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Community and Q&A

Do I need radiant heated floors

SPLITorradiant | Posted in Energy Efficiency and Durability on

I am converting a screen-in porch to a four season porch. I live near Albany New York. Dimensions 36′ by 10′ with slopping roof line. 2,400 Cubic feet.
Walls are all Marvin Ultimate windows double pain gliders with argon, knee walls and ceiling will have foam insulation. I’m considering two Mitsubishi hyper heat units on each of the short walls, each will generate 12,000 BTUs. The floor is a slab which will have luxury vinyl tile on it.
MY QUESTION: Do I need radiant heating under the tiles?

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  1. this_page_left_blank | | #1

    I would say no, you don't need it. If you don't want to wear shoes in the winter, then what you definitely need is insulation on the slab, floor heat or no. Floor heat without insulation means you'd just be heating the ground.

  2. walta100 | | #2

    The first question that comes to mind is if your new room started life as a patio without a footing set below the frost line? Generally patios just set on the surface and don’t worry if the patio moves when the ground freezes but things get ugly when you start building on the patio.

    Have you done a manual J calculation for the room? 2 tons sounds like too much for 360 square feet.


    1. SPLITorradiant | | #4

      Thanks for responding Walta,
      What is the manual J calculation? The slab is about 7 inches thick which sits on top of a cinder block surround.
      thank again!

      1. Expert Member
        NICK KEENAN | | #9

        Manual J is a book published by the Air Conditioning Contractors of America. It contains a technique for looking at the construction of a house and the local weather and calculating how much heating is required to keep it warm on the coldest days and how much cooling is required to keep it cool on the hottest days. Many jurisdictions how require Manual J calculations before installing a HVAC system.

        The technique is simple but it can be a lot of work: go through a house room by room, and for each room measure how many square feet of exterior wall, windows, door, roof and exterior floor it has. For each of those types you measure or estimate the insulation level. Then you look at the local weather history and figure out how hot or cold it gets where you live. Given the insulation level, the square footage and the temperature you can calculate the heating and cooling demand. Then make adjustments for things like solar heat gain from the windows and air infiltration.

        Construction varies so much that things like floor area and volume are essentially useless in estimating heating and cooling load. It's like saying "how big a crane do I need to lift a rock that is four feet wide?" Without additional information there's no way to answer that.

        You can get Manual J software to let you take a first crack which will give you an idea of what kind of system you need.

  3. paul_wiedefeld | | #3

    Do you have an existing boiler?

  4. SPLITorradiant | | #5

    Thanks for your response Paul,
    We have a furnace with forced air tubing in our crawl space. Porch sits on its own slab which is about 7 inches thick on top of a cinder block surround.
    If we go with radiant heating floor it would have to be electric.

  5. walta100 | | #6

    Manual J is a calculation to determine how much heat is required to keep your room at your desired temp given your location, windows and insulation.

    Cinder block sounds like you may have a foundation.

    Generally electric floor heaters do not putout enough heat to keep a room warm they do take the chill off ceramic tile


  6. Jon_R | | #7

    +1 on putting some rigid foam on top of the slab. If this isn't possible, then insulating the perimeter and heating the floor would provide better comfort.

  7. SPLITorradiant | | #8

    Thanks Jon!

  8. Expert Member
    Akos | | #10

    Albany has around a 0F design temperature.

    Assuming your porch has windows on 3 sides, will have at least R20 in the rafters and R15 in walls, knee walls around 3' tall.

    This means around 80sqft of walls, 360sqft of roof and 200sqft of glass. Assuming code min windows of U0.28 (~R4).
    You loose:
    Ceiling: 360*(70F-0F)/20=1260BTU
    Walls: 80*(70F-0F)/15=370BTU
    Glass 200*(70F-0F)/4=3500BTU

    Slab losses depend a lot on soil and construction, but say 2000BTU.

    You are still very very far from needing two one ton heat pumps.

    As for the floor heat, for floor heat to work, you first need to insulated the slab. But once you insulate the slab, it will be warm enough that the floor heat won't do much if you are also heating with the mini split. Save your money on the floor heat and get even more rigid over the slab.

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