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radiant sub floor insulation

hoff Member Posts: 1
I built a new home 5 yrs ago. I have a sunroom with radiant floor tubing in a 4inch concrete slab. I installed 2inch blue board insulation under the slab but did not  insulate on the side wall of the slab. How bad is this going to kill my efficiency?  Will it help at all to dig down outside and insulate the exterior of the block?  Thanks


  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479

    Most of your heat loss is through the vertical side of the slab. If  you can insulate it, you should. 
  • Peter Turk
    Peter Turk Member Posts: 10
    Yes, you will lose most of your heat thru the side wall

    I agree with Wayne. The side insulation is the most important. It is difficult to quantify how much the loss is. It will depend on the outside temperature and the ground temperature. The bottom line is that the heat from the slab is being sucked out from the foundation walls.

    Instead of trying to insulate the exterior of the block wall, try to do it from the inside. If you insulate the exterior, you fighting a losing battle. With the concrete floor connected to the wall, unless you complete insulate the exterior wall (both above and below grade) you will never win. The exterior wall will continue to draw heat away from the floor. Assuming and hoping that you kept your PEX tubing at least 6-8" away from the exterior wall, you should be able to rent a gas-powered concrete saw, cut the 4" slab 2" away from the exterior wall and fill the 2" gap with extruded polystyrene insulation. That will 100% solve your problem, provided you can do it without creating any other ones.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    your diagram

    is a bit misleading about underside insulation though. it's not a good practice to omit underslab insulation, and it's certainly not "about the same" as just insulating the edges.

    the edges are definitely most important but that doesn't mean bleeding heat to dirt forever is a good idea either.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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