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Community and Q&A

Lap Siding Over Plastic Drainage Mats (Delta Dry, Sure Cavity, Gravity Cavity)

treigle | Posted in General Questions on


Does anyone here have experience installing lap siding over plastic drainage mats (not mesh) such as Delta Dry or MTI’s Sure Cavity or Gravity Cavity? If so, how did it turn out? Did the mat compress noticeably around nails, causing the siding to look wavy?

I’m considering installing LP Smartside lap siding (probably back-primed) over a rainscreen, and I like the idea of the added back support that a plastic mat would provide the siding compared to furring strips. I realize this support isn’t really necessary, and that I’d have to mark stud positions carefully if using a mat. My main concern is that maybe the mats are a bit squishy (I can’t handle them locally without ordering a roll or pallet).

I realize most people just use furring strips under lap siding to create a rainscreen. Are there any major disadvantages, other than added cost (and possibly installation time) to using these plastic mats under lap siding?

Thanks much!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Treigle,

    If you will not be installing exterior insulation, there is no reason not to use an entangled mesh drainage mat behind lap siding other than the things that you noted: cost, labor/time, difficulty of finding stud locations, etc.

    If you will have exterior rigid foam, Tara Murray from Benjamin Obdyke told me that they'd want you to install nailbase outside of the foam before using one of their mesh style rainscreen products. I have not checked with other manufacturers but I would imagine they'd reply similarly as this is about fastening the siding adequately.

  2. taramurray | | #2

    Hi Treigle,
    Based on this application and your concern on squishiness, we'd steer you to our Slicker MAX product. It has a filter fabric with rainscreen attached. It's installed with the fabric facing out, giving you a more compression-resistant nailing surface...and also lays down a nice chalk line for plank as well.

    Also, to clarify on the exterior insulation application - in terms of layering we would recommend Sheathing > WRB > Slicker > Exterior Foam > Furring strips (nailbase) > Siding.

  3. Expert Member


    The products you are considering are impermeable, which means unlike most rain-screens you will not have any drying to the outside. Depending on your wall assembly, that may be a problem.

    1. GBA Editor
      Brian Pontolilo | | #4

      I should have started my first reply by mentioning that I don't know of builders using any of those rigid rainscreen products for lap siding installs, even though manufacturers say that it is an acceptable application, which is why I jumped to the possibility of a mesh product.

      But I'm not sure if you are correct about the outward drying Malcolm. The products themselves are impermeable, which prevents inward vapor drive from reservoir siding materials like stone and stucco, but they create ventilation channels between themselves and the sheathing/WRB, which allow for some outward drying. And, some of the products, like the ones from MTI are perforated for permeability.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        Brian, maybe I should have qualified my reply. A typical rain-screen has the sheathing drying through a WRB of somewhere in the region of 70 perms into a ventilated cavity. These cavities can also dry through the siding.

        The drainage mats like Delta Dry, have some of the attributes of a rain screen, in that they provide a capillary break, and drainage plane, but they don't provide a significant path or mechanism to move moist air to the exterior, and being impermeable do not allow drying through the siding. Is there some drying to the outside? Sure, but perhaps not enough to be relied on as a strategy for walls that can't dry to the inside. You would have to be very careful what level of vapour retarder you chose.

        My take is you have gained a bit of backing for your cladding - which really hasn't been seen to be a problem when installing over furring - at the expense of a much less effective rain-screen.

        1. GBA Editor
          Brian Pontolilo | | #6

          I agree with you, Malcolm. And I would look for more certainty in a vapor-open exterior assembly if it were my project, which again is why I went right to the mesh products as a better potential option in my first reply to this question. (By the way, the mesh products meet the same standards for compressive strength as the rigid products, I believe.)

          I just thought that I should mention, to be fair to the manufacturers, that while the materials may be impermeable, the rigid products are designed to allow for outward dying of sheathing into the rainscreen gap and bi-directional drying of the siding, outward (obviously) and inward, again into the materials' corrugations or dimples, etc.

          But as you pointed out, there is no drying of the sheathing or wall through the siding. And how well the theory of this design actually works, I don't know.

          Thanks as always for your thoughtful responses.

          1. Expert Member
            MALCOLM TAYLOR | | #7


            Recently I've noticed a bit of creep in what manufacturers are recommending these sorts of products for. Mesh mats for rain-screen walls are suggested as suitable underlayments for metal roofing, and dimple mats seem to be following the same path by marketing themselves as Rain-screens - both without much I can see in the way of solid data to suggest they are useful in the new situations. They may well work better than I think, but being risk adverse, I'd rather stick with what experience has shown to be effective.

  4. treigle | | #8

    Thanks everyone!

    Malcolm: Your point about drying through the siding is a good one. I hadn't considered moist air stagnant behind the drainage mat. I had only thought about how liquid water on the sheathing could drain out between the dimples of the mat (I planned to use Cor-A-Vent above and below the drainage mat).

    Brian: Your point about the mesh is also good, but the one drainage mesh I handled in person was awfully squishy. I wouldn't hesitate to use it under shingles, but I wouldn't use it under lap siding. That product was NOT Benjamin Obdyke's homeslicker, however, so I'll take a look at that.

    Overall it seems that furring strips may be the most reasonable solution. The flexure of even thin lap siding supported every 16 inches is fairly minimal, although my OCD nature compels me to investigate a few more options. I could fur between studs to add more support. I might build a mock-up wall with lap siding nailed over one of the drainable house wraps to see how they perform, but I'm not quit convinced.

    One benefit I see of using a vapor impermeable drainage mat is that it provides an added layer of protection for the sheathing from outside moisture in case of siding leaks or failure, but I suppose this is a fairly negligible benefit with a rainscreen wall, and as Malcolm pointed out, may be detrimental if your walls can't dry to the inside.

  5. Expert Member
    Peter Engle | | #9


    I think you're missing a major moisture removal mechanism - condensation. I have seen quite a bit of condensation on the inside of dimple mats used behind siding. Moisture from the wall system moves through the sheathing/WRB as vapor and condenses on the cold (in winter) dimple mat. The dimples let it run down and drain. It makes more of a double-sided rain screen. And, since the moisture is condensing and running down, it is being removed from the wall system, probably as effectively as drying through the siding. Plus, you have the advantage of not soaking the siding and potentially damaging the paint. I'm seeing (and using) dimple mats behind stucco/stone claddings for just this reason. The impermeability prevents vapor drive in both directions, and whichever side gets wet is still able to drain. Add a bit of venting top and bottom and it seems like a nearly bulletproof solution.

    But back to the OP, on my own house I've got furring strips behind the clapboard and Slicker behind the wall shingles. Those are currently my favorite treatments.

    1. Expert Member
      MALCOLM TAYLOR | | #10


      You may be right, although I'm a bit fuzzy how that works. If enough moisture was moving through the assembly to create condensation in the small gap formed by the dimple board, wouldn't the space quickly exceed the moisture levels of the sheathing behind, and wouldn't that moisture then equalize with the wall by moving back through the WRB? Maybe I'm just not getting the mechanisms in play.

      Either way, one of the two sides gets short-changed. If the gap is between the WRB and the mat, the siding gets no advantage - which is one of the benefits furring affords, that you don't need with stone or masonry.

  6. Expert Member
    AKOS TOTH | | #11

    Maybe it is just me, but I'm really not seeing what benefit these products bring.

    For panel and lap siding, simple strapping makes for cheap rain screen that we know works great and lasts. I can't see spending money on anything else.

    I'm not familiar enough with stucco, from what I gather, a drainage mat there can be a real benefit, so that could be a good place. Even there, something that is vapor permeable makes a lot more sense.

    I've pulled off enough 100 year old cedar shingles with tar paper behind on walls to know there is no need for anything else high tech there. If painted, some back ventilation would extend the life of the finish, drainage mat in that case would a good option.

    1. GBA Editor
      Brian Pontolilo | | #12


      I’m with you when it comes to furring strips as the smartest option for many siding types and a mesh product for cedar shakes, particularly if they are going to be finished.

      And you are right that most experts agree that an air space behind stucco is absolutely necessary and these products are generally the best way to get there. I wrote about that recently:

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