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Community and Q&A

Choosing Slab Insulation for Radiant Floor

ronletourneau | Posted in General Questions on

Hello all,
I’m seeking some guidance regarding slab insulation in Berkeley, CA.
The house will be on a slab due to height above grade concerns as the owners are elderly.
The owners are insisting on in-floor heat.
The owners will not allow tubes in concrete due to seismic shifting concerns and they don’t want to live with the concrete being their finished floor.
So, we’ll use sleepers and something like Warmboard.
How much insulation would you like to see under the slab?
How much on top? What type here?
Thank you very much,
Ron Letourneau

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  1. user-6184358 | | #1

    Your Title 24 consultant should be able to help.

  2. Jon_R | | #2

    I'm curious, how will you heat the water?

  3. ronletourneau | | #3

    I forgot that that our Title 24 guy might have some suggestions. Usually, he just approves what we submit.

    They'll use an electric on-demand water heater. Tied to a solar array. This will be separate from the electric boiler for floor tubes.

    Berkeley is the first city in the country to forbid any new natural gas or propane hookups, so everything needs to be electric now. I'm a little nervous about the spec'ed electric boiler. It's a first for us. But the owners said no to wall-hung or ducted minisplits.

  4. Expert Member
    Akos | | #4

    If you have the heat load for the house and the soil temperature you can figure out how much insulation you need.

    For example, a 2000 SQFT of heated floor with a 20000BTU heat loss needs the slab at 75F for 70F room.

    If your sub soil is at 55F:
    with R5 you loose 2000(75-55)/5=8000BTU to the soil

    That is about 1/2 of your heating load, so you probably want around R10 to R15 of rigid under the warmboard.

    You can put the rigid under the slab as well but that couples the slab to floor heat making it very slow to react, can work but best to avoid it.

    If you are insulating under the slab, make sure the slab is decoupled at the edges. Since concrete is highly conductive, it is very important to fully insulate it to avoid thermal bridging.

    Electric boilers work great. They are cheap and extremely reliable.

    P.S. Cost wise even around me which is zone 5, floor heat does not make much sense in a reasonable well insulated and sealed house. A ducted heat pump is cheaper to install and run and provides same comfort and also does cooling.

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