity. The good news is that, in that case, the dehumidifier only has to run a little to keep it dry once it's dried out. Some dehumidifiers run their fans continuously when they are set to operate on a humidistat. That can waste a lot of energy. So if you go that route, make sure it's one that shuts off completely when the humidity is below the set point.

  4. agurkas | | #4

    I am in Boston area.

    Condensation I found was on TOP of the plastic sheathing in the crawlspace. 1st I thought it was mildew, but on closer inspection it was just water.

    House is definitely leaky, because walls don't have insulation in them yet. Zero. Just plaster, 2X4 studs, T&G, and then clapboard siding. So probably that contributes. AC only runs about 2-3 hours per day in my house, since we have lots of trees around and house doesn't really heat up. Also all HVAC equipment is new, so it cools everything in a jiffy.

    We will be doing insulation work, but not until fall. My concern is all that moisture could cause mold. It really smells musty in the basement.

  5. Expert Member
    Dana Dorsett | | #5

    Dew points have been running in the low 60s recently in Boston (it's actually been drier than average year dew points most of the summer to date). So, sure, with ground temps in the 50-53F range the temp at the slab is below the dew point of your ventilation air, and a ground vapor retarder would have moisture on the top.

    On a concrete slab the moisture would be adsorbed into the concrete, and would not show up as liquid water until the concrete was saturated.

    I have about 1500' of basement slab and 150' of dirt-floor crawlspace (with a poly vapor barrier) in my home in Worcester, and the water table is rarely more than a foot below the slab (and sometimes above, in spring, ergo 4 sump pumps.) A single 70 pint dehumidifier set up to drain into a sump is more than sufficient to keep it dry. I monitor the RH with a cheap Acu-Rite checking it whenever I'm down there, and as long as it's 60% or lower at whatever temp the basement happens to be, I leave the dehumidifier off, but when it rises I run it. The dehumidistats on room dehumidiers are the opposite of precision instruments (crummier than the AcuRite), so when the RH reads below 60% I just turn it off. If you don't have much outdoor air leakage into the basement the duty cycle of the dehumidifier will be quite low.

  6. agurkas | | #6

    So from what I am hearing, AC registers aren't really an option. I should spend couple hundred $ on a dehumidifier?

  7. Expert Member
    Dana Dorsett | | #7

    If you run AC into the crawlspace it'll lower the temperature, then on days when the cooling loads upstairs are low any air infiltration is even MORE likely to condense.

    Unless you air condition the house to a dew point under 50F (about 40% RH @ 75F) there is going to be some condensation risk at the ground vapor retarder even if ventilating it with conditioned space air (and not a cooling supply duct.)

    Sometimes the direct approach is the right approach. A dehumidifier converts that latent cooling load (humidity) into a sensible cooling load (heat), so it'll warm UP that space a few degrees, lowering the condensation risk even after you turn it off.

    If you insulate the exterior walls of the crawl space, that too will raise the average temp a bit. If you insulate the floor and pour a thin rat-slab of concrete over it it'll be warmer still. A primary rationale for insulating basement slabs (and not just walls) in a Boston climate is to keep the basement warmer (= drier) in summer, thus limiting the "musty basement" risk. The wintertime heat load & energy lost to an uninsulated basement slab while measurable, is pretty small.

  8. agurkas | | #8

    That all sounds like $$$$$ compared to $200 for dehumidifier.
    I looked at data log for my house and my temps are at a 76F mean and 60% RH.

  9. Expert Member
    Dana Dorsett | | #9

    The dew point of 76F / 60% RH air is 61F, so ventilating a crawlspace with a 50-55F vapor barrier floor with 61F dew point air will indeed produce condensation on top of the vapor barrier. You need a dehumidifier (like almost every other house in MA with an uninsulated basement slab.)

    If the walls of the crawl space are not insulated & air sealed all the way up to the top of the band joist it's well worth it on an energy-use basis. Code-min foundation insulation in MA is now R15 continuous insulation or R13 studwall + R5 continuous, but even R10 is a HUGE improvement.

    There are multiple vendors in MA selling used-once &/or factory-seconds foam insulation that could make that pretty cheap & easy (cheaper than R15 + 5), assuming you can get decent sized sheets into the crawl space. It also takes remarkably little insulation to fix the cold floor problem. An inch of EPS would do it. If you go that route used polyiso (fiber or foil faced) is fine on the foundation walls, but EPS or XPS would be better for anything horizontal on top of the vapor barrier.

    I did my crawlspace with somebody's leftover XPS from an unvented attic project that were already the right height for the crawlspace (a craigslist find), and the rest of the foundation was done with 3" fiber faced reclaimed roofing polyiso at $20 per 4'x8' sheet. (I've seen it since both cheaper, and more expensive from Nationwide Foam in Framingham, and Green Insulation Group in Worcester, the two largest reclaimed foam vendors in the region (but there are a handful of smaller operators too.) I'll eventually get around to insulating the crawlspace floor, but probably not the basement slab (due to headroom issues.)

    But whether you insulate crawlspace floor or not you'll still probably need the dehumidifier unless you also insulate the basement slab.

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