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Community and Q&A

Foundation dimple mat

oldbungalow | Posted in Green Building Techniques on

Hi, in the article on dimple drainboard,

it describes how the dimples (presumably facing in) create an air space between the backfill and the foundation wall for water to freely flow. I’ve seen products that have the dimples facing out, with geotextile filter fabric facing out. Is this wrong? For example-
which says “dimples and filter fabric always face backfill”
or this

what happens when water gets behind this kind of installation, can it still free-flow without the air gap?

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  1. KauaiBound | | #1

    Dimples face out. But wouldn't really matter so long as you created a drainage plane for water to escape to your drainage system around your foundation

    The drainage mats really aren't necessary so long as you provide a free-draining backfill material (like WASHED drain rock with a suitable filter fabric to prevent fines from contaminating/clogging drain rock/drainage pipes).

    Synthetic drainage mats were originally used to avoid importing and placing costly drain rock and to protect applied waterproofing barriers during backfilling.

    Personally, I'm not a fan of drainage mats - they are relatively costly to buy (there are some less expensive, but quite flimsy low-end products out there) and a struggle to apply - back in the day we used powder-actuated nails to affix the mats. Some use spray adhesive. Some prop the mat against the wall and use the weight of the backfill to hold them in place. A termination bar at the top of the mat is needed to prevent dirt/debris from getting behind the mat.

    If you don't use drainage mat and are applying some sort of waterproofing on your walls, need to consider how to protect them against damage when you backfill.

  2. Expert Member
    Peter Engle | | #2

    With a mat that has no scrim, the dimples face in, creating an air space between the mat and the waterproofing on the foundation. If the dimples faced out and the waterproofing was sticky, there would be no real airspace. The mat would still help protect the waterproofing and add to its effectiveness, but that's about it.

    If a dimple mat has a scrim layer, then the dimples can face out. The airspace is created between the mat and the scrim layer, and the mat itself still protects the waterproofing layer.

    You still want to use a freely-draining backfill if possible and like Kauai says above, use a termination bar at the top of the system to both hold the mat in place and prevent soil, debris and water from getting behind the mat.

    Drainage mats are just one option here. Rigid insulation board (fiberglass or rockwool or more exotic materials) is generally free-draining and stiff enough to resist soil pressure, while adding much-needed insulation to the exterior side of the foundation.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi K T.

    You may find this article helpful: Using a Dimple Mat to Keep a Basement Dry.

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