We are getting ready to pour a new concrete slab over top of an old slab where a house was demolished. The old slab is 4-6 inches thick, and the new slab will be 3-4 inches thick. We would like to run PEX lines for a hydronic radiant heat system in the new slab.

From the research I have done, everyone says to use insulation under the slab to prevent the heat from going into the ground instead of into the house. I am wondering how critical that is and how much it will affect the efficiency of our system since we don't have the option of putting insulation under the slab. Should we abandon the radiant heat idea?

Any advice is appreciated!

3 Answers 3


If you want to do radiant infloor heating you will need to insulate under the slab. If you don't you will constantly be rejecting heat to the ground underneath the slab, and it will suck excess heat out of the system. If your not going to insulate beneath the slab, abandon the idea of radiant infloor.

Your also going to have to use HE Pex for radiant infloor. Standard waterline Pex is not allowed for hydronic heating systems.

  • Thanks for the advice. I was wondering if the slab underneath might act as an insulator against the ground. There will be about 1.5 inches of concrete above the pipes, but 6-8 inches beneath it. I would think that would allow the heat to escape upward more easily. Also, does heating the ground under the house really cause that much heat loss? Wouldn't the warmth still eventually rise back into the house?
    – BWDesign
    Feb 22, 2015 at 22:39
  • You will have a substantial amount of heat loss in my experience. Especially if you have a high water table, the system will never maintain a decent slab temperature. Every system i have ever worked on but one was insulated. The single house that i went for a service call to had the ground loops installed by the contractor. The system never worked from day one, however it was operating as it should, something was just sucking so much excess heat from the floor. I suspected that the slab was not insulated, and later found out i was right by pictures the homeowner had.
    – Mnc123
    Feb 22, 2015 at 23:35
  • Either insulate under the slab or scrap the idea. Its quite costly to finish a house and realize that the infloor heating is not working.
    – Mnc123
    Feb 22, 2015 at 23:38
  • Ok and phooey :P Thanks for the advice! I guess we will stick with a geothermal forced air system.
    – BWDesign
    Feb 23, 2015 at 11:37
  • 1
    We definitely live in a cold climate. We are in central Ohio and this weinter we hit -20 with a wind chill of -40. Can you provide any evidence that a geothermal system will not pay for itself? We can get it installed cheaper that usualy (around $7k) because we can use our well as the supply instead of a closed loop system. Also we can get a 20% tax credit for the system.
    – BWDesign
    Mar 3, 2015 at 15:53

Concrete is not an insulator. It is thermal mass; virtually the opposite thing. You absolutely need to insulate your slab on all sides that you don't want heat escaping (i.e. all but the top).

If you can't put insulation under the new radiant hydronic slab, you should abandon it. 50% of the heat will he lost under the house. It will run all the time and cost you a ton of money.

Radiant slabs are often abandoned in my experience. They often don't work right, and this is a surefire way to torpedo the project.


Heat moves from hot to cold. The more insulation you put in walls, roof lines, or attics, the more heat you push into the uninsulated ground. It makes sense to insulate and isolate the ground from your inside environment even if it doesn't have floor heating.

Never install radiant without insulation, the insulation is more important than the radiant heating system.

  • That is an interesting point. You would recommend installing insulation under the slab even when not using radiant heat?
    – BWDesign
    Apr 25, 2015 at 0:52

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