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Community and Q&A

Capillary break in deep footing way below slab level?

Jerry Chwang | Posted in Green Building Techniques on

I have a deep footing (7ft below bsmt grade) and foundation wall on south side of my excavation due to poor soil there.

Do I need to put the capillary break between footing and foundation wall down there, or does it make more sense to put the paint on the inside of the foundation wall, just below the slab grade?

Am also insulating under slab, and gravel beneath that. I understand that the gravel is a capillary break as well, so do I need any capillary break at all on the foundation wall?

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  1. GBA Editor
    Martin Holladay | | #1

    Just because you have a capillary break under you slab, doesn't mean you don't need one between your footing and your foundation wall.

    A capillary break is cheap -- as simple as a coat of elastomeric paint. I think you should definitely include a capillary break between the footing and the foundation wall, even if your footing is very deep.

    Of course, you should also include a capillary break at the top of the foundation all; closed-cell sill seal is the usual product to use between the foundation and the mudsill.

  2. Jerry Chwang | | #2

    Thanks Martin, I just thought it would make more sense to have the break closer to the slab / foundation wall contact point. Also, my contractor is ready to lay down concrete block on the footings, but easy enough for me to tell him about the paint. I thought it might need to be some kind of special material, but masonry paint is all I need, eh?

    One other thing regarding location: If there were rising water table during the spring for example, the break at the footing would be completely bypassed...I think? but I suppose I could do both: paint the top of footing / foundation interface, and the inside of the foundation wall where it touches the slab. Or maybe I don't need the slab part.

  3. GBA Editor
    Martin Holladay | | #3

    Usually, there is some vertical rigid foam insulation at the perimeter of the slab. That provides your capillary break at the slab perimeter.

  4. Jerry Chwang | | #4

    I think that might go back to a question I asked in another Q&A (or blog...can't remember): Whether the slab is structural in bracing the foundation walls from earth load. If so, they should be in contact with the foundation walls.

    My engineer wants this, but it doesn't seem clear though, because many builders use XPS around the perimeter as you said. To make matters more complicated, in my application, I am using something called Durisol ICF. Its a wood fibre / cement mix pre-formed block that is filled with concrete. It provides some insulation value and would be in contact with the slab. I'll probably end up putting some rigid mineral wool there.

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