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Community and Q&A

Conditioned Crawlspace with Helical Piers

Deanna | Posted in GBA Pro Help on

I live in Ottawa. I lifted my house onto helical piles and created a skirt for the crawl space. The City is creating road blocks and isn’t satisfied with my solutions to the problem they identify. They want the heated crawl space vented (I have an HRV by code in the house and offered to include the crawl space). I read here that venting is a bad idea! The City wants a continuous building envelope and we suggested to perhaps spray foam the floor on top of the gravel sitting on poly, in addition to the skirt frame cavities. The skirt framing is currently affixed above to the beams and rim joist, and is 3″ off the ground. Cement board is on the exterior frame sitting just below ground level.  The City thinks the cement board will push up during frost heave and move the house. It’s 1/2 ” board! They asked for a slip joint for movement, and I think dimple board will avoid frost heave (not pictured), however we think by adding rigid foam on the inside – not attached to the frame – the floor can move independently of the frame, while maintaining a continuous envelope by having the floor spray foam affixed to the rigid insulation. The City also wants batt insulation under the skirt frame – under where it sits 3″ above the ground. I say it is a bad idea and mice love it. The City doesn’t know what’s best and want me to provide a solution. My structural engineer says my house on plies will not be impacted by skirting moving from frost (my property is all sand). A geotechnical engineer said I’m crazy to spray foam the floor, as it will be heated and never heave (except it was suggested by us to the city only to keep a continuous envelope!). Plus he said that sand expands the least – so bonus for me. This drawing is what I have so far. We also think waterproof paint on the cement board will appease their concern for water being wicked by the cement board during rain, but also the dimple board wrapped underneath will keep moisture from being wicked up the board. Tyvek ends above the drip edge. The City thinks Tyvek should continue past the drip edge and down behind the cement board…but that would just keep the cement board wet if water ran down. Does anyone have advice? Apparently this is the first ever crawl space not built on concrete foundations 😉

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  1. Expert Member
    DCcontrarian | | #1

    What was the motivation for using helicals? Was it because the soil doesn't have the bearing capacity?

    There's two ways to do a crawl space. One is to make the walls and ground the building envelope. The other is to make the underside of the floor above the building envelope. With a crawlspace on piers of any kind it's tough to do a conditioned crawl because you're starting without walls.

    How do your piers come out of the ground? Does the house sit directly on them? My house has piers for part of the house but there's an 18" block of concrete sitting on the ground with the top of the pier embedded in it and the house sits on the block.

    1. Deanna | | #8

      The house was sitting on concrete blocks since 1948. I was quoted 40K to lift and move the 540 sq ft house so I could install a foundation or slab. We settled on concrete piers, but the engineer had a better idea to use helical piles...for which the city issued a permit. The city never asked what I was going to do with my crawl space. In hindsight, I wish they did. The house is sitting on 3 ply lvl beams which sit in saddles at the top of the helical piles. The piles were drilled directly into the ground (sand). Helical piles were faster than concrete piers that we were going to dig. Based on what I need, I cannot spray foam the ceiling of the crawl space since I have my mechanical equipment, water pump, and plumbing in the crawl space.

  2. MartinHolladay | | #2

    Satisfying your code officials probably won't be easy. You might want to read this article: "Crawl Spaces vs. Skirts."

    1. Expert Member
      1. Deanna | | #9

        Thanks - it's very detailed. My mind is blown. Makes me wonder why leaving out the crawl space details wasn't a red flag with the city. There is a lot here to wrap my head around. In hindsight, I should have razed the house and started from scratch...on slab. (I'm in a flood zone but not in the 100 year flood plain, so thought it was smart to sit higher than the ground on piers).

  3. maine_tyler | | #3

    "The City wants a continuous building envelope"

    I would agree with them there. The question is where are you envisioning the envelope? It's not really clear from your description where, in your design, the envelope line is. I cannot see the picture as it's too small.

    DC said it:
    "There's two ways to do a crawl space. One is to make the walls and ground the building envelope. The other is to make the underside of the floor above the building envelope."

    1. Expert Member
      MALCOLM TAYLOR | | #5

      "There's two ways to do a crawl space. One is to make the walls and ground the building envelope. The other is to make the underside of the floor above the building envelope."

      And the second one presupposes there are no exposed services in the crawlspace, and that there is an insulated core to bring plumbing up from the frost line to the conditioned envelope.

      1. Expert Member
        DCcontrarian | | #6

        Yep. And in Ottawa that's going to be a real issue. And the core is going to have the same constructability issues that the crawl space walls have.

    2. Deanna | | #10

      Walls and the ground will be the only option at this point. But not sure if spray foaming the framing cavity and a foot inside the perimeter will create a continuous envelope.

    3. Deanna | | #13

      Do you think insulating my skirt frame with spray foam and spray the foam inside the perimeter one foot on the gravel create a continuous envelope??

  4. Expert Member
    Akos | | #7

    As others have said the issue here is there is no such thing in the code as an insulated skirt. So first step is to pick something that is in the code and work from there.

    You can insulated the floor, doesn't need any spray foam, batts with a layer of rigid under the floor joists works just fine. The rigid becomes the main air barrier which will make the inspectors happy. If you have critters, a layer of plywood over the rigid is a must.

    The other option is to build a proper foundation around the perimeter. I'm guessing you don't want to dig down to frost depth, you can look at a shallow frost protected foundation setup with something like a PT grade beam instead of concrete. The crawl stem walls can now sit on this grade beam and since the beam is protected from frost heave by the rigid wing/skirt insulation you won't have issues with movement. You would need to cover the dirt under the house with a heavy duty poly and tie it into the air barrier of the crawl walls. This crawl will need to be conditioned so that is probably a dehumidifier plus a baseboard heater. A stale air pickup to your HRV is also a good idea.

    You can also pass inspecting by installing just batts in the floors and install the skirting afterwards. In this case you will still need the poly and conditioning as above.

    1. Expert Member
      DCcontrarian | | #11

      This is why we need more information about why helicals were used. Here in DC, there's a strict tree preservation law, you can't damage the roots of a mature tree, so helicals are often used for additions in the root zone of a tree. But if you do that you can't do an enclosed crawl space, that's going to kill the tree too, you have to leave it open.

      If the reason for helicals was that the soil can't bear a conventional foundation then you can do a stem wall, but it needs to be tied to the helicals so it can't move independently, this seems to be the issue the inspector flagged. What we do in that situation is grade beams, which are 18x18 blocks of poured-in-place concrete between the helicals. These are reinforced with rebar and the rebar is tied to the helical and then the whole thing is encased in concrete. So the grade beam is tied to the helical and can't move independently. If you need more height you can go higher with concrete block on top of the grade beam.

      But all this needs to be done before the house is built. It sounds like Deanna already has a house.

    2. Deanna | | #12

      I think if it was July, then I'd just do the crawl space ceiling to pass inspection and do the skirt later. My time is running out - we're headed closer to winter now. Sigh.

      1. Expert Member
        DCcontrarian | | #14

        You said you talked to an engineer who said that sand heaves the least. Could you get him to quantify that? It's possible to design a skirt that allows the floor to move a certain amount.

        I'm thinking you ring the floor with 4x6's or 6x6's. The skirt hangs from the underside of the house with an air gap between the bottom of the skirt and the ground. The skirt rests against the side of the 6x6's, but isn't attached. The floor is a layer of foam board with two layers of 1/2" treated plywood over it, joints offset and screwed together and screwed to the 6x6. The whole floor and rim can float independent of the house.

        How much r-value do you need in the walls? I'm thinking 2x6 walls filled with rock wool. I'm trying to think of a sheathing for the walls that is allowed to be that close to the ground and I'm drawing a blank.

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