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I am looking to retrofit a hydronic radiant heating system to the first (ground floor and basement of my house. For the first floor the plan is to install the system under the subfloor by snaking the tubing between the joists. The basement floor is slab concrete without any insulation below the slab. I don't have any ceiling height to lose so adding insulation does not seem like a good idea. Further, there is a mild moisture issue with the floor so this just seems like a bad idea.

I was thinking of running additional tubing in the ceiling. I am completely unfamiliar with the concept of radiant ceiling heating and not sure if it can be combined with radiant floor heating. Can I essentially run one set of hydronic radiant heat tubing to heat the ceiling of one room and the floor of the room above it.

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  • I would think if you are using radiant floor heat you would have some ceiling heat already. Cable heating was big here in the Pacific northwest when power was cheep. It will work to add ceiling heat but remember heat rises so your upstairs floor heat may need to be turned down.
    – Ed Beal
    Commented Apr 13, 2017 at 22:03
  • @EdBeal Heat does not rise. Heat moves from high temperatures to cold temperatures. Hot air rises. In a subfloor/ceiling cavity, there's little room for convection, so not likely a problem there. You also don't rely on convection for a radiant ceiling, which is why they are much hotter than radiant floors.
    – Hari
    Commented Apr 14, 2017 at 21:58
  • Ok try and not insulating above a cable or radient heat system in the ceiling and see how much is in the room below.
    – Ed Beal
    Commented Apr 16, 2017 at 22:48
  • If you have a moisture problem at the basement floor, you want a forced hot air heating system. In the heating season circulating hot air is necessary to dry out the floor; in the cooling season a dehumidifer. You cannot cover this floor.
    – Jim Stewart
    Commented Mar 5, 2022 at 13:53

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You'd need custom heat spreaders, and it still won't work well

The heat spreaders used for hydronic systems only spread the heat along one face (so either a subfloor or the ceiling drywall). You would need to get a custom heat spreader for a single tube to do both.

A hydronic radiant ceiling also requires much higher temperatures than a floor does, so you'd have an uncomfortably hot floor or a rather ineffectively tepid ceiling. Neither is ideal.

You could run parallel systems between ceiling/floor joists

Two systems, with different temperatures, could be placed in parallel in the same space between joists. I can only speculate that it could work, but I've never heard of this before, so you might be venturing out a bit here.

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  • lol, I just had this crazy idea and am now considering it. @strongbad did you do it? Anyone ever heard of the parallel systems within the same joists since 4 years ago?
    – jpx
    Commented Oct 28, 2021 at 21:27
  • I still haven't heard of this specifically, but radiant heating is fairly common for outdoor dining in California. Some places have large radiant heaters along a pergola that face downward, toward the tables. These are pretty effective, but they also operate at ~800 °C rather than conventional radiant heating temperatures. I still can't quite imagine why you'd want this, except to provide hydronic heating to a slab-on-grade storey (like a basement).
    – Hari
    Commented Oct 30, 2021 at 5:12
  • exactly. my scenario: a retrofit where I may add radiant heat floors to main level, but it only makes economical sense if I can also use it to heat other spaces, such as my basement with uninsulated slab (hence radiant ceilings combined with staple-up for floor above)
    – jpx
    Commented Oct 31, 2021 at 6:19
  • @jpx, if you have the headroom for it, you could look at installing on-grade insulation with PEX guides. This would raise the existing floor height by at least 1-1.5", but that may not be too much of an issue and would be a better solution. We don't do product recommendations here, but searching for "on-grade hydronic retrofit" should be useful.
    – Hari
    Commented Nov 1, 2021 at 7:16
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Radiantec.com had been instructing the diyers on radiaint ceilings for years. My install was accomplished with pex and heat fins stapled up under sub floor and pex and heat fins under drywall. R19 batts were installed in the joist bays to keep the instals separate. The cement basement floor is covered with rigid foam and plywood (r8). Left as bare concrete, I believed all that radiant heat would sink endlessly into the concrete. System works well but I think my success comes first and foremost with the building envelope. I took great pains to air seal and insulate every exterior surface making the radiant system much less agressive in the first place. A poor building envelope needs overly hot radiant surfaces. A good envelope causes radiant surfaces that dont even register as warm to the touch leading people to wonder if you actually put in radiant heating at all. The system radiantec.com suggested to me used a single hot water tank for both hot water and floor heating. The heat needed was so low due to the envelope design. Love love love the radiant ceiling. Dont let anyone tell you otherwise.Excellent piece from an engineer. It says it all- again. http://www.tesmar.com/html/radiant_floors_vs_radiant_ceilings.html

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I put 3/4" fin tubing in my ceiling and placed small fans (computer type) on top of the fins blowing down. These are about every 3 feet; it has been working great for about 15 years. The basement is not finished and the ceiling is open. I also noticed a big drop in the amount of time the furnace was running. The walls are insulated with 1" foam. I oversize the hot water heater (designed for heating) and added a small pump. The fin tubing is connected in the instant hot return lines. In the summer I move a few valves and bypass the tubing. Go for it!

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