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radiant floor over concrete slab

jeff_87 Member Posts: 3
Other question-I was going to use the hot water loop in the boiler (already goes to radiator in another part of the house). THis water is over 170 degrees. Is this water too hot for radiant floor heating. how does it 'cool down' for the system.

It seems like a waste not to use the already heated water for this system as well


  • jeff_114
    jeff_114 Member Posts: 1
    radiant floor over concrete porch

    Live in Northern NJ.
    Have a raised back porch which is concrete that we are taking over to enlarge the kitchen. Was thinking about using radiant heat with warmboard/hot water loop from the oil for steam burner. Architect believe the radiate stuff should be installed below the concrete. My belief is that we are then heating the whole concrete slab (only 1/2 of which will be indoor) as well as the foundation of the slab.
    Needless to say, I have no idea whom is right. suggestions from someone with an interest and experience with this type of issue
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479
    I think

    Warmboard is a good choice. I sometimes use quik trak in such applications. I shoot down plywood 5/8 or 3/4 and attach to the top of the plywood. The plywood acts as an insulator. What is your floor covering? Another method would be putting down an inch of EPS insulating board and do an over pour of gypcrete or concrete. Are there any height differences to account for building on top? WW

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  • Brad White_185
    Brad White_185 Member Posts: 265
    With half the slab outdoors

    you will be hosting a mid-winter flower show but at least the admission fees will defray the heating costs...

    The cost of putting it in/under the slab would be prohibitive for the benefit unless you were demolishing the structure and starting anew. In that case you could do it right. Nothing wrong with using a concrete slab as a radiator- it holds and transmits heat very well, very evenly when properly done.

    But let's deal with your situation.

    With all due respect to Wayne, I would use more than plywood as an insulator; it has an R value of between 0.6 and 0.95 depending on thickness.

    What you really need is at least R-5 if you can get the thickness. Warmboard is essentially a thick structural plywood; I do not have the R value handy but if 1-1/8" thick softwood, it would to my mind fall at about R-1.4.

    A good alternative might be Roth panels, similar in concept (panels with molded grooves covered with a aluminum conductive surface) but these are made of insulation with an R value of 3 to 4 depending on thickness, 3/4" to 1", which can be verified.

    What is most critical is the edge condition in your case. With the slab traversing the outside wall, if there were a way to break that conduction, (I have no idea how), it would be great. Otherwise, think of it as building a thermal bathtub.

    My $0.02

  • Ron Gillen
    Ron Gillen Member Posts: 124
    Roth Panels

    Kitchen? Probably some sort of tile flooring. Roth panels would work well and give you some insulation.
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479

    insulation all around is mo better. Esp on perimeter. I like the thermal bathtub analogy. On slab pours I tell the GC's to pretend the slab is a swimming pool and they are keeping the heat in like it's water using the EXP insulation. (Yes I sometimes get a slack jawed vacuous eyed response) I have seen the Roth panels on their web site, but never in person. It looks like a good product. How thick are they? Who sells them around the mid East US? Inquiring minds want to know. WW

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  • jeff_87
    jeff_87 Member Posts: 3

    You guys are definetly answering some of my questions, so thanks. did some reading last night, apparently radiant floor heating used to be done all the time with concrete (but the tubes were actually in the concrete) not ontop like in my situation.
    I also found a product called 'barrier insullation'- its product with double sided vapor barrier and internally some foam insullation. It seems like this would work as a good thermal/vapor barrier.
    I like your flower show analogy-helps me to think about how this works

    Sooo. . . I probably need a vapor/insullation barrier on top of the concrete slab, then the warmboard or roth board, then the flooring (which will be word or cork). Not interested in using tile in the bathroom.
  • Doug_7
    Doug_7 Member Posts: 233
    Real Insulation and Thermal Break Required

    Jeff - You are not understanding what Brad and Wayne are telling you.

    Concrete is an excellent conductor of heat. With half of your concrete slab sticking outdoors and half indoors, and no insulation, this is a terrible design. Heat losses will be huge and the heating requirement will be enormous. There needs to be a properly designed "Thermal Break" of approximately R-20 between the outdoors and indoors.

    Concrete exposed to the outdoors is normally insulated to at least R-20 on the inside or outside.

    'barrier insullation'- a product with a double layer of aluminum foil with thin plastic sandwiched between is radiant (reflective) insulation - not thermal insulation that provides a barrier to heat conduction. It has no significant R-value to resist heat flow by conduction. Put it inside the windshield of your car to reflect the heat from the sun - but it will NOT properly insulate a concrete slab that is exposed to outdoors.

    How many inches of height do you have to work with between the concrete slab and the finished floor ??

    You will need to either put down several inches of real thermal insulation, such as extruded polystyrene (Blue Dow Board) which is R-5 per inch, on top of the concrete slab and below the floor heating - OR - Can you take the entire concrete slab into the building so it is not sticking outdoors at all ?

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