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Basement humidity

user-6135630 | Posted in General Questions on

I’m trying to zero in on a basement humidity problem I discovered this weekend. I recently built a house in North Central Massachusetts, Zone 5 and am I seeing signs of excessive humidity in my unfinished basement with no obvious sources of moisture. While poking around I noticed moisture on the inside of the poly that the insulators had installed on the “warm side” of the fiberglass batt insulation (The rest of the house is dense pack cellulose with now vapor retarder). At first I though I might have had a leak but then ruled that out as the moisture was consistent along the entire 30′ walk out section. I then notice a couple small puddles had not dried up by the filter on the well line I had changed a week ago. The well line and water softener tank have been sweating aggressively as well.

The foundation has a proper asphalt damproofing , there is an interior and exterior footing drain, a 6mm poly vapor barrier with taped seams, the basement has 30′ of exposed grade so ground water is not and issue and the gutter down spouts are tied into drains. The rim joist has 2″ of closed cell spray foam and all framing joints have received spray foam caulking. Upstairs the house has has been averaging RH of 45-50% which is controlled by the A/C and HRV. The house is considered fairly tight with less than 2 ACH.

Do you think I have a ventilation issue or moisture issue?

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  1. user-36575 | | #1

    The problem is likely the "diaper".
    The wet diaper approach (poly/open insulation/concrete) often/usually results in a permanently soggy diaper condition. This "design" violates two rules. One is don't put fiberous insulation directly against concrete, because this makes the concrete cool, while still allowing moisture laden air to penetrate through the insulation. It hits the cold concrete wall, and when temperatures are below the dew point, the moisture condenses. This problem is exacerbated because a second rule has also been broken, which is using poly to trap the water there.
    Water also comes thru the concrete from the exterior, in spite of damp proofing. The poly is preventing it from dissipating.

    I suspect a third problem might be that new building materials also contain a LOT of moisture, including the concrete, and it takes a while (SWAG, a year, depending on conditions) to get rid of this moisture. Even in well-built houses, portable dehumidifiers are often recommended to help dry out the house initially.

    GBA has articles on basement insulation. The diaper approach is never recommended.

  2. Expert Member
    Dana Dorsett | | #2

    Foundations can't dry toward the below-grade exterior. Concrete wicks moisture- even if the exterior of the foundation walls are perfectly damp-proofed, moisture will wick up from the footing. To make it even more fun, fiberglass in contact with damp concrete will wick moisture.

    Bottom line: A fiber insulated studwall up against a foundation walls needs at a minimum a capillary break & vapor retarder between the concrete & fiberglass, and the ability to dry toward the interior. That means no interior side vapor barrier. Ideally it would also have sufficient air-impermeable insulation between the fiberglass and foundation for wintertime dew point control on the above-grade portion at the boundary between the fiberglass & air impermeable insulation. In your location would be ~R5 of something like rigid foam insulation for ~R13 fiberglass. Since rigid foam is also a pretty good capillary break and is fairly vapor retardent, that allows you to skip the interior side vapor barrier.

    Humidity issues in the basement interior are almost certainly from the ventilation air's high dew point. Up until this past weekend there was a full week where outdoor dew points in your area hovered in the 68-75F range. Unless you have more than inch of foam under the basement slab the temperature of your basement slab is well below that temperature, and so is your incoming water line. Any outdoor air that gets into the basement will condense on/near the colder surfaces, and unless you have mechanical dehumidifcation it won't dry quickly (or at all.)

    The reported 45-50% RH upstairs is meaningless without the upstairs temperature (to which it is relative.) The dew point of 80F / 45% RH is 56F, which is probably about the same temperature of an uninsulated basement slab this time of year in your location. The dew point of 75F / 50% RH air is 55F, not much different. If that is the same body of air in the basement, puddles would not dry from a 55F slab.

    If you're air conditioning to 70F / 50% RH the dew point would be 50F, and if upstairs air was circulated into the basement 55F-56F puddles on the floor would be able to dry. If the basement air is somewhat isolated from upstairs air, you may need to run a dehumidifier to bring the dew point of the basement air below the slab temperature.

  3. GBA Editor
    Martin Holladay | | #3

    Andrew and Dana are correct. You made two big mistakes:

    1. You insulated the interior side of your basement wall with fiberglass insulation, which is a no-no. Instead, you should have used rigid foam or closed-cell spray foam installed against the concrete.

    2. You installed an interior polyethylene vapor barrier, which trapped the moisture. You never want to include polyethylene on the interior of a basement wall.

    Here is a link to an article that explains what you should have done: "How to Insulate a Basement Wall."

    By the way, the puddles near the water meter indicate that your incoming water temperature is cold enough to encourage condensation on the outside of your water pipes. The solution is to install pipe insulation on your cold water lines. (You may still get a very small amount of insulation on the exterior of your water filter -- but insulating your water lines will help.)

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