I have a carport that was closed in to become a 3rd bedroom in our house. It has a low ceiling of approx 8 or 9ft. I gutted the whole room so it's down to the slab right now.

I'm in the process of getting an on demand water heater (Navien CH240) for my house for my radiant floor and domestic water. I already have radiant floor heating in the original part of the house but want to add it in the carport area.

My question, would I be better off getting an electrical 120sqft heating pad from build direct (everwarm) for $1200 and hook that up, or should I tie into the new navien combi unit and add another zone of hydronic heating for my 11x17 room in the carport. That would make my radiant floor heating area 1400 sq ft, and with 2 adults in the house we're not as worried about pressure so really a question of ease of installation and cost.

And also, since the ceiling is so low, if i were to choose a system, which one would require less floor room (ie. subfloor, ect.)


1 Answer 1


After checking what type of instant water heater that is - since it's gas-fired, your operating cost will be MUCH lower with gas providing the heat than with electric resistance heat. Perhaps 30% (not 30% less, unless you have remarkably low electric rates and high gas rates) - typically near 30% of the cost of running electric resistance heat, for the same heat output.

Electric can be a shade thinner, but it's on the order of 1/4" .vs. 1/2". In either case, you should build an insulated floor on the slab unless the carport slab was insulated (very unlikley) or you'll be trying to heat the outdoors via an uninsulated slab, so the effective difference in height is none, really. Depending what you want for a floor, lay 2" XPS (extruded polystyrene insulation) and embed your tubes in gypsum cement or something like that on top of it, or buy pre-grooved subflooring to drop tubes into (and then pay attention where not to drive nails, or just don't choose a nailed down flooring.) There are a number of common systems for adding radiant heat on top of an existing floor.

  • The problem is the ceiling height in the room is rather low so if I got hydronic radiant floor heating, then I would have to insulate the floor [2x4 and rigid + 3/4" plywood] then lay down the pex tubing, and then pour concrete on top of that? (That's my assuption) Not leaving a lot of head room.
    – OnlyHumain
    Commented Mar 25, 2014 at 17:28
  • The other way, would be to lay a roll of electric radiant heat and save at least 1"-3" of space on the sub floor.
    – OnlyHumain
    Commented Mar 25, 2014 at 17:29
  • 1
    If you don't insulate the floor and lay down electric, please let us know your power company so we can buy stock in them ;-). To insulate the floor for either form, you can simply fasten down XPS sheets, attach the tubing or electric wire and pour gypcrete to cover it all. No plywood or 2x4 needed.
    – Ecnerwal
    Commented Mar 25, 2014 at 17:32
  • 8 feet is not a particularly low ceiling unless you are abnormally tall. Half my current abode is around 7 feet, the other half around 8. You'll only lose 3 inches or so to insulating and embedding, done right. You could do a skeevy job in an inch and a half, but you'd pay for it the rest of the time you spent heating this house...
    – Ecnerwal
    Commented Mar 25, 2014 at 17:40
  • Pardon my ignorance, but wouldn't I need to still lay a subfloor on top of the XPS and Gypcrete? (Good one about the electric company, made me lol).
    – OnlyHumain
    Commented Mar 25, 2014 at 21:20

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