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Community and Q&A

Thermal break for new radiant slab?

Chris Hoy | Posted in Mechanicals on

We are retrofitting a slab on grade portion of our home that has an existing 4″ thick concrete slab with a thinnish layer of insulation underneath. We are going to install radiant heating over top of this existing slab. There’s an existing fireplace in the room and the hearth is 3 inches above the existing slab.

So to keep the new radiant overall thickness to roughly 2 1/2 inches to allow for 1/2 inch for wood flooring I was going to install 1/4 inch thick extruded poly (Owens Corning fanfold) then Hilti wire mesh, then 5/8 tubing, then gypcrete.

My concern is whether I can use the fanfold material in contact with the radiant gypcrete slab. Thoughts?


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  1. Richard McGrath | | #1

    I suggest using Uponor Fats Trak 0.5 ( Part # A5090313) . Fast Trak 1.3 is 1 1/4 " and you could pour 1 1/4 gyp over it and get better response and more insulation value . You won't need to staple it down , use wire mesh and you can pour whatever you desire over it . A supplier that carries Uponor material should not be hard to locate . What type of boiler / water heater will you use ?

  2. GBA Editor
    Martin Holladay | | #2

    A radiant subfloor panel like Warmboard is 1 1/8 inch thick. If you want to install 1/2-inch-thick finish flooring, these two products will be 1 5/8 inch thick. That still leaves room for 1 inch of XPS or EPS foam under the Warmboard, while still keeping the total thickness of the assembly well below 3 inches.

    With this approach, you end up with thicker insulation. Skimping on insulation makes no sense -- especially for a radiant floor over a slab on grade that has what you call "thinnish insulation underneath."

  3. Richard McGrath | | #3

    Martin brings up a good point . Exactly what type of wood product are you considering for finished floor ? Warmboard has another product called Warmboard R . Check pricing for Warmboard products as they are not cheap . At 12" center tube spacing they are simply the best the market has to offer . 6" spacing or 8" spacing allow for lower water temps however while offering the same BTU sq ft output . I apologize for not catching the wood finish last night . Remember to keep your surface temps at or below 85* to protect your flooring from adverse effects . You can get 30 BTU sq ft out of Warmboard they claim with 100* SWTs and a finish floor R value of point 5 .
    This type of system as opposed to the overpour will be faster to respond and not add to overheating and solar gain issues also . It also will allow you more choices as to which type of products you use in the end . You will see the tubing while fastening so there is much less of a chance that tubing damage may happen .

  4. Expert Member
    Dana Dorsett | | #4

    When you have that little insulation you're probably better off going with Roth panels than with WarmBoard:

    Roth panels are made of high-density EPS, which is ~4x more insulating than either the doug-fir plywood or aspen/pine OSB versions of WarmBoard. The 1" thick Roth panels designed for half-inch PEX run about R4.5 in the field between channels, and still over R2 at the deepest part of the channel, which is way better than 1/4" XPS under a wood-based PEX channel system.

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