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Community and Q&A

Concrete slab details

swmurray | Posted in Green Building Techniques on

My architect  has drawn the attached detail for a house in Madison County NC just above Asheville, so in climate zone 4A-5A. We have chosen AAC walls as we are somewhat isolated and far away from any fire departments. We will have an outdoor wood boiler with radiant heating in the concrete slab. My initial questions are: Since the radiant heating pipes and all of our plumbing is in the slab, would a 6″ slab offer any benefits over a 4″ slab? With radiant heating in the slab, how much insulation should we have under the slab?

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  1. Expert Member


    I'm sorry I can't be of much help with your question. I don't know enough about under-slab foam.

    What did spring 0ut at me in the sections you posted are the roofs which are low-slope and shown as being insulated by either batts or foam. You can not effectively insulate and vent 1/12 roofs with batts, not can you ventilate even higher pitched roofs when there is no venting provided at the top of the roof.

    I also don't know of any exposed-fastener meal roofing the can be used at that pitch. Most need at least 2/12 to work.

    1. swmurray | | #2

      Thanks Malcolm. We are planning to use closed cell spray foam insulation to create a conditioned attic (contrary to what the architect has shown). We have not decided on a roofing product yet, but there are some metal roofing systems that are approved for a 1/2" in 12" slope. We are undecided as to which is better for our 1/12 pitch roof; a metal roof (which may offer more fire protection) or a liquid-applied membrane (like Hydrostop). If we decide on a liquid-applied product, we may install a non-combustible sheathing (like Armoroc or Nocom).

      1. Expert Member
        MALCOLM TAYLOR | | #4

        Good luck with y0ur build!

  2. GBA Editor
    Brian Pontolilo | | #3

    Hi Stuart,

    I watched a lot of architects and builders of high-performance homes install all different arrangements of radiant floor heating about 15 years ago. Some homeowners like it for the comfort, but anecdotally, I've heard many times that it's too expensive to install, not as efficient as promised, and fails to respond to temperature swings quickly enough.

    That's not to say that it doesn't work, just that there are some issue to be aware of, many of which are discussed in this article: All About Radiant Floors

    If you do continue down this road, and perhaps if firewood is going to be your primary heat source, it is a good option, then you can use this article to learn more about how much to insulate the slab: Determining Sub Slab Rigid Foam Thickness

    1. swmurray | | #5

      Thanks Brian. Being somewhat isolated, we have had several instances in the past when winter storms have knocked out the power and it can be two weeks or more to get it back. I have found that living here works best if you have several heat sources; some that can function without power or with limited power provided by a small generator.

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