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Climate zone 7a building on helical piles – how to insulate floor?

TomScott | Posted in GBA Pro Help on

Hi experts,

I’m building a house in climate zone 7a on helical piers. Ideally, high enough up to allow for parking and a patio underneath.

We’re going for a hybrid wall of continuous exterior insulation (rigid mineral wool R15) > vapor/air/water barrier> and mineral batt (R13) in the 2×4 stud wall. That’s not complicated, but how do we incorporate the floor system into the building envelope and how do we insulate and airseal it? I have looked at diagrams for the floor, but I haven’t found any for climate zone 7a or with continuous exterior insulation.

An occupant is highly, highly sensitive to indoor molds.

Thanks,
Tom

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Replies

  1. Expert Member
    1. TomScott | | #4

      Thanks Malcolm, those are good diagrams to have.

  2. Andrew_C | | #2

    The Cold Climate Housing Research Center, based in Alaska, has information about building on permafrost foundations. They have several approaches that they've looked at with raised floors. I haven't read all the articles, but if they raised the floor, they must talk about how to insulate it.

    1. TomScott | | #5

      Great resource! I'll look the Center up.

  3. nickdefabrizio | | #3

    Virtually all houses built on the NJ coast in the last 20 years are built on pilings above floodplain. While this is zone 4, there are many things that one could learn from these structures as there are tens of thousands built...Here are a few things I have observed:

    Most homeowners enclose the ground floor to use as a garage or storage area (this helps keep wind out but creates a huge mess when hurricane waves wash through). Whether or not the ground floor is enclosed, if the main floor is high enough for cars to park below it, double 5/8 sheet rock is required by fire code (use exterior grade sheetrock if exposed). This creates a pretty good air barrier if taped right.

    Also, straight floor trusses are becoming popular because they can be very deep, span long distances, and offer chases for HVAC/plumbing to go through. I suspect the best insulation fill for this is a blown product (cellulose, fiberglasss or even rock wool), but most people in coastal areas just use batts.. Some people add a layer of foam board, so that is something to look into. Some high priced homes use spray foam.

    One detail to carefully consider is the plumbing chase: it must be highly insulated. If the ground floor is not enclosed, you may want to create and heat a small utility room or stairwell for this purpose so the pipes have a protected path from below the frost line into the heated main floor. Also a very large corregated pipe filled with foam or rockwool might work as a chase...

    Lastly, even though you are above grade, squirrels and other rodents can be a nuisance, so take precautions.

    1. TomScott | | #6

      Yep, gonna insulate the heck out of that chase. Hadn't thought about the double drywall or squirrels. We won't be enclosing it so I'll have to give a good think to those aspects -- thanks for those tips.

      1. Expert Member
        Akos | | #7

        +1 On critter proofing. You want a solid critter barrier that is continuous across any cross section of your envelope. The ones up north in cottage country are pretty tenacious and will even chew through spray foam between rough openings and doors/windows. You don't need a blower door to tell you where there are leaks in the envelope as they will find even the smallest hole. Plywood and wire mesh are your friend, don't skimp on it and make sure all transitions are fully protected (floor to wall, wall to roof).

        No amount of insulation in a floor will make up for the fact that the outside is connected straight to say -20F air instead of ground which might be at 50F. This means the floors will always be slightly cold.

        For example, I'm in zone 5 with my bedroom partially in an overhang with 16" of insulation in the floor. If I turn off the floor heat, the floor will be cool. Not freeze your toes but also not barefoot comfortable.

        A wood burning stove that can throw off a lot of IR can compensate for some of this but a bit of resistance floor heat is not a bad idea. Doesn't have to be the main source of heat, just enough to raise the floor temperature above room temperature.

        1. Expert Member
          PETER G ENGLE PE | | #9

          Joe Lstiburek wrote an article a few years ago about this. He advises using oversized floor joists and providing a few inches of airspace between the top of the insulation and the floor. When he started recommending this, people's heads started to explode since he was one of the first to point out how important it is for fluffy insulation to be in contact with an air barrier on all six sides. Turns out that's not true for floors, and leaving the airspace above the insulation means that space approaches room temperature rather than the outdoor temperature. Of course there are specific caveats and techniques involved. The point is that the floors are warmer with the airspace and you can run plumbing through that space as well. I don't have a link to the article but I'm sure a quick search on the Building Science Corp. website will find it.

          1. Expert Member
            Akos | | #10

            Peter,

            Take a look at my post #22 on the discussion here. I've only used Therm a couple of times, not sure how much to trust it.

            https://www.greenbuildingadvisor.com/question/insulating-an-exposed-floor

  4. rockies63 | | #8

    I did a lot of research on cold climate buildings on piers and I eventually looked at what they do in Alaska. They build most of their houses up on piers because of melting permafrost and their primary problem is how to keep the water and waste pipes from freezing between the surface of the ground and the underside of the floor system. Then there's the problem of keeping the pipes WITHIN the floor system from freezing since they can be only a few inches above the floors bottom sheathing.
    What I decided to do for the pipes within the house was use 12 1/4" thick SIP floor panels on top of the foundation beams, build the exterior walls on top of the SIP panels and then build a raised floor within the exterior walls in which to run the pipes. All the pipes are now within the conditioned environment.
    For the pipes between the ground and the house I built a utility shed on a concrete pad next to the house (the exterior walls were connected to each other via an insulated "picture frame" box) and brought the underground pipes up into the shed and then through the attached side walls and under the raised floor system. The pipes are never exposed to the outdoor temperature - there's a small direct vent propane heater in the shed that is connected to a thermostat the comes on if the shed's indoor temperature gets too cold.

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