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Community and Q&A

Insulating a House on Piers

MALCOLM TAYLOR | Posted in General Questions on

Kiley – in response to a question on insulating a house on piers, you posted a link to a FHB article showing a well thought out house with details on the floor system and how to bring services up in an insulated core. I’ve somehow lost the link. Any chance you remember it and could post it again?

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Malcolm,

    Is this the article I cited: Air-Sealed and On Piers?

  2. Expert Member

    Hi Kiley,

    No it as a whole house where they built a 2"x4" framed floor under the regular one so they could sheath the top, rather than work from below.

    Thanks for looking. It's not a big deal, I'll poke poke around and see if I can find it.

  3. jlipkowitz | | #3

    Hi Malcolm,

    The document you are asking about can be found here:

    While we are at it, I am pondering my option for my insulated core to get into my cabin 2 feet above grade in Maine. I was considering building a foam box and sheathing in PT Plywood, but I have some extra 12" culvert pipe. Similar to what Daniel suggested in a different post and combining with their approach in the document I am attaching here, I am considering having the 12" culvert run from below frost line (4' below grade) up to the underside of the floor. Inside the culvert I would run a 4" pvc conduit. That 4" conduit would be open to the inside of the space for conditioning. Inside the pvc conduit would be my 1 1/4" line from the well. I would insulate between the outside of the 4" conduit and the wall of the 12" culvert pipe. I thought about doing this with spray foam, but it seems difficult.

    Do you think mineral wool would be a reasonable application for this? Do you think a 4" thick layer of mineral wool would provide sufficient insulation? Finally, in this application, would I want the top of the 12" culvert exposed to the inside of the space, or just the top of the 4" pvc conduit?

    Thanks so much for your help,


    1. Expert Member
      MALCOLM TAYLOR | | #6


      Thanks so much!

      Part of why I'm thinking about this is I am also building a small recoding studio on piers right now.

      I've done several insulated cores for services - which is a lot easier in our temperate PNW climate. They have been larger enclosures, with an area open to above of about 2 sq ft. My concern with a smaller core is that even with a highly insulated enclosure around it, the open area around the pipe needs to be a certain size to connect properly with the heated area above. If there is no heat transfer down the middle of the core, any insulated box will eventually allow the services to freeze.

      I think there is an optimum ratio for each climate between the amount of insulation necessary, and the area of open core - but I don't know what it is.

      To finally get to your questions:

      - I'd be fine with mineral wool as insulation (and am thinking of adopting your idea on my current built). My one concern is that with your frost depth, depending on how wet the site was, the sub-grade mineral wool might get saturated and freeze. Somehow the bottom of the culvert needs sealing against water infiltration.Either a cap or maybe a plug of spray foam?

      I don't see any advantage to exposing the whole 12" culvert to the inside, just the open core.

      1. jlipkowitz | | #11


        Thank you for your reply! That and the idea of pourable foam are super helpful!

        I hear your concern - I would effectively have a 4" core around a 1 1/4" poly pipe; a pretty small core. I could use heatline's product on it with a thermostat and see how it performs and run a small bit of juice if needed. If I were to go for a bigger core as you suggested I would have to change approaches from the double wall conduit I was describing.

        You are correct about the seasonally high water table and the need to make a water tight enclosure. Water is 16" below grade right now.

        It seems easier to me to imagine making a culvert water tight than than a box. (easy to buy a watertight cap for it, drill a hole for the 1 1/4" line, and spray foam). I could run the 12" culvert as the core, 1 1/4" pipe running up the inside of it, and then frame a pt box wrapped in foam around the culvert. Even if water infiltrated the inside of the box at times, it would be inside the conditioned space and thus shouldn't freeze. Does this seem like a resonable approach? I am happy to change it up, just not sure what will work. I guess the culvert might not even be necessarry and I could just run the pipe on the inside of the insulated box?

        Thanks so much,


        1. Expert Member
          MALCOLM TAYLOR | | #12


          I think you are on the right track. I'd stick with the culvert as the outer layer, and add heat tape as insurance. If you can foam in between the two that's probably the best bet.

          1. jlipkowitz | | #13

            Thanks, Malcom. Sorry for the followup here, but just to verify, are you saying your would trend towards sticking with my initial plan of the conduit inside the 12" culvert, no foam box outside the culvert?

            I will certainly pour foam inbetween the two if I took that plan, but, what about the concern with the core being too small to effectively be conditioned by the space above and below? Also, if I seal the space off at the bottom to prevent water infiltration, that would make conditioning from the bottom less impactful, yes? I wonder if I if I could leave the inside of the 4" pvc uncovered on bottom. If water came up inside the conduit and sorrounded my poly pipe it would theoretically be conditioned water and not freeze?



          2. Expert Member
            MALCOLM TAYLOR | | #14


            Bear in mind I'm speculating here, with not a lot to back it up...

            The setup you are suggesting is the same as the one used in the article you found the link for - and in a similar climate - so that's a good start.

            I'm tempted to say it might work better with a 6" inner core. That gives you less insulation, but more connection to the interior.

            The real wildcard is the connection at the bottom. I doubt there is enough heat in the ground to meaningfully help keep the pipe warm. What I don't know is what water in the lower end of the pipe would do. Act much like air? Freeze if temperatures dropped? My inclination would be to try and exclude it so that if additional heat did end up being necessary to keep the inner core warm, you could use heat tape, or even a very small fan blowing conditioned air from the house downward.

  4. mdb_az | | #4

    Malcom is this what you're thinking of (via the South Mountain website)
    was using this as a reference for one of our projects the other day.

    1. GBA Editor
      Kiley Jacques | | #5

      I think that’s it! I’m so happy you solved the mystery; it was driving me bonkers. I know I was the editor on that story and I could not recall the details. Thanks a million.

    2. Expert Member
      MALCOLM TAYLOR | | #7



  5. Expert Member
    PETER G ENGLE PE | | #8

    FWIW, there are "pourable" grades of SPF. They would make insulating the annular space between the culvert and core pretty simple.

    1. Expert Member
      MALCOLM TAYLOR | | #9


      I didn't know that. Do you know where you can get them?

      Edit: I found them at marine stores near me.

  6. rockies63 | | #10

    In building a service chase from the ground to the underside of the floor system and connecting it to the interior space (in order for heated air inside the cabin to circulate into the chase and help prevent pipes from freezing) risks radon entering the structure, as well as cool moist air and bugs.

    Why not build a small slab on grade next to the cabin and put a mechanical room/shed on it? Have the exterior walls of the shed and cabin touch (an insulated connecting 2x4 "picture frame" filled with insulation can be between the two layers of exterior sheathing). Run all the water/well/septic pipes up into the shed through the concrete floor and then pipe them through the connecting walls into the cabin.

    If it gets really cold put a direct vent propane heater in the shed connected to a thermostat so it only comes on if the temp gets close to freezing. All your other mechanicals can go in the shed as well, freeing up space in the cabin.

  7. user-6623302 | | #15

    If it was me, I would build a chase with PT wood, designed like a small frost protected foundation. So, wing insulation, insulated walls, air sealed to the floor above.

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