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Hydronic tubing above or below subfloor?

Tim Brasel | Posted in Mechanicals on

I’m getting ready to do renovate my 1956 cape cod in Delaware. The house is heated with hot-water baseboard radiators throughout (supplied by a 120,000 BTU gas-fired boiler at 180F). In the remodeled kitchen, I’ll be putting cabinets where the existing radiators are. Floor will be ceramic tile rather than the current vinyl sheet, over 3/4″ plywood.

While I could install toe-kick radiators under the cabinets, I’d prefer in-floor hydronic heating throughout the kitchen. I’ve had it in a couple of prior houses, and liked it a lot.

My question: what are the operating cost and performance tradeoffs of an “above-the-subfloor” system like Warmboard, vs running tubing under the subfloor (in aluminum extrusions such as Wirsbro Joist Trak)? How much performance do I lose by the tubing being under the subfloor rather than above? If I can avoid it, I’d prefer not to add the floor thickness of a Warmboard or similar product (I’ll already be adding thickness with cement board and tile).

The kitchen is currently above a non-conditioned, non-vented crawlspace, but as part of this project I plan to insulate the crawlspace walls per the excellent instructions on this website. In general, though, the house is insulated as well (or I should say as poorly) as might be expected of a 1956 house.

Thanks for any assistance and comments – I’ve done a fair amount of searching on this particular question without success, but a link to a previous post would be great as well, if one exists.

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  1. Chris | | #1


    1. Rip out the kitchen sub-floor and install Warmboard S (

    2. If you were already planning to fill the floor joist bays in your crawlspace with spray foam, staple-up from underneath.

    3. Mud job instead of cement board.

    4. Go with the hydronic toekick heaters and electric in-floor for comfort rather than space heating.

  2. Expert Member
    Dana Dorsett | | #2

    Above the subfloor solutions are more expensive to install, but are more responsive, and can operate at a lower temperature, gaining a few percent in efficiency if you have a condensing boiler, but less than a 1% improvement in efficiency if it's a cast iron boiler.

    An under the subfloor solution is probably going to work out just fine,but without some critical information there is no way to tell for sure. To start with, get a handle on the whole-house heat load using wintertime fuel use against heating degree-day data using the methods spelled out here:

    Then divide the total heat load by the amount of available floor are for heating. That is enough information to ballpark the water temperatures needed for either WarmBoard or below-the subfloor solutions.

    If the room to room temperature balance is pretty good with the existing baseboard, we can use the baseboard length in each room to come up with the approximate heat load in each room, and come up with a room by room load/square feet ratio to fine tune the water temperature information.

    Odds are pretty good that the existing boiler is 3-5x oversized for the actual heat load. Depending on how the water temp requirements work out it's possible (likely) that swapping out the beastie boiler for a condensing water heater or a small modulating condensing boiler. For a primer on getting mod-con boiler sizing, see this bit:

    Toe kick heaters basically suck. If you need more radiation by cabinets, use cast-iron baseboards. New cast iron baseboard is ridiculously expensive, but there are miles of it ripped out and scrapped or recycled every year by renovators, and it can be had in salvage yards (or craigslist) at a fraction of the new price. It's pretty bulletproof compared to fin-tube baseboard.

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