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Musings of an Energy Nerd

Using a Dimple Mat to Keep a Basement Wall Dry

Dimple mats help reduce hydrostatic pressure near basement walls

Most dimple mats are made of high-density polyethylene. These semi-rigid three-dimensional membranes come in a roll. [Photo credit: DMX Plastics.]
Image Credit: Image #1: DMX Plastics

Homeowners with problematic basements often post questions on GBA. They generally complain that their basements are damp, and that basement moisture problems aren’t easy to fix.

Solutions to damp basement problems are only indirectly connected to the topic of green building. In recent years, however, green builders — that is, builders who focus on energy efficiency — have become experts on moisture problems in buildings, in part because insulation problems are intimately tied to moisture problems.

In short, green builders are more likely to build a dry basement than a run-of-the-mill builders — and in this way, building a dry basement has become a green building concern.

One elegant and simple way to reduce the chance of moisture problems in basements is to install a dimple mat on the exterior side of the basement walls.

What is a dimple mat?

A dimple mat is a sheet of semi-rigid plastic (usually high-density polyethylene) that comes in a roll. The plastic has an egg-carton shape, with protruding dimples that make the membrane fairly thick — generally between 1/4 inch and 1/2 inch thick.

When a dimple mat is placed against a concrete wall, the dimples make the plastic stand out from the wall, leaving a free-draining air space between the plastic and the concrete. If any water reaches this air space, it easily flows to the bottom of the dimple mat.

It’s part of a system

While a dimple mat helps convey water to the bottom of a basement wall, it’s not enough to keep a basement dry. The dimple mat is intended to be part of a drainage system that includes a footing drain (see Image #2, below). Ideally, the footing drain system includes perforated 4-inch piping surrounded by crushed stone and a filter-fabric “burrito.” The footing drain needs…

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  1. David Williams | | #1

    Probably Overkill
    I'm building my cabin (zone 6b) using Faswall blocks, a hybrid ICF, and these blocks are designed to work without a vapor barrier. My current plan is to construct the foundation wall, apply a parge coat (probably with Quikrete surface bond cement), coat that with Thoroseal, then secure a SuperSeal dimple mat up to grade, then 3" of foam board or Roxul insulation before backfilling. I'll be using metal siding above grade. My hope is that any water vapor moving out of the foundation wall will be able to escape up or down through the dimple mat air space.

    The perimeter drain will be constructed as outlined in Martin's article and with the drain line laid in with the septic drain and daylighting slightly beyond the tank location.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to David Williams
    Faswall blocks are made from 85% mineralized wood chips and 15% cement. While you note your hope that "any water vapor moving out of the foundation wall will be able to escape up or down through the dimple mat air space," your hope is unlikely to be realized.

    Most foundations walls start out damp and stay that way forever. I assume that Faswall blocks are designed to handle moisture, especially if the manufacturer advises builders to use the blocks below grade. I advise you to get used to the idea that the below-grade wall will always be damp, with no need for drying in any direction.

  3. David Williams | | #3

    Thanks for the reply, Martin.
    Thanks for the reply, Martin. I guess I'm missing something. One of the supposed advantages of the Faswall block system is the ability of the block wall to serve not only as thermal mass but also as a "moisture sink" as well, providing a stable interior relative humidity as water vapor moves in, out, and through the block. Why would the foundation wall end up being any more or less damp than any other wall in the building interior?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to David Williams
    For an above-grade wall, the Faswall blocks are probably capable of acting as a moisture buffer. (I'm a skeptic, however, when it comes to the purported advantages of moisture buffering; for more information on this topic, see "Hygric Buffering and Hygric Redistribution.")

    When it comes to a wall that is below grade, I don't think that the Faswall blocks will behave in the same way. A below-grade wall is always cool and damp -- because one side of the wall is always cool and damp. Try as you might to isolate the Faswall blocks from the outdoor environment -- with Thoroseal and dimple mat, for example -- the high humidity environment on the outside of the wall will influence the temperature and humidity levels of the Faswall blocks that are below grade.

    But don't despair. The same can be said of a conventional concrete wall. If the interior and exterior of the wall are properly detailed, your basement will be dry and comfortable.

  5. David Williams | | #5

    Thanks. Martin. I'm going to
    Thanks. Martin. I'm going to relax a bit just knowing the environment I'm building in. I'm in south-central Montana at about 4400'. We get an average of about 14" of precipitation a year, the temps swing from -20+ to triple digits, and it's generally "drier than a popcorn fart" around here. Plus the "soil profile" at my location is about 6-10" of topsoil before moving into loosely consolidated, poorly sorted glacial till and sandstone/siltstone. Short of a biblical flood, I imagine I won't lose a lot of sleep over a wet basement. :)

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