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Community and Q&A

Basement floor insulation

fKWgHHCRg4 | Posted in GBA Pro Help on

A section of my house still has an earth floor in a space that’s approximately 4 1/2 to 5 feet tall. I had put a sheet of heavy plastic over the earth and then had some crusher dust added to keep it in place. Needless to say, this did not stop the moisture from coming up through the ground. The space is always damp and consequently the rooms above it always have a damp smell in the warmer months and a very cold floor in the winter.

I did put 1-inch white styrofoam on the outside walls but that made no discernable difference. Is there any way to insulate the floor without pouring concrete?

And do you have any suggestion as to what I could do to insulate the ceiling in this space to try and make the floors above warmer in winter?

I’d appreciate any advice. Gwen

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  1. GBA Editor
    Martin Holladay | | #1

    In order, here are the steps necessary to address a damp crawl space:
    1. Verify that the grading around the perimeter of the house slopes away from the foundation, not toward it. Make corrections as needed.

    2. If the house is on a hill, create a swale on the uphill side of the house to divert water away from the foundation.

    3. Install roof gutters connected to a conductor pipe that carries the roof water away from the foundation.

    4. If the crawl space is still damp, it may be necessary to install a French drain around the inside perimeter of the crawl space. This trench should have a perforated drain pipe leading to a sump. The trench should be filled with clean crushed stone.

    5. The sump needs an airtight lid. Install a sump pump connected to a pipe that conveys the water away from the foundation or (where permitted by local regulations) into your sewer pipe.

    6. Install a heavy-duty plastic sheet on the crawl space floor, with taped seams. Secure the edges of the plastic to the crawl space walls with a pressure-treated batten secured to the foundation wall with masonry anchors.

    7. Perform air sealing work at the rim joist. Seal all crawl space vents.

    8. Insulate the walls with several inches of XPS, EPS, polyiso, or closed-cell spray polyurethane foam. Protect the foam with a thermal barrier as required by your local building code.

  2. Riversong | | #2


    You don't indicate what kind of foundation walls you have. If the floor is dirt, I suspect that the walls are dry-stacked stone and leak like a sieve.

    You can follow all of Martin's recommendations to try, possibly in vain, to make such a cellar dry - or you can vent the space and insulate and seal the floor above, providing the floor framing is not covered with wires and pipes and there is no ductwork in the cellar.

    The best way to keep the space above warm and dry is to install 2" of foil-faced polyisocyanurate rigid foam board under the floor joists. This will serve as both a conductive insulation layer and will radiantly isolate the floor from the cold ground in addition to acting as an effective vapor barrier if all joints are taped and edges sealed with canned foam.

  3. fKWgHHCRg4 | | #3

    Thanks so much for the advice. I realize that I probably did not provide enough information in my initial question. I don't believe there is any water getting into the basement from the rain. I did have the original concrete block foundation removed about eight years ago and replaced with solid concrete foundation walls.

    My house had had numerous add-ons over the years and the original portion has a concrete floor in the basement, where the furnace and hot water tank are located. I recently added a vent in the basement door which is in the area of the earth floor, and also put a vent in the one small window that is in the basement in the furnace area in order to hopefully draw some air through the space. I am not sure if these vents should be open all year round, or closed in the winter months.

    I am wondering if there would be any benefit to putting rigid styrofoam on top of the earth floor and maybe something to hold that in place as a way of keeping the moisture from coming up from the ground. The space is not used for any purpose, so there would be no need to walk on it, except for a path directly through the doorway and into the furnace area.

    I had thought of insulating the ceiling above the earth floor but have been told that if I were to put batts of fibreglass insulation on the ceiling and then use a vapour barrier to hold it in place that it would trap moisture and the floor above may rot. Someone else advised that I should do the batt insulation and use Tyvek or house wrap that is not moisure-proof to hold the insulation in place and possibly cut down on the coldness.

    Maybe Robert's idea of the foil-faced product is the best plan. I assume this would not require any vapour barrier or building paper. I am determined to have a warm kitchen floor this winter, and do appreciate the advice I have received. Gwen

  4. Riversong | | #4


    You still left out the most important piece of information: where are you, what is your climate?

    But a couple of things: You had originally mentioned "white styrofoam". White foam board is EPS, not XPS which is what the proprietary material Styrofoam is made from. And EPS is more water vapor permeable than XPS, but neither is a true vapor barrier. And most heat loss from a basement is through the above ground walls, not into the deeper earth, so insulating the earthen floor will do little to keep the house warm. A durable plastic film, like Tu-Tuf, on the ground and well sealed with caulk to the bottoms of the walls will take care of ground moisture, but you'll have to put something on top in the walkway, such as sand or bricks or paving blocks.

    If the furnace is in the basement, then you don't want to leave the vents open in the winter, and often humid summer air will cause condensation in a cold basement. So open vents may be counterproductive unless it's more humid in the basement than outdoors in the summer.

    Fiberglass batts is the worst thing to put into your floor framing in a damp basement, and neither a poly vapor barrier nor housewrap would be wise below the insulation since they both trap moisture - the former traps all moisture and the latter traps liquid moisture and lets water vapor pass through.

    Short of damproofing and insulating the entire basement, including all walls, sills and rim joists, the simplest and most effective solution to both dampness and cold is to use 2" foil-faced polyisocyanurate insulation board (Thermax, High-R, etc) under the floor joists, well sealed at all joints and edges. The foil is a perfect vapor barrier, an excellent barrier to radiant flow between the floor and the ground below, and high-R insulation.

    In any case, sealing the earthen floor is essential. You could also put down a vapor barrier and pour a thin 2" layer of concrete to make it durable and walkable.

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