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Insulating Open Pier Foundation

big__o | Posted in General Questions on

Im in DFW (Climate zone 3a) where 99.9% of all houses built in the last 50 years are on a concrete slab foundation.

I have a screw pile foundation. the perimeter is steel W-beams, and wood I joists spaced at 24″ o.c support the floor

since this is so rare in my area there is no one to go to to find out how to best insulate the floor. so I thought I would ask here. The bottom of the joists sit 24 inches off grade, and are approx 12 inches tall. the floor sits approx 36 inches off grade.

Code is only r19. please note that this is an open foundation so there are no walls to create a traditional crawl space. This is an intentional decision and won’t change.

thanks

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Replies

  1. Expert Member
    Peter Engle | | #1

    You can meet the code R19 by installing 6" fiberglass batts (or other "fluffy" insulation) between the joists. If you hold the water supply piping up close to the floor and install the batts towards the bottom of the joist cavities, then the pipes are on the "warm" side of the insulation and protected from freezing. If you do this, it is very important to provide good air sealing around the perimeter (band joist, sill, whatever) so that outdoor air does not enter the space above the insulation. In any open crawl like this, I recommend a rugged air barrier on the bottom of the joists. That could be a layer of foil-faced foam like Dow Thermax or at least a layer of plywood or OSB sheathing. Again, it is important to tape the seams and make this layer airtight. If you want more R-value, just increase the thickness of the batts or the foam layer if you're using one. In your climate, the risk of condensation inside the cavity from interior vapor drive is low. There is always some risk of condensation in cooling weather if you let outside air get into the floor cavities. With open piers, the risk of condensation on the underside of the OSB/foam layer is also low, even in humid Texas.

    1. big__o | | #4

      I hadnt even thought of doing a hybrid approach with fluffy insulation and foam boards underneath. THANKS!

  2. charlie_sullivan | | #2

    Where are you in the construction process?

    I would suggest filling the joist cavities with dense packed cellulose, if you have someone locally who can do that. That will get you a lot better than code R-19, but won't cost much extra so you might as well do it while they are there.

    1. big__o | | #5

      The house is already almost dried in, so we will have to insulate from underneath. can that be done with dense packed cellulose? also, we would still have to air seal so moisture doesnt get into the cellulose , correct?

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Max,

    What may inform your choice of insulation and how you install it, is the sequencing of construction, and the need to keep the insulation from getting wet until lock-up.

    1. big__o | | #6

      we are practically dried in, so no concerns about the insulation getting wet. thanks

  4. Expert Member
    Peter Engle | | #7

    Dense-pack cellulose can be installed from underneath, either with netting or through holes in your coverboard.

    1. big__o | | #8

      ok,great to know. thanks. if we do that we essentially have to cover the full depth of the i joist, right?

      then after that cover and air seal the bottom of the joists with a layer of taped foam?

      1. Expert Member
        MALCOLM TAYLOR | | #9

        Max,

        If your subfloor is detailed as an air-barrier, that is the penetrations are sealed, there isn't much reason to air-seal whatever you use on the underside.

        I'm not sure you need to worry about adding a layer of foam below the joists in your climate.

        1. Deleted | | #10

          Deleted

        2. big__o | | #12

          ok, im listening. I didnt air seal the subfloor but I assume thats as easy as taping the joints and caulking nail holes and foaming around openings?

          we are in a heavily wooded area with lots of critters. if we air seal at the osb subfloor and just insulate with the fluffy stuff wont the critters find that to be a nice cozy place to live?

          1. Expert Member
            MALCOLM TAYLOR | | #16

            Max,

            If you used subfloor adhesive and the plywood is t&g, there isn't much benefit from worrying about the joints or nail holes. Just seal the penetrations.

            Pest will try to get in, and foam won't stop them. Plywood seems to be the best practical solution, although even with that I've had problems on several projects with rats. They need both a place to work from, like perching on the top of diagonal bracing, and some evidence it's worth doing, like seeing a small hole they suspect leads somewhere. So it's worth trying to eliminate paths of access, sealing around penetrations carefully, and keeping the seams of the plywood tight.

        3. jlipkowitz | | #35

          Hi Malcom,

          Very grateful for your time. More questions from me on a different aspect of the same project. Zone 6 in Maine. My floor system is on piers, and I have 2x6 joists. Subfloor will be OSB. (floor joists and subfloor are part of kit and can't be changed). I am planning on filling the joist bays with cellulose. I am building high enough above grade to have access to the underside of the joists.

          My options are:

          A. sheath it with a plywood, tape the seams, and insulate with cellulose from the top. (doesn't deal with thermal bridging)

          B. Wrap the joists with foam, tape foam, then sheath with plywood. If I used the foam, I would like to use 1" of EPS from a cost perspective, but, that would violate the recommended ratio for my climate which could lead to condensation issues?

          C. Cut 1 1/2" strips of foam and adhere them to the underside of the joists, and then sheath (air sealing the sides at the rimjoist as well). Would that be a way to deal with thermal bridging without spending much on foam and creating condensation issues?

          Thanks again,

          Josh

          1. Expert Member
            MALCOLM TAYLOR | | #36

            Josh,

            Floors are the most sensitive assembly from a comfort perspective, because they are the one you are in constant contact with.

            Bearing that in mind I doubt you will be satisfied with the performance of option A.

            1" EPS is permeable enough that you will get adequate drying to the outside even though the ratio isn't met, and the stack effect means that the primary air movement will be upwards, alleviating a lot of the concern. So option B is workable.

            - The only downside to option C is the labour involved.

        4. jlipkowitz | | #38

          Hi Malcolm,

          I'm revisiting this thread from last year as I am about to begin the insulation project and have a few questions. I'm reposting some details here with my questions. Thanks so much for any help you can give!
          I am in midcoast Maine (Zone 5) and have recently constructed a round cabin on piers. The cabin was from a kit, so the r-values were all baked in due to material choice other than the floor assembly. Walls are R 18 and Cieling is R 21.

          The floor assembly is supported on piers at the exterior and a steel ring in the center. The floor is framed with 2x6 PT joists, spaced 48" at the exterior of the house and about 6" at the central steel support ring. Essentially they create large pie- shaped wedges between the joist bays. The subfloor is 3/4" ply.

          My conundrum is figuring out the best way to insulate this setup. I have about 2' between the bottom of the joists and grade.

          A few potential options:

          A. Wrap the underside of the joists with 1" EPS and tape the seams, protect it from the underside with plywood, and then blow in cellulose from the top.

          With the subfloor already being on, I would drill through the subfloor from the top to insert the hopper hose to blow the cellulose. My concern is that the pie wedge shapes would make the cellulose have a hard time filling the whole cavity and I would have to drill multiple holes per joist bay for that to work. It also involves the labor of attaching the foam/plywood under the structure in a tight space and the cutting difficulties of applying square sheets to a round structure.

          B. Hire a spray foamer to foam from underneath - probably 3" of foam and then apply 1" along the remainder of the side and bottom of the joist for thermal bridging.

          A few questions - GHG potential aside for the closed cell (I'd have the soy-based applied at least, but I realize it is a high-impact material choice)- Is there a reason from a building science standpoint to not apply the spray foam?

          My concern is that puts the vapor barrier on the outside of the assembly. Would this be problematic? If I went this route, would I want to make sure my finished floor wasn't acting as a vapor barrier so any moisture that did make its way into the floor assembly could then dry to the inside? Would the spray foam be a reasonable route and if so, what else do I need to keep in mind to make sure the performance and moisture issues are managed?

          Thanks and best wishes,

          Josh

  5. 1869farmhouse | | #11

    Even if one is generally opposed to (closed cell) spray foam, the two areas where it seems to me to be the ideal solution are:

    1. Unvented/conditioned attics where exterior insulation is not an option

    2. Between floor joists on an open pier foundation.

    3 inches would get you r-21 quickly and easily or flash and batt with 2” foam could put you in the neighborhood of r-30. Air sealed and nice a cool July!

    1. big__o | | #13

      interesting. I am going with closed cell spray foam for the unvented roof but why do you like it for the foundation. Im not opposed to spray foam except for cost but I think with 2 feet of clearance the spray foam guy might charge me even more :)

      1. 1869farmhouse | | #15

        https://www.lsuagcenter.com/nr/rdonlyres/d33f711d-dc4b-4e4c-9ed6-a97dce9db026/79805/pub3187insulatingraisedfloorslowres.pdf

        Check out this study from LSU. They tested 12 houses, each with different insulation styles in vented crawlspace/piers. Their conclusion was anything fluffy was generally less than ideal. Plus that’s and awful job. If I’m working in two feet, I’d much rather spray and be done than piddle with batts, rigid board, and seam tape.

  6. Expert Member
    Peter Engle | | #14

    Austin,
    There are a number of issues with using spray foam on the underside of houses, in addition to its global warming potential. It does nothing to prevent thermal bridging of the joists, so the bang for the buck is actually rather low. It also leaves the bottom part of the joists exposed to the weather. In poorly managed crawl spaces, this can lead to dramatic moisture issues and rapid decay of the exposed lumber. In the OP case, that's less of an issue b/c the crawl is fully open and vented. Adding even a small amount of foam (1" or so) to the underside of the joists keeps them on the dry side. Even a layer of plywood/OSB changes the microenvironment enough to keep them substantially dryer. Having a coverboard on the bottom also helps to protect against critters and that's not a small thing with an open foundation. If it were my house, I would be tempted to use about 1" of recycled EPS insulation and then a cement board cover - Hardiboard or equivalent. Possibly even fiberglass-faced gypsum like DensGlass Gold. In Zone 3, maybe just DensGlass Gold.

    Max, the fluffy insulation does not need to fully fill the joist cavities. In fact, leaving an air space above it can make the floors more comfortable. See this article from BSC: https://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance. Again, if you do this, make sure to insulate and air seal the perimeter. You don't need a ton of insulation in your climate. I would aim higher than code, but going any higher than R-30 for the floor is probably a waste of money.

    1. 1869farmhouse | | #17

      Having done this work in a similar climate zone, I’ve worked from this LSU study and had tremendously more successful results than my previous work that included fiberglass:

      https://www.lsuagcenter.com/nr/rdonlyres/d33f711d-dc4b-4e4c-9ed6-a97dce9db026/79805/pub3187insulatingraisedfloorslowres.pdf

      Air sealing being so much more important than r-value, I think the thermal bridging is the price we pay for simplicity and a long lasting solution. I don’t like anything fluffy exposed to weather, potentially even more concerning when connected to a home that’s going to be cranking a/c and then covered with something with such a low perm rating as EPS.

      1. Expert Member
        Peter Engle | | #22

        Austin, The LSU study you linked to was a well-done study, and it included both spray foam and rigid foam solutions. In the summary it says, "Foil-faced rigid foam and closed-cell sprayed polyurethane foam exhibited good performance, keeping subfloor moisture content within acceptable levels." So both work. The LSU study did not examine rigid foam+fluffy insulation as an option, since 2" of rigid foam met the local code requirements. Still, with two systems that both "work," we're left with the other factors: cost, simplicity, effective R-value/$, and GWP. Closed cell spray foam generally costs more, has a higher GWP, and lower overall R-value/$. It has the advantage of simplicity. While many contractors choose the simpler solution, I generally value the other factors more highly. To each his own.

    2. big__o | | #18

      thank you for the insight Peter. Ive already learned a lot from the responders to this thread but I have more new questions - which way should the floor dry? to the inside? or the outside? Im thinking to the inside but my question is - does this limit the floor type I use? its an osb subfloor, which is already low perm, if I add say vinyl flooring over that, Im guessing it essentially becomes vapor impermeable and will not be able to dry to the inside. does that then mean i cannot use the 1-2 inch layer of foam, or even spray foam as Austin suggested?

      1. 1869farmhouse | | #19

        I tend to agree with your logic Max, although I’m not sure if the building science supports it or not. Years back, I did a pier type foundation using mineral wool and rigid board, seams taped. And for the exact reason you mentioned, I used a penetrating oil based finish on the hardwood instead of a vapor impermeable poly.

        I’ve read exhaustively online, and worked in the field more than that - but I’m certainly not a building scientist.

        Edit: My mentality on my newer method (closed cell) is that although the exposed floor joists are a thermal bridge, they also allow drying to the exterior.

        1. big__o | | #20

          for Austin and Peter, how much of a themal bridge is it though. Wood has an r-value per inch of 1 or so, and the main part of the joist is 3/8 inch or so. even if its only 2 inches of foam and then some batt insulation, I feel like the amount of thermal bridging is almost immaterial. am I wrong?

          1. 1869farmhouse | | #21

            It adds up over the course of an entire wall, floor, or ceiling assembly - so I wouldn’t say immaterial, but more the price of admission. Personally, I think closed cell foam is villainized more often than is fair because it is admittedly coming with a greater carbon impact than other materials. But in my mind, building an assembly that is efficient, realistic, and most importantly long lasting is still for the greater good in the end. If the material was all scrapped in 10 years, that would sure be a greater carbon impact than nearly any assembly!

          2. Expert Member
            Akos | | #23

            Mostly correct. Depending on the width of the I joist flange, the first 1.5" of SPF will get very little effective R value (~R3.5). Anything after that will have much better performance, pretty close the the LTRR of the foam.

            Since closed cell foam is a vapor barrier, it will keep outside moisture out of your assembly, you can put any type of flooring over it.

            P.S. Having dealt with critters and floors, I would make sure that there is a solid critter barrier first. Depending on how hungry the local fauna is, carefully installed plywood works, backed up with metal mesh even better. Once you installed the plywood, at that point might as well skip all the foam, tape the seams and fill the joist bays with cellulose.

  7. big__o | | #24

    Akos, thanks for your thoughts. if I go with plywood cover and cellulose, I would definitely want some type of breathable floor. I know t hat plywood has a good perm rating of approx 10 I dont think id want to risk it.

    what are your thoughts on hardware cloth in place of plywood?

    1. DuPro | | #26

      What are your thoughts of using plywood to seal the bottom and secure the insulation, then drill 2" vent holes at some pre-determined spacing in each bay and cover the holes with a screen material?

      1. Expert Member
        MALCOLM TAYLOR | | #27

        DuPro,

        By "breathable" Max means vapour-open, so he needs to use a material the that a highish perm rating. Drilling holes makes the assembly air-open, which isn't something you want.

        1. DuPro | | #28

          Copy that, so insulating the bays ( I will have 14" tall joists) and sealing the rim joists and then cover the bottom of the joists with plywood, is that reasonable?

          1. Expert Member
            MALCOLM TAYLOR | | #29

            Dupro,

            It sounds reasonable to me. The only thing you might want to consider are whether to substitute pt plywood, or use some other more robust material instead.

  8. DuPro | | #25

    MaxE we are building the same pier & beam floor design and have the same dilemma as to how to insulate and seal the floor system. The information in this blog is very insightful to a possible solution path. I was leaning toward Rockwool bats and then sealing off the bottom with plywood, have same concerns on moisture issues. Will be very interested to see how you solve this issue. BTW we are building in the panhandle of Florida on a lake with a slope, pushed us to a pier & beam plan.

    1. big__o | | #32

      Hi, dupro, I'm considering one of two methods.
      1. Flash and batt. 2 inches of ccspf, and the rest with some type of batt for cost. Then to rodent proof, either plywood, corrugated metal, or hardware cloth, depending on my further research

      2. The second method is to use recycled roof xps foam. They come in 3" thickness (r15),use two layers,and I'm thinking we could rip them down on a table saw and seal any gaps with spray foam
      Rodent proofing would be the same as option 1

      1. DuPro | | #33

        Appreciate the response, we are planning on using I-joists as well, there is some discussion on the drawbacks of using ccspf, still researching this. Let me know what direction you take!

      2. rickreed | | #37

        Insulating a subfloor prior to drying in the house would create a swamp amidst the floor joist, so not an option for me. Insulating after dried in would be the only option. Problem is how to attach a zip panel from underside unless there's the clearence and a level grade below to operate a drywall panel lift or you've a few guys muscling the panels up.
        I like your suggestion of some type of rodent proof lightweight corrugated panel that would solve the problem but wouldnt it be difficult to seal at the perimeter to prevent air and critter infiltration, wouldn't it be best if it was a lightweight flat panel. Any ideas on which this would be?

  9. Expert Member
    Peter Engle | | #30

    DuPro,

    Your plan should be very rugged and durable. In very humid FL, using PT plywood would make it even more so. Just make sure to seal all of the holes to keep those enormous snakes out ;-)

    1. DuPro | | #31

      Thanks Pete! we were really running out of options, for years off-grade foundations in Florida were just left open air, no closure. that's before HVAC, house envelopes, energy efficiency, now Code wants a sealed flooring system, hard to achieve in a high humidity environment. This method is high on our list for a possible solution.

  10. FADLLC | | #34

    What do you think about using zip board and taped seams as the soffit? I don't have any hands on experience with zip tape-would it be a forever material in this horizontal application?

    Also, how do you go about keeping water out of the floor while you're building? Mine is too low to be fun to add the [comfortboard and zip] system on my back, so we'll be building little swimming pools, adding fluffy insulation to the bays, installing the advantech subfloor and just making sure there is no chance of water getting in that we can. I'll try to tent it when it looks like rain until we've got a roof.. any other tips/experiences?

  11. ESBB | | #39

    Anyone with experience using zip board with taped seams? I've raised an 1850's farmhouse placing it on concrete piers [required by code] and will be adding a new addition, also on piers. The entire first floor - old and new - will have the Warmboard radiant heating system. { I'm living in a PHIUS certified passive house with radiant heat in the slab [available for purchase] and won't give up the comfort in our new residence. } The foundation is open and about 4' above grade. I'm on the Chesapeake Bay, in southern MD. Any advice appreciated.

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