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I am looking at building a slab-on-grade house

jb487 | Posted in Green Building Techniques on

I am looking at building a slab-on-grade house. I am also trying to avoid having any ductwork (or other utilities) in my attic. Where is the best place to route ductwork, plumbing, electrical, and sewer if I am slab on grade?

I wish to avoid a basement or crawlspace and their associated challenges. I also want to have the ceiling free of penetrations and no ductwork in the attic. Is it OK to run plumbing/sewer/electrical conduit under a foundation slab? I already know I want to avoid running ductwork under the slab, so I’m considering a ducted minisplit with minimal ductwork.

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  1. JC72 | | #1

    It's common for electrical, sewer, and plumbing to be placed under the slab.

    As for placing the HVAC system inside the building envelope you can either place the mechanicals inside a large utility closet within the house and run duct work via dropped ceiling, or you can place the mechanicals in the an unvented attic space (the attic space becomes part of the interior space), or you can install minisplits in various rooms and forgoe ducts altogether.

  2. GBA Editor
    Martin Holladay | | #2

    John has provided a thorough answer. And if you don't like any of the options listed by John, you should switch to a crawl space or basement foundation.

  3. jb487 | | #3

    Thanks for the info - just what I was looking for! By the way, I just completed Musings of an Energy Nerd and found it very helpful!

  4. this_page_left_blank | | #4

    Our slab on grade house has the plumbing under the slab (not really much choice there), but everything else is either in a utility room (doubling as a rear entrance way and storage area), or in the ceiling. On the main floor, that's in the floor joists for the floor above (16" open web trusses, so plenty of room). On the upper floor there is what we have referred to as a 3.5" service channel, which is probably the same as a dropped ceiling. Except I picture a dropped ceiling as having removable tiles, whereas our ceiling will be drywalled once all the utilities are up there. The walls were built taller such that the finished ceiling is still the proper height.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for the feedback. Glad you liked the book.

  6. Expert Member
    Dana Dorsett | | #6

    Be aware that hot water distribution and hydronic heating plumbing needs to be insulated to a least R3 if it's under a slab, independent of the size of the pipe, per R403.5.3 of the IRC 2015:

    (Look about half way down that page, or search for "R403.5.3" )

    Above slab hot water plumbing in side of conditioned space only needs to be insulated if it's 3/4" or larger in diameter, or part of a recirculation loop (including the return side of the loop), unless it's a demand-type loop.

    Under IRC 2012 there were other more complicated requirements regarding diameter & length, etc. In California pipe insulation is covered in both the California Plumbing Code and the currently active 2016 Title 24 code, which is even more complicated to figure out.

    If you happen to be in a state governed by IRC 2012 rather than 2015 (or some other code), use this as a handy planning for compliance guide:

  7. Expert Member


    By all means run electrical service and sewer under your slab. For water I would try and route it through interior walls if possible. For the portions you can not do that with, I would run the supply lines in 3" PVC conduit sleeve under the slab. It makes replacement or repairs a lot easier.

  8. GBA Editor
    Martin Holladay | | #9

    If you take Malcolm's advice and run your water supply pipes in your walls, just make sure they aren't exterior walls. In a cold climate, pipes in exterior walls can freeze.

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