Using a Dimple Mat to Keep a Basement Wall Dry

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Using a Dimple Mat to Keep a Basement Wall Dry

Dimple mats help reduce hydrostatic pressure near basement walls

Posted on Jun 22 2018 by Martin Holladay

Homeowners with problematic basements often post questions on 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 to lead to daylight or a sump equipped with a sump pump.

Footing drains are required by building codes, so most new homes have them. But builders should remember that a dimple mat won’t do much unless it is properly connected to a well-detailed footing drain system.

Before dimple mats were invented, many builders backfilled their basements with free-draining material like crushed stone or gravel; the free-draining material played the same role as a dimple mat.

Dimple mats aren’t required by code. If a builder has had years of success using crushed stone or gravel as backfill, there isn’t necessarily any reason to install a dimple mat.

Some geographical regions are blessed with sandy soil that drains readily. If you are lucky enough to be working in such an area, you may have no need for special backfill material or dimple mats.

Filter fabric is an available option

In addition to plain dimple mats, some manufacturers offer an enhanced version of a dimple mat — one with a layer of filter fabric bonded to one (or in some cases, both) sides of the dimple mat. If you use a dimple mat with filter fabric on one side, remember that the filter fabric should be installed facing the soil. (If the dimple mat has no filter fabric, the little round bumps should face the concrete.)

In theory, a dimple mat with filter fabric provides two layers of channels: one layer is between the filter fabric and the dimple mat, and the second layer is between the dimple mat and the concrete. This approach is supposed to provide redundancy. However, cynics (including building scientist Joe Lstiburek) assert that the filter fabric side of the dimple mat soon gets clogged with fine soil particles, so there is little or no benefit to paying extra for dimple mat with filter fabric.

Who makes dimple mat?

Manufacturers include:

Installation instructions

Dimple mats can be installed on most types of foundation walls, including poured concrete walls, concrete masonry unit (CMUConcrete masonry unit. Precast concrete block used to build walls. CMUs have hollow cores that can be filled with concrete onsite for additional reinforcement. The use of stronger, more lightweight types of concrete such as autoclaved aerated concrete (AAC) is becoming increasingly popular in CMU manufacture. ) walls, or insulated concrete form (ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure.) walls.

Most manufacturers advise that their dimple mats be installed with mechanical fasteners (concrete nails, Hilti fasteners, or TapCons), usually in conjunction with some type of proprietary washer. Different fasteners are required for CMU walls or ICF walls than for poured concrete walls; in all cases, builders can consult the dimple mat manufacturer for fastening advice.

In most cases, a dimple mat is terminated at grade. Most dimple mat manufacturers sell an accessory for this horizontal joint, variously called a termination strip, a termination bar, or flashing. This accessory is important: it’s necessary to prevent dirt from entering the air gap behind the dimple mat and clogging the drainage channels.

What if I want to install exterior rigid foam?

If your foundation includes exterior rigid foam, a question arises: Does the dimple mat go between the rigid foam and the concrete, or on the exterior side of the rigid foam?

Experts provide conflicting advice. The choice is yours.

If you decide to install the dimple mat between the rigid foam and the concrete, here’s what you need to know:

  • Fastening the dimple mat to the concrete wall will be simpler than it would be if there were an intervening layer of rigid foam.
  • The performance of the rigid foam will probably be slightly degraded by the presence of the air space between the rigid foam and the concrete. (Even below grade, there will be some air flow through the drainage channels of the dimple mat.) According to Joe Lstiburek, if you install dimple mat between the rigid foam and the foundation wall, it’s reasonable to de-rate the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the rigid foam by about 5%.

If you decide to install the dimple mat on the exterior side of the rigid foam, here’s what you need to know:

  • Fastening the dimple mat is complicated by the presence of the rigid foam. You’ll either have to use extra-long fasteners, or special fasteners (for example, the Delta-Grip Fastener from Dorken) designed to hold the dimple mat to the rigid foam layer.
  • While the dimple mat will probably be terminated at grade, the rigid foam needs to extend to the top of the concrete wall. That means that the installer needs to come up with a horizontal flashing system that takes these facts into account. In most cases, the rigid foam should be terminated at grade, at the same level as the dimple mat, so that Z-flashing can be installed at grade to protect the top of the dimple mat. After this Z-flashing is installed, an additional strip of rigid foam, spanning the distance from grade to the top of the foundation wall, should be installed. Finally, the above-grade rigid foam will need to be protected from UV light and physical damage by a layer of metal flashing, stucco, or some type of weatherproof panel. In most cases, additional Z-flashing will be needed at the horizontal joint between the bottom course of siding and the top of the rigid foam. (As an example of a flashing detail for this type of installation, see Image #3, below, which shows a detail provided by Dörken, a manufacturer of one type of dimple mat.)

When I asked building scientist Joseph Lstiburek where he preferred to see the dimple mat, his answer was succinct. “Put the dimple mat against the concrete,” he said. “The whole idea of the dimple mat is to protect the concrete, not the insulation.”

Chris Gallagher, the national sales manager for Mar-flex, a manufacturer of dimple mats, disagrees with Lstiburek. “The dimple mat belongs on the exterior side of the rigid foam, because the extruded polystyrene is there for insulation,” he said. “You want to get the water away from the foundation.”

When I asked Peter Barrett, a product manager at Dörken, where the rigid foam belongs in relation to the dimple mat, he answered, “In my opinion it doesn’t make that much difference.”

If the concrete is covered by a dimple mat, do you also need asphaltic dampproofing?

Building codes require that in most cases, a basement wall must be protected on the exterior with a dampproofing layer. In the 2018 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code., this requirement can be found in section R406.1.

According to a different section of the code, section R406.2, a small subset of basement walls — those built “where a high water table or other severe soil-water conditions are known to exist”— need to be protected by waterproofing, not dampproofing.

For readers unfamiliar with the distinction between dampproofing and waterproofing, here's a simplified explanation:

  • The classic example of dampproofing is an asphaltic compound applied to the foundation with a roller or a spray rig.
  • Examples of waterproofing include peel-and-stick membrane or an expensive liquid-applied compound that includes styrene-butadiene rubber.

According to Dörken's Peter Barrett, about 90% of single-family homes in North America are built on sites where dampproofing is adequate. Only 10% or fewer are built on sites requiring waterproofing. Barrett notes that dimple mats meet the code definition for dampproofing. So from a code perspective, a basement wall protected by a dimple mat does not need asphaltic dampproofing on the concrete.

Not all manufacturer’s reps agree with Barrett, however. According to Claudia Bravo, a representative for JDR Enterprises, the manufacturer of J-Drain 200, “The purpose of our product is to relieve the hydrostatic pressure. We advise installers to include a tar layer or an EPDM layer — whatever material is driven by the architect’s specification.”

Some builders, including Matt Risinger, a builder in Austin, Texas, believe in a belt-and-suspenders-and-rope approach to keeping basements dry. In a Journal of Light Construction article titled “Foundation Waterproofing That Works,” Risinger explains his approach: “Effective foundation waterproofing is more than just one product; it’s a system with three critical components: a membrane to protect the concrete; a drainage mat to relieve hydrostatic pressure and allow water to drain down, instead of in; and a French drain at the footing level to carry water to a daylight drain or to a sump pump.” Risinger advocates upgrading from asphaltic dampproofing to either a peel-and-stick membrane or a liquid-applied material that includes styrene-butadiene rubber.

With his usual bluntness, Lstiburek pooh-poohed the Risinger approach. “If you have a dimple mat, you don’t need waterproofing membrane,” Lstiburek told me. “You don’t need peel-and-stick. It’s a waste of money.”

I asked Chris Gallagher of Mar-Flex whether builders should install asphaltic dampproofing on a basement wall if the wall is protected by dimple mat. “That’s a loaded question,” Gallagher answered. “If you go to the eastern half of the U.S., most low-end builders just put on the dimple board, with nothing underneath.”

When I asked Peter Barrett of Dörken whether asphaltic dampproofing is required on a basement wall if the wall is protected by dimple mat, Barrett said, “No. Some people do that, but it’s not required.”

I asked Ross Carmen, a SuperSeal rep, the same question. He said, “The SuperSeal has an ICC rating. It’s approved as dampproofing. That means it’s approved for builders to do nothing to the wall. In my opinion, I would comfortable with just the dimple mat. I would just detail any wall penetrations with some type of sealer. People think you have to spray dampproofing on the wall, but if the wall cracks, the dampproofing doesn’t do anything.”

Dimple mats are used on the interior, too

To address water entry problems in a problematic basement in an existing home, some contractors install an interior French drain at the perimeter of the basement slab. The French drain directs water to a sump. In this situation, it often makes sense to install a dimple mat on the interior of the basement wall before installing rigid foam or spray-foam insulation.

For more information on this topic, see Fixing a Wet Basement.

And sometimes on floors...

If you are installing finish flooring over a basement slab, you may want to install a horizontal dimple mat between the concrete slab and the OSB or plywood subfloor.

A full discussion of the use of horizontal dimple mats under subfloors in basements is beyond the scope of this article. Whether this approach makes sense depends on many factors, including the condition of the footing drain system, whether or not there is a layer of polyethylene under the slab, whether or not the slab is insulated, and the local climate.

Correcting drainage problems is expensive

If you are wondering whether the cost to install dimple mat on the exterior side of a basement wall is worth it, you should remember that repairing problems in two or three years will be far more costly than the relatively minor expense of installing dimple mat. Ideally, once a foundation is backfilled, no one should need to excavate down to the footings ever again.

If you compare the cost of the dimple mat to the cost of hiring an excavator and repairing landscaping, dimple mat is cheap.

Martin Holladay’s previous blog: “Where Can I Find Good Advice?”

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Tags: , , ,

Image Credits:

  1. Image #1: DMX Plastics
  2. Image #2: Fine Homebuilding
  3. Image #3: Dörken

Jun 26, 2018 10:59 AM ET

Probably Overkill
by David Williams

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.

Jun 26, 2018 11:08 AM ET

Response to David Williams
by Martin Holladay

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.

Jun 26, 2018 11:24 AM ET

Thanks for the reply, Martin.
by David Williams

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?

Jun 26, 2018 11:50 AM ET

Edited Jun 26, 2018 12:24 PM ET.

Response to David Williams
by Martin Holladay

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.

Jun 26, 2018 12:01 PM ET

Thanks. Martin. I'm going to
by David Williams

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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