GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Radiant floor in very thick slab

Daniel Stewart | Posted in Energy Efficiency and Durability on

I’m working on a project (climate zone 4C) that will use a thick (~16″) raft slab due to poor soil conditions and the client is interested in a radiant system as well.

If the pex tubing were held approximately 2-3″ below the top of the slab and the entire perimeter and sub-slab were insulated to R-20 would it make sense to use a radiant system?

I imagine at the very least it would be a huge flywheel that a very long time to heat up or cool down, so you’d have to just set the temperature and leave it alone.

Any thoughts?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Mike Haskell | | #1

    Hi daniel, what is the area of the slab, and the heat load calcs of the house?

  2. GBA Editor
    Martin Holladay | | #2

    Such a slab would be very hard to control. When the weather changes from cold to hot, this slab would take many days to cool off -- and the house could become uncomfortably warm. Low-mass floors are easier to control than high-mass floors.

  3. Expert Member
    Dana Dorsett | | #3

    If you're going to do a radiant concrete slab make a sandwich out of it with at least 3-4" of EPS over the structural slab below, and ~3-4" of radiant slab for the flooring on top of that. It means two concrete pours on different days, but it brings the thermal mass of the radiation to within manageable levels, with at least some ability to track the heating load using PID algorithm thermostat controls and a "set & forget" strategy (no setbacks.)

    At 16" it's hopeless, as Martin correctly points out.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |