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Community and Q&A

Insulating/ventilating a crawl space

jgold723 | Posted in General Questions on

We are in southern Maine.

The previous owners of my house (which we purchased in February) built a 22 x 22 addition on sonotubes. The floor is about 18 inches over dirt. Only access is through a basement window in the original portion of the house.

I finally got brave enough today to pry the plywood off the window and take a peek in.

There does not appear to be any vapor barrier on either the soil (which appears dry) or the underside of the joists (2 x 8 pressure treated). There is 6 – 8 inches of fiberglass batt between the joists, although it is falling down in some places.

We have not experienced any problems (the space appears dry, no smell of mold and as far as I can tell, no critters), but I’m thinking this is not the best situation from either a construction or energy efficiency standpoint.

What would be a reasonable way to keep the space dry and also to insulate the living space above? I say reasonable because our budget is limited (thus no spray foam) and we don’t intend to excavate and pour a frost wall, unless we have to.

And, should I introduce some kind of ventilation? The walls of the addition extend down to ground level and a little below.



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  1. GBA Editor
    Martin Holladay | | #1

    1. There is no need for ventilation.

    2. Install a 6-mil (or heavier) layer of polyethylene on the soil, held in place with bricks or rocks.

    3. Install a continuous layer of foil-faced rigid polyisocyanurate insulation, at least 2 inches thick, on the underside of the floor joists. Tape the seams of the polyiso sheets, and seal the perimeter with caulk between the polyiso and the joists.

  2. user-1091831 | | #2

    Just curious. Since the walls extend below grade, why not insulate the foundation walls, since it is less surface area? Or Is it more cost-effective to beef up the existing insulation already existing in the floor, rather than have insulation in the walls and floor?

  3. jgold723 | | #3

    That's a great question. I assume the underside of the floor needs to be done because the soil will transmit cold. But Insulating the walls would be much easier than crawling around on my back trying to nail insulation a few inches over my face.

  4. dickrussell | | #4

    John, describe the construction of those addition walls that "extend down to ground level and a little below." I hope they aren't wood or something that has no business being in the ground.

  5. jgold723 | | #5

    Oh, they are... unfortunately. From what I can observe from the single access point, it does appear that they are made of 2x6 pressure treated and then faced on the outside with the same cedar clapboard that's on the rest of the addition.

  6. GBA Editor
    Martin Holladay | | #6

    The cedar clapboard (and the sheathing it is nailed to) will eventually rot. It doesn't sound like the walls are anything close to airtight, so if you want to improve the insulation, it needs to go on the underside of the floor joists, not the rickety wooden walls.

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