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Can I put poly under the underpinning to create a capillary barrier?

Jamie B | Posted in General Questions on

So I bought a century old Row home. and I’m about to pour the first sections of the underpinning.

Its sitting on very firm clay.

However, the groundwater can get pretty high now that I’ve dug lower, and I’m starting to see some water come in to the open pits. I’m not concerned about this as I’ll have a good weeping system in place.

However, what I am concerned about is Rising damp, aka capillary action. The basement will be living space, so I don’t want any moisture issues.

Usually in a new build, you pour the footings first, then you can make a key groove and continue with your sub-slab poly or use Delta Footing barrier to create a barrier between the footing and the foundation wall.

I am also made aware of Fast foot by fabform, a Canadian company which specifically creates a capillary barrier inherently by its design.

So with all this in mind, I was thinking of lining the back side and bottom of the underpinning forms with 10mil poly to create a sub footing capillary barrier. This means that the new concrete footings will be sitting on poly, instead of directly on the soil.

Does anyone see any issues with this?

I wonder how the building inspector will take this. To my understanding, the footing is supposed to be poured directly on “undisturbed soil” I’m afraid of the inspector not approving since it’ll be poured on poly.

Please let me know your thoughts, or if anyone has any experience creating a capillary barrier with underpinning.



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  1. GBA Editor
    Martin Holladay | | #1

    A sketch would be useful, to see exactly what you mean by "underpinning." It sounds like you are lowering the floor of a basement or crawl space in an older house.

    It would also be useful to know where you are located. (Canada or Europe, I'm guessing.)

    What do you mean by a "weeping system"? Do you plan to install perforated drainage pipe that leads to daylight? If so, will the perforated drainage pipe be at or slightly below the level of your new footings?

    To answer your question briefly: There isn't any reason why you can't install polyethylene under your footings. But unless you have a drainage system that includes perforated pipe leading to daylight, and unless that drainage system is installed below the level of the polyethylene, the polyethylene will be defeated by rising ground water.

    For a thorough discussion of these issues, see this article: Capillary Breaks Above Footings.

  2. Bob Irving | | #2

    poly is typically installed above the under slab foam insulation and directly under the concrete. The reason for this is that if water enters under the slab and leaves water above poly, it will take a long time to dry. Water needs to have a direct route to drain off.

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