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How to Insulate Piers With EPS Foam

Lindaloowho | Posted in General Questions on

I know this isn’t what is considered best practice, but I am attempting to build an air sealed skirt around a home on piers. One story home, no insulation in exterior walls.

As I put a continuous layer of EPS on the interior of the wood skirt, do I insulate the piers on the” interior “ side?  Plywood skirt covers the pier on the exterior side, but no room for insulation between the skirt and pier.

Also, no sill plate. Rim joist sits on piers. Do I insulate the top of the pier when insulating the rim joist areas between each floor joist?

Thanks for to any Weekend Warrior reading here today!

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Replies

  1. Charlie Sullivan | | #1

    Might be a dumb question, but what are your piers made of? Also, how many piers, and how high is the space you are trying to close off?

  2. Lindaloowho | | #2

    Hi Charlie,

    Concrete piers. There are 24 piers. 16 around the perimeter and 8 in the middle which I will vapour barrier but not insulate. The space is 30-36 inches tall.

    Thanks in advance for sharing any thoughts. I know the best thing would probably be to pull up the skirt and insulate the floor assembly, but the insulated skirt is in place (has been for 4 years), and we are trying to make this work for a few more years.

  3. Expert Member
    Zephyr7 | | #3

    My first thought is how well is that skirt air sealed? If you have a lot of leaks around the ground, for example, that will drastically reduce the effectiveness of any insulation you add. It might actually be easier to just insulate the floor instead of the skirt.

    If the skirt is pretty well air sealed, then I would try to insulate on the outside of the piers to limit how much thermal bridging you could potentially have with all that concrete. If you don't have room for that, get as close as you can with the rigid foam and finish the rest off with spray foam which won't have a problem insulating the contoured piers (I'm assuming they are round piers). Ideally you want a capillary break between the top of the concrete pier and the structure too, which you might already have. Even if you don't have a capillary break, your risk should be minimal here, especially with the exterior part of the pier remaining open/unsealed.

    Bill

  4. Lindaloowho | | #4

    Actually the piers are square so I was going to build “half boxes” with rigid EPS and adhere them to the interior side of the concrete. Then spray foam the seams.

    Any thoughts about insulating the rim joist without sill plate? Blocks of EPS with spray foam? I will allow an overhang of the EPS and connect the joist assembly to the skirt assembly EPS with spray foam.

    There is no capillary break and can’t jack up the house or slip any thing over the top of the piers. Glad you said there would be little risk, Bill. As you said, the only thing between the outside and the exterior of the concrete pier is a layer of treated plywood. Hopefully everything will dry to the outside.

    Don’t shoot, but after applying the continuous layer of EPS, I was going to leave an air space and then install a layer of “double bubble double foil“ membrane on the interior of the entire skirt assembly and over the rim joist, overlapped and sealed over the ground vapour barrier. Hopefully this will ensure an air barrier and I can take advantage of passive radiant heat to help keep pipes from freezing. Since I am allowing for an air space, I also hope to gain a little R-value too.

    I have been see-sawing on your suggestion regarding the ground insulation myself, Bill. I was looking at a rolled thin insulation (not sure of R-value) that has a radiant barrier on top. Do you think I could apply this to the 6mil poly on the ground later?

    Thanks so much for your help.

    Linda

    1. Expert Member
      Zephyr7 | | #6

      EPS is a good choice for the rim joist since it is slightly vapor open and will allow for a little bit of drying. You can probably use canned foam here and not two part spray foam to seal in those pieces of EPS. Note that the concrete piers themselves don't need to dry, but you don't want moisture wicking issues with your wood members on top of the piers.

      I don't think your radiant barrier is going to be worth the effort, you'd be better off using thicker rigid foam. On the ground, I'd just use a thick liner and tape seal it to the perimeter to get you an air seal. The bubble insulation is only around R1 or so as I recall in terms of actual performance when you don't factor in optimized theoretical radiant performance.

      Is there any particular reason you can just insulate the underside of the floor/joists? I think that would be easier, and would certainly involve less potential issues.

      Bill

  5. Expert Member
    Akos | | #5

    Keep in mind there is no such thing as an insulated skirt, what you are building is a crawlspace with perimeter insulation. Make sure you follow the best practices for that from here:
    https://www.buildingscience.com/documents/building-science-insights/bsi-115-crawlspaces-either-or-out

    Your plan to box around the exterior piers is probably the best in your case. For insulating the rim joist, the easiset is to fill in the gap between the joist with pieces of ridig to get it to the same plane as the inside of your skirt and run the rigid on the skirt up to the bottom of the floor. Seal in place with canned foam.

    You don't have to insulate the interior posts, those can stay as is. You do have to get a decent seal on your vapor barrier around them.

    The biggest problem with this setup is critters. Without a rat slab, you are making a very cozy home for most. Sometimes, just because of this reason, it is best to insulate the underside of the floor (batts in the joists, ~1.5" of rigid bellow) and cover with plywood or wire mesh. You can than do a vented skirt to discourage larger critters.

    1. Lindaloowho | | #10

      Thanks Akos,

      I have read this article and we are going with the configuration in figure 3f: “encapsulated” crawlspace with dehumidifier. Thanks for your suggestion re: building in the rigid insulation so everything runs in one straight line (in my case, I have to build in with wall insulation to match rim joist), floor to ground. That would make things easier since Bill has convinced me to throw on another layer of insulation on the walls anyway.

      Can I ask: How does the sealed vapour barrier on piers act as a capillary break when there is no physical barrier between the wood member and concrete? What keeps the moisture from moving beyond the height of the seal within the pier?

  6. Lindaloowho | | #7

    Thanks everyone,
    Bill and Akos, the main reason for not insulating the floor assembly is the pipes below and the 1000 square feet of insulation and plywood we would need. Can’t find wood anywhere these days. The insulated skirt is already in place.

    Akos, we have buried galvanized metal lath with openings smaller than 1/4” around the perimeter of the skirt so hoping that takes care of critters.

    Putting up the radiant bubble barrier just seems so much easier to run around the inside perimeter of the skirt and because it acts as a vapour/air barrier too which will hopefully catch any places we miss with rigid/spray foam. It’s also cheaper than another layer of foam. Is it really as useless as everyone on GBA says? Even if I leave the recommended air space?

    If I forgo the bubble and foil barrier, should I bring my vapour barrier up to the floor assembly? Or should I stop just under the rim joist? The property is in Southern Ontario.

    1. Expert Member
      Zephyr7 | | #8

      You get about R1 from the radiant barrier part alone, at least that’s what’s usually used as a reasonable estimate. The crazy high R values you sometimes see in promotional materials are not really realistic.

      Bill

      1. Lindaloowho | | #9

        Thanks Bill,
        I edited my question to ask about poly vapour barrier on the walls instead of bubble/foil. How high would you recommend I come up? All the way to the floor?

    2. Lindaloowho | | #11

      Thanks Akos,
      I have read this article and we are going with the configuration in figure 3f: “encapsulated” crawlspace with dehumidifier. Thanks for your suggestion re: building in the rigid insulation so everything runs in one straight line (in my case, I have to build in with wall insulation to match rim joist), floor to ground. That would make things easier since Bill has convinced me to throw on another layer of insulation on the walls anyway.

      Thanks for the reminder that I can leave the inside piers sealed with vapour barrier only. I’ve asked Bill this question too: How high can I go with the vapour barrier? I’m not in termite country but I am in carpenter ant country.

      Can I also ask: How does the sealed vapour barrier on piers act as a capillary break when there is no physical barrier between the wood member and concrete? What keeps the moisture from moving beyond the height of the seal within the pier?

  7. Deleted | | #12

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