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Tunnel mat as a basement floor drainage plane in CZ5

Zampano77 | Posted in General Questions on

I am in the process of preparing to finish my old basement in order to utilize it for dry storage. I have gotten most of the design details from contributors here. My questions are:

1.) What is the preferred type of foam and facing to use on the walls that will act as an airtight barrier ?and drainage plane for the leaky stone walls. All types of Used foam is avail here in WNY. What should the vapor permeance of the product and how does that permeance work in the stone foundation wall stackup? Can it be 2 top 3″ of unfaced EPS, or is XPS the best choice?

2.) Would like to not have to excavate for an inside of the foundation wall drain tile and was hoping to use a sealed boat of 3/4″ HDPE tunnel mat to route all wall and floor water flows to airtight sump. Does anyone see a problem with this solution ( attached is a drawing of proposed setup).

Hoping to get an answer so I can get the tunnel mat ordered if needed or should I gird myself for the task of perimeter drain excavation.

All input appreciated

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  1. Zampano77 | | #1

    Not shown in attached drawing is a foam block on top of the stone wall to seal the mud sill to the top of the foam wall board to air seal the insulation system as prescribed my Mr. Maines in a previous post.

  2. user-2310254 | | #2


    Does the water enter from the foundation wall and the existing slab? If it's through the foundation only, wouldn't it be simpler to install a perimeter drain connected to the sump?

  3. Expert Member
    Michael Maines | | #3

    John, you now have three threads on essentially the same topic. it would be easier to keep track of your project if you kept additional, related questions in the same thread. This board is fairly active so if you don't get an answer just post a "bump." When you post overlapping threads like this one we all lose track of the situation. A moderator can delete the "bump" when they notice it.

    As I recall, you have a leaky stone foundation and want to use rigid foam instead of sprayed foam on the interior. I advised you on the pros and cons of various foam types. All of them can be used as air barriers. Technically light-density EPS may not be a good air barrier, but if you choose a higher-density product or a foil-faced product like Amvic Silverboard, it can be taped as an air barrier.

    The lower the permeance the better, in your case--you want moisture to stay on the foundation side of the wall. The lowest permeance materials have foil facings.

    I don't see anything wrong with your drainage solution, as long as there is not significant water flow. If there is, the system might clog.

  4. Zampano77 | | #4

    Michael and Steve,

    Thanks for your replies. I understand now about keeping posts related to a single project on a single thread, my apologies for the scattershot approach. The only forums that I have ever visited prior were automotive questions which tend to be episodic and do not need a continuing dialogue. I also now know what " bump" means, never knew and now I do, thanks also.

    As for the perimeter drain, my foundation has full height stone walls intersecting the foundation at three points along the perimeter ( another foundation wall from a previous iteration of the house as well as a cistern wall ) , and would require digging below them to have a continuous drain tile to the sump. The work for a single person seems too much as it is just me. I was hoping with the tunnel drain I could forgo the large amount of labor and materials needed by installing the matting.

    Does the airgap that is created by the metal channels to affix the foam create any kind of problems ( around +/- 1" of air) if it is properly detailed and sealed against infiltrating the inner envelope? I know the SPF sprayed debonds from the rough/ dusty walls and allows water to flow behind, does a larger airgap cause any forseeable problems?

    Other than that I appreciate all input and find the GBA site and its contributors to be invaluable to me on all aspects of green building.

    Thanks again

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