GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Crawlspace insulation

oldhouse7 | Posted in Energy Efficiency and Durability on

Hi everyone – Looking for some clarification around how to best re-insulate a crawlspace that has falling fiberglass batts.

The crawlspace is under an enclosed 3 season porch (no heat or AC), the crawlspace has insulated steam pipe and h/c water pipes running through it. The vents were previously covered with plywood (but not air sealed). The house also had termites, so I need to be able to continue annual termite inspections.

Am I correct to use 6mil poly on the floor as a vapor barrier and rigid foam insulation on the walls, while leaving the rim joist uninsulated to allow for termite inspection? When coming up the wall, should the poly be attached over the rigid foam, or between the foundation/ exterior walls and the rigid foam?

Lastly, the exterior of the crawlspace appears to be stucco applied over some sort of chicken wire/board. Looking from inside the crawlspace, exterior wall has a footing and then the chicken wire/board is recessed about an inch – would I use the rigid foam on the footing and the wall, and tape the seam so it’s sealed?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    Your location/climate-zone is... ?

    Is the crawl space accessible from a hatch to a basement, the porch floor or some other location inside of conditioned space, or is it exterior-access only?

  2. oldhouse7 | | #2

    Interior access is through a hopper style window in the basement, exterior access is through a hopper window sized opening (covered w/ plywood.

  3. GBA Editor
    Martin Holladay | | #3

    Your description is unclear. Does this crawl space have conventional poured concrete walls? CMU (concrete block) walls? Or some type of wood-framed walls with exterior stucco?

    If the crawl space is built conventionally, with concrete or CMU walls, you would want to insulate it according to the recommendations in this article: Building an Unvented Crawl Space.

    The termite inspection strip that needs to remain uninsulated is the top three inches of the concrete or CMU wall, not the rim joist area.

    If you are installing polyethylene on the floor, and you also want to extend the polyethylene up the wall, the polyethylene is usually installed first, and is later covered by the rigid foam that is attached to the wall.

    If your crawl space walls are wood-framed walls, you have several issues to address (mostly concerning the durability of this detail).

  4. GBA Editor
    Martin Holladay | | #4

    1. You didn't answer Dana Dorsett's question: Where is this house located?

    2. It looks like your crawl space walls lack a masonry or concrete foundation. In other words, the studs extend down to dirt. This isn't a good detail, because any studs or plates in contact with dirt will eventually rot. How far down do the studs extend? Are the bottoms of the studs rotten yet?

    -- Martin Holladay

  5. oldhouse7 | | #5

    Crawlspace has a dirt floor, the exterior facing walls are metal lathe on the inside, stucco on the outside. The basement foundation walls are CMU, with a stucco finish inside the crawlspace. Pics below. There are two hopper windows into the crawlspace from the basement.

    Water/Steam pipes run through the crawlspace. The space above is unconditioned, though it gets some A/C in summer if we leave the doors open to the porch. No heat in winter though.

  6. oldhouse7 | | #6

    House is located in northern NJ.

    I'll need to crawl in to verify, but I believe the studs only extend down to the wood sill plate, which rests ontop of the dirt. Not sure if the sill is rotted.

  7. GBA Editor
    Martin Holladay | | #7

    Eventually, the lowest wooden components of these wood-framed walls are going to rot (if they haven't started to rot already). You might want to replace these walls with CMU (concrete block) walls before you go to the trouble of insulating them.

    -- Martin Holladay

  8. oldhouse7 | | #8

    I believe the walls are only providing structural support to the exterior wall - not the porch above - as there are concrete piers supporting the porch.

    Is there a simple/cheaper alternative to replace the sill only, rather than create new CMU walls?

  9. GBA Editor
    Martin Holladay | | #9

    Of course there are cheaper solutions.

    You can do nothing, and cross your fingers.

    You can excavate down to the bottom plates, and see if the bottom plates are rotten. If the bottom plates are rotten, you can try to replace the bottom plates with new pressure-treated plates.

    -- Martin Holladay

  10. oldhouse7 | | #10

    Thanks for the input.

    Supposing I replace w/ pressure treated plates - Does the fact that the porch above the crawlspace in unconditioned have any bearing on encapsulation/insulation approach?

    Based on the article you linked, it seems with the exterior facing metal lathe/wood framed walls I may need to do closed cell insulation and install an exterior vent while keeping the hopper windows into the basement open to pull air from.

  11. GBA Editor
    Martin Holladay | | #11

    If the room above has no heat, it's hard for me to understand why their are steam pipes and hot water pipes in the crawl space underneath the room.

    It's up to you to decide whether you want to insulate the crawl space walls. Perhaps there is no need to do so -- especially if any adjacent crawl spaces or basements under the conditioned space of your home can be sealed off from this crawl space.

    The presence of the steam pipes and hot water pipes complicates things, however. Considering the plumbing, I would probably advise insulating the crawl space walls.

    -- Martin Holladay

  12. oldhouse7 | | #12

    Agreed, no idea why they ran steam and water pipes through an uninsulated space. Alas, it's my problem now.

    If I chose not to insulate it, I imagine I would still want to remove the fiberglass batts and cover the floor w/ poly? For the floor, it would seem poly could only go up to the sill plate due to the metal lathe.

    If I do go the insulation route, is DIYing rigid foam to the metal lathe an option or is closed cell foam and an exhaust fan is the only option?

    Ideally, I could DIY rigid foam on the exterior facing walls

  13. GBA Editor
    Martin Holladay | | #13

    Q. "If I chose not to insulate it, I imagine I would still want to remove the fiberglass batts and cover the floor w/ poly?"

    A. Yes.

    Q. "For the floor, it would seem poly could only go up to the sill plate due to the metal lath."

    A. That's OK.

    Q. "If I do go the insulation route, is DIYing rigid foam to the metal lathe an option?"

    A. Sure. Better yet, run the rigid foam as a continuous layer on the interior side of the studs and tape the seams of the rigid foam.

    -- Martin Holladay

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |