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Warmboard on slab versus PEX in slab

Charlemagne Member Posts: 1
Basement floor will be lowered two feet and gravel bed, insulation, vapor barrier installed. Exterior walls (half height above grade) will be excavated and waterproofed.

Question: Can/should warmboard be used below grade in lieu of PEX in slab?

Will in-slab require higher water temp input for slab versus upper floor's Warmboard?

Will higher temps rule out use of geothermal water to water exchanger and force introduction of auxiliary heat source to generate additional energy required for slab?


  • SWEI
    SWEI Member Posts: 7,356

    costs as much or more than pouring a typical 4" slab does.  Adding it to a new slab really makes no sense.  The required water temps for an insulated slab are quite low, and with proper design, they can easily be made to match those of the Warmboard floor above.

    Both will work well with geothermal, solar or other low temperature heat sources.
  • RobG
    RobG Member Posts: 1,850

    I agree totally with SWEI, you are already pouring the slab, it would be a waste of money to put warmboard over it. Just make sure that you insulate under the slab (the more insulation means less heat loss). The only reason not to put the tubing in the slab would be if you have a very high water table. If that were the case I wouldn't bother excavating anyway.


  • alotlikeearl
    alotlikeearl Member Posts: 68

    Don't forget edge insulation!
  • heatpro02920
    heatpro02920 Member Posts: 991
    in slab

    this is a no brainer if you are already pouring a slab put it in the slab, insulate it correctly and install the pipe correctly and you will have decades of worry free efficient heat...

    Note- I don't like putting wood board on top of cement, I use the roth panels and like the outcome..
  • Gordy
    Gordy Member Posts: 9,546
    Tighten it up

    Agree with others. One suggestion tighten up the tubing layout. Helps lower water temps a bit.
  • Bob Gagnon plumbing and heating
    I agree with Gordy

    Close pipe spacing and good insulation will make the slab heat up in a couple of hours, instead of a couple of days.

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Gordy
    Gordy Member Posts: 9,546
    I will add

    Since it's geothermal you should be a with in reach with supply temps to the slab. But then we don't know your heat loss to the space. Also keep your loop lengths with in reasonable design lengths depending on the dia pex you use.