Insulating a Radiant Slab

Dan asks: I am renovating an older house and am installing radiant heating on top of the existing slab-on-grade. I am planning to use 1.5 inches of foam board, which gives me R-7.5. Is that going to be enough R-value? Also what should the density be of the insulation foam board be?  On top of the insulation and radiant tubing, I am thinking of pouring self-leveling Gyp-Crete. What do you think of my plan?

Steve Bliss of responds: Most builders use extruded polystyrene foam under a slab as it has better compressive strength than expanded polystyrene (common called “beadboard”). Examples of extruded polystyrene are Dow Styrofoam and Owens Corning Foamular.  Standard Styrofoam is rated at 25 psi compressive strength, which is fine for your application, although 40 psi Styrofoam can be special-ordered if desired.

What R-value to use depends on your climate and whether the existing slab already has insulation. In warm climates, a minimum of R-5 is recommended below the slab; R-10 in cold climates. Also, make sure that you insulate the slab edge as that’s where the heat loss from a slab is greatest. Some people use twice the R-value at the slab edge as they do under the slab.

Gyp-Crete and similar lightweight concrete mixes, typically made from concrete and gypsum plaster, are well suited for your application. Gyp-Crete is a brand-name product that must be installed by licensed applicators, but many (often less expensive) lightweight cement underlayments  are available. Check with your local ready-mix concrete company. For small areas like a bathroom, you can mix and spread the topping material yourself.

A couple of things to keep in mind:  “Self-leveling” underlayments need some help with a trowel to spread evenly and set level. Also, if you are planning to put ceramic tile on top of the lightweight concrete, bonding and cracking can be an issue. A crack-isolation membrane such at  Ditra, Kerdi, or Nobleseal under the tile is highly recommended. Each type of membrane has its own installation requirements which must be carefully followed for good performance.

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