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It seems there are two main product types for house foundation dimple boards with fundamentally different approaches.

  1. A naked hdpe sheet where the dimple peaks are installed facing wall. Hydrostatic pressure builds on exterior of plastic and any intruding water will run down the air gap between the plastic and wall to the drain. enter image description here
  2. An hdpe sheet that has a layer of filter fabric affixed to the peaks of the dimples. The sheet is installed so that the peaks/fabric face outward. Hyrostatic pressure is on the fabric. Intruding water will run down the plastic sheet. There is no longer an air gap between the plastic and the foundation. enter image description here

My questions is what would be the benefits and additional considerations of one approach versus the other? It seems the fabric approach would reduce pressure faster, but if the fabric ever clogs or fails then the system loses effectiveness.

2 Answers 2

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Neither your Option #1 nor your Option #2 are ideal.

Note: Mastic on the wall is used to keep moisture OUT of the wall, not the HDPE membrane.

Note: The HDPE membrane is used to provide an air gap at the wall so moisture can drain DOWN the wall. It eliminates hydrostatic pressure so moisture is not forced in through the wall causing a leak.

Your Option #1: Without a filter fabric the system will clog in a short amount of time. (Btw, you didn’t mention having a mastic waterproofing applied first, but I assume you understand it’s required.)

Your Option #2: Installing the HDPE sheet with the dimples away from the wall allows the membrane against the wall and defeats the purpose of the dimples. (With filter fabric applied to the dimple side of HDPE sheet, it will flex to infill the dimples and moisture will create the hydrostatic pressure it’s designed to eliminate.)

Recommended Option: Install the mastic waterproofing on the wall, then install the HDPE sheeting with dimples toward the wall to create an air gap that allows moisture to drain down the wall to the perf pipe, then install the filter fabric to keep soil particles out of the system and extend the longevity, then CAREFULLY backfill with drainrock. (We use a 1” fiberglass board so rocks can’t accidentally puncture the fabric and membrane.)

Likewise we wrap the perf pipe with filter fabric to insure the moisture that drains down the wall reaches the perf pipe and soil does not plug the perf pipe.

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  • Yes, i understand a sealant would be required, but i wasn't trying to document the full system, but rather provoke thought and generate feedback specific to the comparison of dimple out vs dimple in. Commented Jun 28, 2020 at 22:48
  • @BillTarbell asking dimple out v dimple in shows you don’t underdstand the function.
    – Solar Mike
    Commented Jun 28, 2020 at 23:02
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    @SolarMike i'm not sure that you fully read the question or have sufficient knowledge on the topic - The dimple out implementation is a common approach whereby filter fabric is adhered to the dimples by the manufacturer and is in direct contact with the backfill. There are a great number of sites online for you to learn more about this type of product, but here's one that was near the top of my google search: berkeysupply.com/sheet-drain.html Commented Jun 28, 2020 at 23:13
  • The logic against option #2 makes sense. However, if it's true, why is the product manufactured this way and the mfgr recommends installing it this way? Surely they don't intentionally recommend installing their product in a way that would lead to product failure, warranty claims, and potential class-action lawsuits, do they?
    – FreeMan
    Commented Jun 29, 2020 at 14:40
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I'll wager the two product styles address different needs. The sheet with the integral filter fabric installed facing the soil might work well in a soil that does tend to get saturated with water. It provides a drainage channel for that water to run down, rather than build pressure against the foundation wall.

The sheet with no filter, installed so the dimples face the wall, might be more appropriate for a soil that already has decent drainage (a more-or-less sandy soil). Here the sheet would provide a drainage path for whatever water does find its way to the wall, as might happen with rain or irrigation overspray.

Part of the difference might be explained by something simple, such as patent encumberance of one style leading to competitors making the other style.

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