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Crawl Space versus Slab on FIll

bauchin | Posted in Energy Efficiency and Durability on

Building a house in Climate Zone 6 with a full basement under main house but either crawl spaces or slab on fill for one section where there is ledge.  Trying to decide whether it is more cost effective and/or more energy efficient to build this section as insulated slab on fill or wood-framed floors over crawl space.  There are some plumbing/sanitary runs for a bathroom in one portion of the wing . . either underslab or in the crawl space.  Slab on fill option better for insulation but crawl space better for cost/plumbing access.  Any thoughts for which is better and how to best insulate that option would be appreciated!

Thanks

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Replies

  1. Expert Member
    Akos | | #1

    If you already have the basement, I would go for a crawlspace.

    The extra cost of floor joists is offset by not needing infill and sub slab insulation. Plus you'll end up with warmer floors and a way easier way to run mechanicals plumbing and electrical. Keep in mind that a crawlsapce is essentially a mini basement, so it needs to be treated the same. Insulation on the walls and conditioned air supply.

    Crawlspaces also make for a great long term storage where items go to die before being thrown out.

    1. aunsafe2015 | | #2

      "where items go to die before being thrown out"

      Hah hah. The trash can before the trash can, so to speak.

  2. Expert Member
    BILL WICHERS | | #3

    Crawlspaces also permit for future maintenance issues, so something like a leak in a water pipe is a minor repair instead of the *major* rework that would be needed if that pipe were under a slab.

    Personally, I prefer to go with a full basement if you have the option, since you can put mechanicals down there and also store stuff in a more accessible space. If you're already excavating for a crawl space, going a little deeper to get a full basement isn't usually hugely more expensive -- and the construction part is pretty much the same, just with higher walls.

    Either way, if you go with a basement or a crawl space, insulate the walls, not the underside of the floor. Ideally you would do this with rigid foam on the exterior of the masonry foundation walls.

    Bill

    1. Malcolm_Taylor | | #5

      Bill,

      "Crawlspaces also permit for future maintenance issues, so something like a leak in a water pipe is a minor repair instead of the *major* rework"

      Having spent the best part of a day tearing out and replacing half the t&g ceiling in our dining room to get at a leak, got me thinking about what the most flexible house would look like.

      Building on a crawlspace with a trussed roof goes a long way. Having services centralized and exposed in a mechanical would be nice too. Even using a suspended ceiling system below a second floor would make changes or repairs a lot easier.

  3. Expert Member
    Michael Maines | | #4

    I prefer slabs when possible. Less material, reduced risk of moisture-related problems, usually lower cost. Ideally with a concrete-free slab but that's not always practical. The main drawback in a situation like yours is the amount of interior fill required. If you have to bring in a lot of it, you might be better off with a crawlspace.

  4. bauchin | | #6

    Hi all . . . thanks so much for the insights and feedback. After talking with my structural engineer and reviewing everyone's feedback, I am going to pursue the crawlspace/basement option and, per your collective advice, detail the walls as a basement. My structural suggested that I draw it as a full basement given that we don't know the extent of ledge or its removability and we make adjustments after excavation reveals the extent of ledge. We can then decide where and how best to create/insulate the crawl space. The two factors that tipped me in this direction were the "warmer wood framed floors" and the "plumbing access" in the crawl space for future maintenance/reconfiguration. Thanks all for your helpful feedback

  5. walta100 | | #7

    I think living on a slab is hard. It is hard on your back and your feet the imperceptible bounce in the joists makes a huge comfort difference.

    Note a large excavator with a jack hammer often can remove the ledge rather quickly consider asking for a bid.

    Walta

  6. TracyS | | #8

    This is a bit older thread, but there was an original question that i don't think was addressed:
    Is a slab on grade or a crawlspace more energy efficient? Or maybe a better question - Is one or the other easier or more feasible to get higher air sealing and effective insulation values? In my head, i've been able to make arguments both ways.

    1. Malcolm_Taylor | | #9

      Tracy,

      Much like whether a one or two story house more efficient, it's an open question with too many variables to give conclusive answer. As nothing precludes making either option very energy efficient, I think other considerations are probably what should drive the decision.

    2. Expert Member
      Michael Maines | | #15

      I agree with Malcolm, but would add that with a slab-style foundation, there is less surface area and so less material needed to insulate and air-seal the foundation.

  7. TracyS | | #10

    Thanks Malcolm.
    The reason i ask is i am debating between the two options for a new build (climate zone 8). I have a list of my personal pros and cons of each. The air sealing and insulation difference is the one i could not definitely put on one side or the other, and so put the question out to see if there was something basic or obvious that would push it to either side.

    My current thinking is that the rim joist space over a crawl space is easier to get high levels of insulation than rigid on the exterior of the foundation walls. And i can insulate the floor joists bays to a high level without the need to use foam. And i think air sealing is probably the same, carefully detailing in either case.

    From my experience with my current house - the crawl space insulation is on the interior of the foundation walls, and my thermal camera picks up a significant cold line along the mud sill. So either way, i would put foam on the exterior of the foundation walls.

    1. Expert Member
      BILL WICHERS | | #11

      You can insulate and air seal the exterior walls of a crawlspace, and then you can insulate the floor too if you want to go all out. At that point, you have insulated as well as a slab would be, but you maintain the ability to access mechanicals for future service work. That ability to make repairs without ripping things apart to gain access can not be overstated!

      Remember that a slab loses energy too, but since it's losing it to the ground, you won't be able to see it on your IR camera. Just because it's not easy to see or measure those losses does not mean they aren't there.

      Bill

  8. TracyS | | #12

    Taking the advantage of accessing mechanicals one step further, and add that it more easily allows for modifications to the floorplan during construction. For example, after the structural framing is in place, you may decide to steal a foot of closet space to make a bathroom bigger. From experience, moving a toilet flange and/or shower drain in a slab is no fun. The advantage of construction flexibility along with no obvious energy performance disadvantage is probably going to make this decision for me.

    1. Malcolm_Taylor | | #13

      Tracy,

      There is a school of thought that says crawlspaces are just short basements, with all their weaknesses (primarily susceptibility to flooding), but you don't get to use them as living space.

      Against that is the benefit you bring up as both a location for services and equipment, and how much adaptability they give to the spaces above. A house with a trussed unconditioned attic on a crawlspace affords a lot of room for easy changes over time.

      You may be interested in Ted Benson's work. He takes these ideas much further, making sure the elements of a building which may be changed are easily accessible.
      https://www.greenbuildingadvisor.com/article/the-bs-beer-show-open-building-systems-with-tedd-benson

  9. TracyS | | #14

    Thanks Malcolm, I'll take a look.

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