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Community and Q&A

Foundation drainage mat (dimpleboard) not required?

Jerry Chwang | Posted in Green Building Techniques on

I am considering using mineral wool rigid insulation below grade on outside of the foundation wall. It would be exterior of the concrete wall, ICF form, waterproofing layer.

Because mineral wool drains, I wasn’t sure if dimple board (drainage mat) is needed and if it is, whether it sits in the foundation wall assembly.

Appreciate any advice on this, and also whether a piece of rigid insulation needs to be used as a ‘roof’ for the weeping tile.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Jerry,
    I have never installed mineral wool insulation on the exterior of a foundation. According to my understanding, however, mineral wool drains well. No additional dimple mat is required when using mineral wool insulation.

    Surprisingly, neither the Roxul web site nor the Thermafiber web site includes installation instructions for installing mineral wool insulation on the exterior of a foundation wall.

    I would welcome any advice from a GBA reader who has installed mineral wool in this location.

    Concerning your "weeping tile" question (which I assume refers to the perforated drain pipe installed near the footing): it is important to install crushed stone under, around, and above your perforated drain pipe, and to wrap the entire assembly (crushed stone and pipe) with landscape fabric to prevent silt from clogging the drain. This means that you need to lay down a strip of landscape fabric on the soil before placing your crushed stone or installing the pipe. Once these materials are in place, the landscape fabric is lifted and folded to create a "burrito" that protects the drainage system from migrating silt.

  2. Jerry Chwang | | #2

    Thank you for this. I have left a message with Roxul technical staff on this. Might be another benefit of their product.

    Yes, here in Toronto, we call it weeping tile (presumably from the days when the pipe was actually a series of clay tiles). I have forwarded the 'burrito' to my builder for consideration.

  3. Jerry Chwang | | #3

    Just got an answer back that the dimple mat is not needed. Anyone else actually try this though?

  4. Steven O'Neil | | #4

    I just installed roxul drainboard on my foundation directly over the waterproofing, no dimple mat. Roxul's sell sheet suggests that this is the way to do it. I can't comment on performance yet, however.
    Against convention, I made a burrito without the tortilla. I just used crushed stone and pipe.

  5. Nick T - 6A (MN) | | #5

    The builder of my home did just also. foam at/above grade...then mineral wool (nonRoxul iirc).

    not sure i would call it 'supporting data' ...but it is done.

  6. Jerry Chwang | | #6

    Thx everyone. I think I will try it.

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