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Community and Q&A

Overpour or not?

IHeartSikka | Posted in Green Building Techniques on

I’m preparing to build a super-insulated spec house (R-36, R-60) in far northern Wisconsin. The site has an insulated 28’x40′ slab-on-grade already there from a previous owner, and I’m pondering the best way to incorporate in-floor radiant heat into the house. (There is no need to inform me that most energy-interested folks think that in-floor heat is overkill for a super-insulated house.) I just built and sold a similarly-insulated house with a heated slab and no heat source on the second floor, and am looking to do that again.

The two methods I’m considering are these:

1) Put down 4″ sleepers for all main-floor walls; put 3/4″ XPS around the sleepers; fill in the floor area with 2″ of XPS, staple down 1/2″ PEX and 2″ of concrete. I like this method because it doesn’t involve the heating of a monolithic footing because the heated concrete will stop at the inner edge of the exterior wall, and the relatively small amount of mass will minimize the slingshot heat effect my last house suffers from while still enabling me to have a stained concrete floor, which will go well with the industrial/modern interior design.

2) Put down 3″ sleepers for all main-floor walls; put 3/4″ XPS around the sleepers; fill in the floor area with 2″ of XPS and 1″ of particle board or other sheet product; route out tracks for the 1/2″ PEX in the particle board and cover it all up with some type of engineered flooring. I like this method because it involves heating very little mass, which I assume will make the system more responsive. And because I won’t have to pour concrete.

I welcome any thoughts on these or other assemblies. Please keep in mind that I’m not trying to build the absolute most efficient house possible, I’m trying to build something that balances good energy performance with UPFRONT cost and sale-ability.

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  1. Expert Member

    Josh, A lot depends on how the existing slab is insulated. If it is already adequately insulated you might only need to add another bottom plate to all the walls and then use either a wood or concrete based system.

  2. davidmeiland | | #2

    What is the heating fuel?

  3. GBA Editor
    Martin Holladay | | #3

    Which ever method you choose, make sure that your heated slab has adequate vertical insulation at the perimeter -- a critical area. If I understand correctly, you are planning to use 3/4-inch vertical XPS for this purpose. That would provide only R-3.7, which isn't enough.

  4. IHeartSikka | | #4

    Mr. Meiland: heating fuel: propane burned in a Navien combi boiler. Mr. Holladay: perimeter insulation: existing slab has 1 1/2"vertical XPS around the perimeter, "tucked in" to the 28x40 footprint; I plan to put 3/4 vertical XPS on the inside of the2x6 exterior walls (and around all the interior walls, just so I'm not heating the aforementioned sleepers); another 3.5" of continuous XPS on the outside of the framing, extending down to the bottom of the existing slab footing insulation.

    So the existing slab will have R-25 around the perimeter, but what I perhaps didn't adequately convey is that I want the new slab/engineered wood subfloor to be thermally separated from the existing slab. If I did an overpour, the new, 2" slab, which will have 2" XPS between it and the existing slab, will have the R 3.7 of the vertical XPS on the inside of the sleepers, the roughly R-5 of the sleepers themselves, and the R-17.5 of the 3.5" XPS on the outside of the framing, making for R-25+.

    That might all be a little confusing, but I think I'll be okay at the perimeter. What I'm interested to know is the effect of having the PEX run in 2" of concrete versus routed into some type of engineered wood product--basically, a poor man's Warmboard.

  5. user-2890856 | | #5

    A little late to this discussion but maybe not too late . Take a look at this product Josh . I use their plywood (3/4) for 1/2 tubing with the graphite coating and the response is phenomenal .. Inexpensive and should suit your option # 2 scenario very well with a bit of securing here and there . Save the sleeper , fastener and additional foam money and use this . Remember , water and energy are both lazy and stupid , they will take the path of least resistance and if they have less R on top as opposed to below you won't suffer measurable losses . Taking into account the existing slab and insulation this will serve you very well

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