Dimpled mat fatigue over time?
I’m insulating an existing basement slab using rigid foam and a dimpled mat.
I have an uninsulated concrete slab that has experienced previous flooding and I would like to now insulate the slab. While the sources of the water issues are believed to have been repaired (new French drains, dramatically improved grading and gutters, replaced rotted thresholds, sealed leaks in egress and windows, etc…), I don’t want to take any chances and I want to plan for future water issues. So my question is regarding the recommended layers when building out an insulated slab with a dimpled mat. With that said, in the current slab I have used a concrete saw to cut many grooves that direct water to a newly installed submersible sump pump. My plan going forward is to put a dimpled mat with rigid insulation, followed by ¾” subfloor – as recommended in the ‘Insulated Slab Floor’ diagrams in GBA’s Strategies & Details section of the website.
So my question is: does the dimpled mat go UNDER the 2×4 PT sleepers – or – between the sleepers? I ask because I would think that over time the combination of the dimpled mat placed directly under the sleepers may cause the subfloor to squeak as the mat gradually fatigues.
Thanks!
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Replies
Ben,
I have reviewed the three "Insulated slab floor" details in GBA's detail library, and none of them include a dimpled mat:
https://www.greenbuildingadvisor.com/cad/detail/treated-2x4-sleepers-one-layer-1-12-rigid-insulation
https://www.greenbuildingadvisor.com/cad/detail/treated-2x4-sleepers-two-layers-1-12-rigid-insulation
https://www.greenbuildingadvisor.com/cad/detail/mechanically-fastened-subfloor-over-1-rigid-insulation
So, to answer your questions:
1. Are you sure you need a dimpled mat? If you have used a concrete saw to cut drainage grooves in your existing slab, you may not need one.
2. If you really want to use a dimpled mat in this application, I would skip the sleepers. You could probably install a dimpled mat, followed by rigid foam, followed by 3/4" plywood secured to the old slab with concrete fasteners. However, I'd check with the dimpled mat manufacturer on the advisability of your plan before proceeding.
Thanks Martin. You are correct - there are no dimpled mats in any of the libraries, perhaps that's a suggestion to pass along to create.
1. No I am not sure I need a dimpled. I thought that the mat was playing an ultrasafe approach. I thought the mat would provide a better barrier but it sounds like it may be overkill considering I already made drainage grooves in the concrete slab.
2. I try to practice the KISS principle so if I don't have to use it, I won't. Also, thank you for your thoughts regarding options on this.