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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!

BWDesign
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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.

Mnc123
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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.

iLikeDirt
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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.

Doresoom
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Alex Schuts
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