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Community and Q&A

Crawl Space Retrofit

Erica Downs | Posted in Green Building Techniques on

We just moved into our house in the Boston area a month ago, and we’re working fast to seal and insulate what we can before it gets REALLY cold.

I would like to air seal and insulate an existing crawl space, but I am wondering what the best retrofit technique is, and if seasonal timing makes a difference.

The house is 1950s, and there is a newer addition with a crawl space. The crawl space is entirely above-grade, has a dirt floor and CMU walls, surrounds the fireplace chimney (yes, the addition was build around the chimney), and is adjacent to a deeper basement, tho not directly connected. There is a plywood “door” (more like a hatch cover) that opens into the space, and one vent on the opposite wall. The floor above has some fiberglass insulation, held up by some kind of pressboard (that is falling down), and sort-of concealing a steam pipe. The dirt floor is dry and the walls show no sign of flooding.

1) I plan to add 6-mil poly to the floor, and extend it up the wall. We also plan to seal the existing vent. What materials are typically used to seal a vent?
2) I would like to add rigid foam to the interior walls as well. However, since this will impact the drying potential of the walls, does it matter if I do this now (winter) or, should I wait until the weather is warmer?
2) Also, any thoughts on what should be done around the brick chimney? Should it be treated any differently than the CMU walls? We may remove the fireplace/chimney altogether, but that likely won’t happen for a couple more years.
3) Would you recommend ground stakes to keep the poly in place? Or is this unnecessary?
4) How would you treat the plywood door? I think we need to keep it for access, since there is plumbing and wiring that runs through the space. Maybe cover it with a rigid foam panel?

Thanks for any help!
Erica

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Erica,
    Q. "What materials are typically used to seal a vent?"

    A. A concrete block or bricks would work. Or, if you prefer, you could cover it with rigid foam and plywood.

    Q. "I would like to add rigid foam to the interior walls as well. However, since this will impact the drying potential of the walls, does it matter if I do this now (winter) or, should I wait until the weather is warmer?"

    A. I don't think it matters when you do this work.

    Q. "Any thoughts on what should be done around the brick chimney?"

    A. Is the brick chimney surrounded on one side by the basement and the other side by the crawl space? Or is the brick chimney part of the foundation (and exposed to exterior temperatures)?

    Q. "Would you recommend ground stakes to keep the poly in place?"

    A. No. If you want, you can use a few scattered bricks or stones to hold down the poly. Or you can cover the poly with a layer of pea stone.

    Q. "How would you treat the plywood door?"

    A. The access door should be weatherstripped and should not be leaky. Many plywood doors warp easily. You can order a custom-made pre-hung insulated steel entry door in any size, if you want. Or, if the plywood door is sturdy, you can weatherstrip the existing door and cover the interior of the door with rigid foam.

  2. Erica | | #2

    Thank you Martin!
    To answer your question, one side of the chimney faces the basement, but you cannot actually see the brick from the basement -- the original CMU foundation wall is between the two. The other 3 sides were originally facing the exterior, but are now enclosed by the addtion's crawl space.
    Erica

  3. GBA Editor
    Martin Holladay | | #3

    Erica,
    You don't need to insulate the base of the chimney.

  4. Erica | | #4

    Great -- thank you!

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