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Radiant for a thick foundation?

jcbesser | Posted in General Questions on

The Quick Question: Is putting radiant heat in an 8-inch slab on grade not a good idea for a Marine Climate home?

Background: I’m building a home in Santa Cruz, Ca (3 cold months a year, lows in the 30s (not crazy cold comparatively)) and the slab on grade (850 sqft) needs to be 8 inches thick for earthquake reasons. We’re in a marine type climate and was curious the thought of putting radiant in a thicker slab. It will be hydronic Radiant and the house will be super insulated.  Is the slab too thick for radiant? The house will be 2 stories and upstairs will have radiant in a gyp.

Happy to answer any questions

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  1. Jon_R | | #1

    The standard response will be that the thick slab will be slow to respond to changing load. But with low load/high area, it's also self regulating. Eg, say that the slab is heating the 70F room by being at 72F. Then the sun comes out and load goes to zero. As the room overheats to 72F, the slab will quickly "turn off" and stop providing heat. If load were to quickly increase, room temperature might drop to 68F - and floor output would double. So while lower mass options would be better, you might find the temperature variation OK.

  2. Expert Member
    Akos | | #2

    The only issue with a thick slab is that your heat will be very slow to react. With an 8" slab, it will probably take on the order of 4 to 6 hours for you to notice a thermostat setpoint difference. This is mostly an issue if you have too much solar gain.

    Even in a warm climate, you will still need some insulation under the slab though.

    Check the heat loads for your upstairs rooms. In a super insulated house in cold climate, I doubt you will need more than a very small electric panel heater to heat them. If you are doing radiant in the ceiling gyp, it is probably way overkill.

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