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confusion with heat loss analysis for concrete slab on grade

DaveA Member Posts: 6
I'm in the early planning stage for a detached steel woodworking shop with a 4" slab on grade floor and heated by a hydronic radiant system, using embedded 1/2" PEX. Building will be 24W x 30DP x 8.5' H. The slab will have 2" Extruded foam beneath and along the side going down 2' against the footer (24" frost line ). I'm trying to calculate my heat loss using a couple different sources I found on line. One is taken from the Design and Install manual for Hydronic Radiant Heating Systems, by Vanguard Pipe and the other is at a site called Build It Solar. Both seem to be pretty thorough. The Vanguard worksheet has an additional feature, whereby you enter the U factor for every component and then multiply it by the Design Temp Diff. which gives a Heat Loss Multiplier. You then multiply each components' square footage by that multiplier to get the Btu loss. My confusion is with the input entry for the slab itself. The Vanguard asks for U factor for the slab edge, which I found elsewhere to be .53, applied to the perimeter. The BIS app also asks for the R value of the slab per foot of perimeter, which I converted from the U factor to get 1.88. But both also have an input for a floor R value or U factor. Can't figure out if I need to enter something there as well and what it should be. Any help is greatly appreciated as I haven't found an explanation for this anywhere else. Thanks in advance.


  • Jamie Hall
    Jamie Hall Member Posts: 18,947
    You do need an R or U for the floor as a whole, and it applies to the area of the floor -- not the perimeter. Reason being that the floor will lose heat downwards as well as heating the room. Use your floor construction detail -- 2 inches of foam -- for the R factor
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 12,005
    Your looking for a couple of things, You need to calculate the R valve of the 4" of concrete and then the R value of the foam and add them together. That's for heat loss through the slab to earth

    Then you need to compute an infiltration factor for the linear feet of footage of the outside walls. Known as the "linear feet of crack" that's for air infiltration where the waals and floor meet

    If I remember right the is a reciprocal of the U factor. Or maybe its vice versa
  • DaveA
    DaveA Member Posts: 6
    Thanks to both of you. Exactly the info I need.