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How to Handle Rigid Foam and Vapor Barrier Under the Middle of a Basement Slab with Load Bearing Footings?

jwolfe1 | Posted in Energy Efficiency and Durability on

When insulating a basement slab with a vapor barrier (e.g., 10 mil poly) that will go directly under the slab has a load bearing footing running down the middle of it how should you typically handle the vapor barrier and rigid foam insulation sheets? Virtually every visual example of an insulated basement wall only addresses the slab edge meeting the foundation wall. That part is straightforward. However, there isn’t much attention paid to load bearing footings in the middle of the floor on an insulated basement slab. The house I am building has a 26×40 ft footprint and will have a load bearing footing going down the middle of it for the full length. To detail the rigid foam and vapor barrier should I:

1. Just run the rigid foam and then vapor barrier like the rest of the floor covering the load bearing footings.

a.      With this method would I need a higher PSI rated foam such as NGX 400 over the weight-bearing footings than what is used under the non-bearing slab (e.g., NGX 250)?

2. Stop just the foam at the edge of the weight bearing footing and allow the vapor barrier to step down from the edge of the foam and cover the weight-bearing footing without a splice?  

3. Stop both the foam and the vapor barrier at the edge of the weight bearing footing and do a good job of taping the vapor barrier to the footing? 

4. Or is there another viable method?

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  1. Expert Member


    When we have a center bearing wall, rather than pour integral slab/footings, we form them up like smaller versions of the external ones, with a footing and short stem-wall on top. This allows you to use the same strategies as you would use elsewhere. The foam covers the footing but is stopped each side of the stem wall (which is poured flush with the slab), so you still get a small, fairly insubstantial thermal bridge.

    The other advantage to this is that the code here moves footings that are poured integrally with slabs, and those formed on compacted fill, out of Part Nine and into Part Four, meaning they require an engineer to sign off on their design.

    1. jwolfe1 | | #2

      Ahhh thank you. I did some searching and found examples of what you describe.

  2. John_F | | #3

    I am trying to figure out this same detail. In my case there are 12"x12" pads to support the floor system above. Do you keep the vapor barrier continuous under these pads?

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