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Thermal Bridging at Metal Helical Piers with Metal Brackets

PBP1 | Posted in General Questions on

Have an elevated floor (22′ x 32′) on piers due to existing mature trees.  The piers are metal/helical screw with metal brackets – corner brackets are bigger (10″x10″ or so) as they join two beams.  The floor is flash and batt, with foam sprayed from above onto pressure-treated plywood covering entire underside.  Floor joist are pressure-treated as well.  Above is OSB with 3/4 composite plywood/white oak (10.5″ width).  In winter, the corners of the floor are colder due to geometry/physics (surface to volume) and I’m thinking thermal bridging due to the metal brackets.  The metal brackets are exposed underneath – brackets inset from plywood as they are in direct contact with  support beams.  Looking to use foam board at about 18″x18″ square glued and sealed around edges at the brackets and elastomeric 3.5″ ID pipe insulation (3/4 in wall) on the metal pier posts rising to abut the foam board.  Looking to reduce heat flow to get a couple of degrees reduction in temperature differential between center of floor and corner.  If anyone has insulated metal helical screw piers/combated bridging, would like to hear.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    If the brackets are saddle type brackets, then they penetrate your insulation and essentially also bypass the wooden structure and will be a fairly significant thermal bridge. If your wooden structure sits on top of flat plate type brackets, then you don’t really have a thermal bridge but might have a heatsink-type effect that draws away thermal energy more effectively than the usual structure/air interface does.

    Either way the solution is the same, and sounds like you’re on the right track. If you insulate the exposed steel you’ll minimize the thermal losses. I recommend either using an adhered insulation (most commonly this means spray foam), or allow for some drainage at the bottom of the insulation. You don’t want to keep water in contact with the steel trapped inside the insulating layer.

    I haven’t insulated a screw pile before, but I have had to insulate pipe supports which are similar. In your case, I’d dig out a bit around the base of the pile, just where the column part goes into the ground, don’t disturb the actual anchor, insulate everything down to slightly below grade, leaving the insulation open on the bottom (don’t tape/caulk it), then fill in around the insulation with pea gravel. This allows for some drainage.

    Bill

    1. PBP1 | | #2

      Thank you Bill/Zephyr7 for the helpful tips. The brackets are saddle type. I will use a spray foam to cover the metal brackets to the level of the lower surface of the pressure-treated plywood (shy of 3/4 inch fill) and then use foam board adhesive to glue on a foam board panel over a larger area (about 1.5' x 1.5'). Good tips on the pile/pier insulation, the floor is low to grade so it will be around one foot of exposed insulation per pile/pier, with a few inches below grade back filled with pea gravel (have plenty of that on hand) - drainage is pretty good, no standing water - but will leave open on bottom. Eaves are pretty long so rain exposure shouldn't be an issue. Trying to do one energy saving project every week ;-)

      1. Expert Member
        Zephyr7 | | #3

        You’ll probably find gluing up the foamboard first and then spray foaming will work better and seal everything up. This is a perfect application for one of the small two-part spray foam kits. Just be sure to apply it when temperatures are high enough outdoors — temperature is critical to get a good cure, you don’t want to apply it when it’s too cold.

        Bill

        1. PBP1 | | #6

          Thanks again, I just finished a crawl area with Froth-Pak 210 (no issues, had all the PPE) and I have a small Froth-Pak 12 that I could use, always a good reminder on temperature for those kits. Was thinking alternatively to one-part spray foam the metal bracket, slap on the foam board with foam board glue/caulk in areas not spray foamed (after a few minutes to allow for some curing of spray foam) and then hitting the edges of the foam board with the one-part.

  2. Aaron_P2 | | #4

    On commercial projects we often specify insulative coatings for steel that penetrate insulation and reduce the thermal bridge/chance of condensation. It might be worth exploring something like that in addition to the insulation you are thinking about (especially for any exposed areas beyond the insulation).
    https://www.tnemec.com/products/view/use/insulating/filter/

    Also, I don't know how practical this is since it sounds like you are fully constructed, but there are also thermal break structural shims that can be used in certain situations.
    https://www.armatherm.com/products/frr-structural-thermal-break-material/

    1. DCContrarian | | #5

      That looked interesting until I read the specs, R-value is 0.9 per inch. Pine is 1.4 per inch.

      1. PBP1 | | #8

        Thanks for the specs

      2. Aaron_P2 | | #9

        They list the Aerolon at 4.1 per inch: https://www.tnemec.com/products/series-971-aerolon-acrylic/

        I agree DCContrarian the it wouldn't get anyone to high levels of insulation, but my understanding for a coating it lowers the conductivity and helps to prevent rusting to a good degree on items that might otherwise be a large heat sink.

    2. PBP1 | | #7

      Thanks, next time will be different :-)

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