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Slab Sandwich

I have a customer that is remodeling their house, pouring a slab downstairs and they are considering different proposals from radiant heating professionals. I'm only consulting at this point and they are having a tough time figuring out what is the best and most cost effective way of constructing their slab as recommended by different contractors which are:

Scenario 1: Four inch structural slab, 1" insulation, 2" topping slab with PEX tubing

Scenario 2: One inch insulation, 4" structural slab with PEX tubing

Figuring a mod/con boiler with outdoor reset controls, what would be your recommendation?

Thanks in advance.
Often wrong, never in doubt.


  • SWEI
    SWEI Member Posts: 7,356
    Two is better than one by far

    but I'd use a minimum of 2" insulation underneath - may even be a code requirement, but perhaps not in your climate zone.

    The biggest thing is to make sure the tubing is tied to the top of the rebar, with the rebar on chairs -- keeping it in the upper half of the pour.
  • hot_rod
    hot_rod Member Posts: 16,347
    how large of a space?

    pouring a basement in an exisiting home? How much headroom to work with?

    For a small area that CreteHeat product is pretty slick. Foam, tube holder, and vapor barrier all in one.

    I'd go with 2" also. I wish every job I have done would have been 2", it does make a big difference especially with slab on grade jobs.

    Remember some sort of edge insulation/ expansion joint detail.

    Slabs poured within 4 concrete walls have a tendency to crack when they heat and expand. They call it external restraint cracking.

    Concrete supplier shops have a special foam strip expansion material. Then you slide on this plastic H cap. After the pour, zip the top of that H and fill it with a polyurethane caulk to seal and allow expansion.

    Here is a piece of it. I use it between garage slabs and the driveway slab. Allows some movement and a bit of a thermal break. Use foam adhesive to glue it against the basement wall up to the slab pour line. After the pour, zip the top 1/2 off and caulk. Leaves a nice clean detain without an exposed, raw foam edge.

    My other wish, looking back would be tighter tube spacing to allow 100- 120F supply temperatures. That allows condensing, solar and heat pumps to work efficiently.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • My recommendation

    to the owner was to install the tubing in the structural slab; it's only 4" thick. Maybe not as responsive as a 2" slab, but plenty quick enough with a mod/con boiler and outdoor reset.

    The owner has had five radiant contractors out to bid his job; some don't mention the slab detail with others recommending either a 2-part slab or a single slab.

    Thanks to both of you for great recommendations. Hot Rod: I've never heard of "external restraint cracking" and we may not get that here with our mild conditions.
    Often wrong, never in doubt.