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Is a drainage mat needed on crawlspace walls?

paula_builds | Posted in General Questions on

I’m researching dampproofing products for a 41 inch poured stemwall for a crawlspace (new construction).  This is a foundation set into a sloping hillside, in rainy Oregon.

I’m already planning the following:
-foundation drains – 4 inch pvc with holes down, set on gravel
-gravel backfill protected by landscape fabric
-backfill adequately sloped away from house
-a capillary break (UGL DryLok paint) on the footings to prevent moisture wicking up into stemwall.

So the product I’m eyeing is asphalt (Henry Non-Fibered Asphalt Emulsion Dampproofing 788), which I intend to roll or brush on.  I’ll do two coats, and make sure the result is as thick as advised (60 mil or 1/16 inch).

My question is–should I add a protective board or drain board after the asphalt?  Or just wait for it to dry and then backfill?

The fact that there IS a product called protective board makes me think that the asphalt might be damaged by the backfilling process (is that true?)

The other product that the store mentioned to me is ‘aqua drain’.

My friend who seems to know these things says that if you expect a lot of hydrostatic pressure, the aquadrain/dimpleboard may be useful.

I guess I’m just wondering
1)If the asphalt layer needs protection
2)Are there other ways to protect it?
3) How to determine if my situation calls for the aquadrain product in addition to the asphalt.  (maybe just on the uphill side of the house?)
4)If I do install the aquadrain/dimpleboard product, any tips on how?  The information sheet says washerhead fasteners or construction adhesive.  I’m wondering what adhesive would hold it up.
5)or is aquadrain or the similar products overkill for a crawlspace on a small house?  The videos I’m watching seem to be doing this for basements that need to stay totally dry.  I want a dry crawlspace but…

Thank you in advance!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Paula.

    Sounds like you are doing all of the right things already. There are a number of reasons why designers and builders choose to go beyond what is required when it comes to water management on basement and crawlspace walls. These may include the local climate, site conditions, design of the house, etc. For more on dimple mats, I suggest you read this: Using a Dimple Mat to Keep a Basement Wall Dry

  2. Jon_R | | #2

    I find it interesting that footing capillary break isn't mentioned by UGL as a use for their DryLok paint. When it comes to capillary movement, one pin hole will move a lot of moisture through a thin barrier. I'd hesitate to use it without some hard data (not anecdotes) as to how DryLok compares to other options (thicker asphalt or rubber based paint on, membranes (eg DELTA-FOOTING), concrete admixtures, gravel under footings) for this use. In any case, DryLok needs two coats.

    IMO, removing hydro-static pressure (ie an air gap) is far more effective than barriers. Certainly with walls, perhaps also when it comes to capillary rise into footings (ie, gravel under the footing with the drain below (not beside) the footing)). But the extra insurance may not be necessary.

    1. paula_builds2 | | #8

      I got the drylok = capillary break idea from a post by a builder whom I respect, but I'm open to suggestions otherwise.

      I agree with you about the air break being superior for the wall, but it doesn't seem like an air barrier is an option for a break between the footing and stemwall.

      Riversong wrote:
      "Whatever is used between footing and foundation must not compromise the mechanical connection that resists lateral displacement, or - for practical reasons - not interfere with the subcontractors setting the foundation forms. The material should also be able to resist some hydraulic pressure.

      Neither plastic sheeting nor asphaltic damp-proofing is acceptable. I use UGL DryLock latex masonry paint for this capillary break, as I do for general foundation waterproofing."

      1. Expert Member
        MALCOLM TAYLOR | | #10

        User ...147,

        Unless you pour the footings and foundations monolithically there is no mechanical connection between the two. It's a cold-joint. Lateral resistance is provided by either a V-groove formed into the footing when wet, or more commonly now rebar.

        That opens up a number of possible materials that Riversong rejected. They are listed here in Martin's blog:

        1. paula_builds | | #20

          Now I'm wondering if I even really need a capillary break above the footings. The footings are built on compacted gravel. Is that a capillary break already? Hmm. I'm mentally going back & forth between the DryLock that many of you have discouraged, and an asphalt product which I intend to use on the exterior already, or some other EPDM product.

          But with the gravel, and the fact that I'm on a hillside --hopefully my footings won't be in standing water....

          Should I just not worry about capillary break? Builders around here don't seem to even know about it at all.

          1. Expert Member
            MALCOLM TAYLOR | | #21


            You are right - capillary breaks are still very much a niche technique. Are they a good idea? Sure. Are they necessary? The vast majority of the houses being built today appear to suggest not always.

    2. qofmiwok | | #22

      Drylock has a lot of different products. Some people are using Drylock Extreme but the manufacturer adamantly states that it is not suitable for this purpose. They have another product which is. But be careful when using the term Drylock generically.

  3. [email protected] | | #3

    Why not just make it a conditioned crawlspace and call it a day? Then you don't have to deal with insulating the floor which is extremely time consuming to do correctly.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      I may have overlooked it but I can't see where in her post she says it isn't going to be conditioned.

      1. [email protected] | | #15

        Doh! I confused her question with another I was reading, after another read it was crystal clear she was talking about the exterior of the stem wall! 😂

        1. Expert Member
          MALCOLM TAYLOR | | #17


          Well at least you had a good excuse. I usually just miss the pertinent information that is right under my nose!

    2. matt9923 | | #5

      Im not following your logic? So we shouldn't insulate our houses because there conditioned?

      They are talking about exterior water management systems.

      What type of soil do you have?
      Any reference to the water table in your area?

      In my experience 90% + of water problems can be remedied from the surface. With that said I would err on the cautious side. You can use dimple mat at least on the wet side and free draining back fill. Or just free draining back fill. In my area we get bank run witch is a gravel sand mixture that works well for this task.

      1. paula_builds | | #12

        My soil drains well, slope is about 20% , and I'm on the side of a rather large hill, so the water table must be a lot lower than the house.

        1. Expert Member
          MALCOLM TAYLOR | | #13


          If you wanted to be absolutely safe you might include a dimple matt on the uphill face of the foundation. I don't see any point in having one on the sides or bottom.

          1. paula_builds | | #14

            Thank you Malcolm!

    3. paula_builds2 | | #7

      Hi Ryan,

      I had thought about something along those lines already after reading an article about sealing crawlspaces and supplying them with air from the interior of the home. I haven't dug deep into conditioned crawlspace as an option. I believe that would require insulating the interior of the stemwalls (which I'd be able to do). Would that also require insulating the floor of the crawlspace? (harder).

      Part of me agrees with the notion of a conditioned crawlspace, rather than the seemingly difficult task of keeping the crawlspace air from moving up into the house.

      1. Expert Member
        MALCOLM TAYLOR | | #11


        Many codes now require crawlspaces to be conditioned. This reflects the evidence that they perform a lot better than unconditioned ones in almost all climates.

      2. [email protected] | | #16

        I totally misread your post initially, but wrt going the conditioned route you would just need to put down a nice layer of poly on the ground and tape/detail all the seams and interfaces. Then add rigid foam (EPS likely as you’re in a wet climate, polyiso can absorb water and XPS hates the earth) to the inside of the stem walls. It’s a well detailed design in many codes and lots of good resources on it online. I think it’s way easier to do then properly insulating and air sealing the floor.

  4. Expert Member
    BILL WICHERS | | #6

    I would NOT use a paint as a capillary break. You need a solid material for that purpose that won’t have issues with efflorescence which will damage waterproofing paints. A common material for use as a capillary break is EPDM sheet. I’ve used HDPE sheet too, which is rigid and slippery and easy to slide into narrow gaps.

    Landscape fabric is not the right product to use to separate layers of underground materials. You want what is known as “geotextile fabric”. While landscape fabric is technically a type of geotextile fabric, it tends to be much less durable that products sold as “geotextile fabric”. You need something that will last for decades and won’t be damaged as you pile on fill materials.

    There is nothing wrong with asphalting waterproofing on the exterior of a foundation wall. That’s what’s been used for decades and it works and lasts. A dimple mat is added insurance and not a bad idea if you’re on a hill and are expecting water to run down the hill and “hit” your foundation wall. The dimple may will serve to relieve any water pressure against the foundation IF you have a functional foundation drain to remove any water that comes down the dimple mat.


    1. paula_builds2 | | #9

      Hi Bill,

      I'm interested in hearing your thoughts on the comments I posted above about the drylok.

      Thanks for clarifying that landscape fabric isn't geotextile fabric.

      I do wonder whether the water is expected to run down the front of the dimple mat, or behind it. That would change how I detail it with the foundation drain. Thoughts?


      1. Expert Member
        BILL WICHERS | | #19

        The paint may offer some benefit, but I doubt very much it would hold up over time and it would likely be damaged just during construction. If the foundation wall is keyed into the footing as it should be, the two concrete pieces will be locked together so a plastic sheet will not affect the ability of the wall to withstand pressure. There is a purpose-made gasket for this location that was recently written about right here on GBA but I cannot remember the name of the stuff at the moment.


  5. Expert Member
    Peter Engle | | #18


    It doesn't really matter whether the water runs down the front or back of the dimple mat. In reality, it's often a little bit of both. The mat should run down to or across the footing, with the footing gravel coming up a bit above the bottom of the mat. The geotextile fabric can be generally folded and tucked against the face of the dimple mat. You should also terminate the top of the dimple mat. Some manufacturer's make termination bars that fasten to the foundation. Some simply use a flashing. The point is to keep the top of the mat fastened to the wall and to prevent soil from filling the spaces in the back of the dimples. Do this before backfilling.

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