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Rigid foam slab insulation

Radhead Member Posts: 2
I have a client who has an associate who believes that slab insulation is a waste of resources and that the soil below the slab can be used as a form of mass heat energy storage. I would also be interested in opinions regarding tarp type insulation products and radiant barriers below slabs. What insulation systems are professional installers recommending?


  • scott markle_2
    scott markle_2 Member Posts: 611
    ground as thermal mass

    The old Heatway design manual took a similar position on slab insulation. Although the manual was well written and had a lot of good information about radiant heating and control strategies, I wonder if this opinion was researched by the same people who did the rubber compounding for entran-2?

    I would imagine it's fairly complicated to model the movement of heat through the ground, lot's of variables. Still I don't think you have to be a math head to understand that an enormous 55 degree heat sink in direct contact with a 72 deg slab is going absorb a significant amount of energy. Yes the earth will warm and a temperature gradient will form under the slab such that the rate of transfer to the earth will gradually decrease and stabilize but they will never reach equilibrium, energy will always be moving into the ground. How much energy is lost to different soil types and what is the point of diminishing return on insulation are interesting questions, I have a memory of an experiment that may have been posted here where temperature probes were placed at varying depths under differing insulation types to try and inform this question.

    Also keep in mind that many soils have water moving through them, Imagine how much heat a slab could loose to the earth if (cold) water was moving through the soil directly below. Extruded Polystyrene is our friend, skimping on slab insulation is a false economy.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Go here and be educated www.healthyheating.com


    Lot's of great reading from one of the industries best, Robert Bean.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Gordy
    Gordy Member Posts: 9,514
    Waste of Resources

    Will be the Franklins in his wallet!  Insulate once reep the returns. Insulate nunce, reep the fuel bills.

     Side benefits are response time, and tighter control over the emitter. With out its like opening the coral of wild horses.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    How to tell if my slab is insulated or not?

    I am convinced that insulating of a radiant slab is desireable. Probably even if it is not heated.

    My house was built around 1950, and judging by a lot of things, there were no heating codes in those days. Was rigid foam slab insulation even available in those days? If not, was anything done to reduce heat loss down and sideways? Judging by the amount of snow that melts around the edge of my house, if it is insulated, it is not very well done.

    When I had my oil tank taken out, they checked for leaks and found some oil had gotten into the water table. They dug down to look at it and smell it, and take samples. The water table is about 6 feet down. I do not know how conductve the dirt around here is, but it is probably a lot less than the water would be. So it could be worse.

    Some friends of mine had to put a foundation under a house that was already built. They went through hell to build a new frame under the house, so they could jack it up and dig deep enough to put in footings, and to replace the rotted timbers all around the edge of the house. It is clearly better to put the foundation in before you put a house on top of it. Similarly, it would be difficult to jack up my slab and put insulation under it. Difficult is probably too weak a word to describe it.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,216
    JDB sorry but no

    In 1950 slabs were not insulated and as far as I have seen there was not insulation board. Dry sand was as good as it got back then.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    in 1950 slabs were not insulated

    So I have, at most,dry sand or may be crushed rock under the slab. Sigh!

    Well, let me be glad the water table is as far down as it is.

    It may not have mattered that much when the house was built. If fuel oil cost about the same as gasoline in those days, it was about $0.20/gallon. In 1976, when I bought the house, it was about $0.40/gallon. Now, it is around $4.00/gallon (a little less, perhaps). Of course, in those days a dollar was worth somewhat more than it is now.

    I looked at the part of John Siegenthaler's book in the chapter on calculating heat loss. It is the loss through the perimiter that really kills you., not the straight down part. At least if you do not have the water table too high, whatever too high might be. Because if the dry sand were not dry, that would not be so good at all. And while it matters if the slab has heat in it (perhaps the slab might be 120F where the tubes are), even if it is not heated, it would be around 70F at the top and still lose heat going down.

    I read here a while ago about someone who wanted to make part of his slab uninsulated to increase the thermal mass to reduce short cycling or something. Now my experience with my slab, insulated or not, is that its thermal mass is so high that not only does it not short cycle, except in the warmest weather when I now put only 75F water into it, but it runs cycles many hours long. At least 4 hours, and sometimes 12 hours. So say good-bye to setbacks, for example. Having outdoor reset is a blessing: even if it did not save energy, it reduces temperature swings of many degrees.Imagine a mass great enough to hold a week's heat instead of about a day. If I had a cold week followed by a warm week, The house would be too hot for about a week during the warm week. I already notice this. After a very cold day, if the next day is quite warm, the thermostat remains satisfied for a day or so without calling for heat. And the temperature holds quite well at the set point just using the heat left in the slab.

    It is interesting having a thermostat that records how many hours it was calling for heat for today and yesterday, and for the entire heating season.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    edited May 2011
    be careful

    we're all using various models to determine basement heat losses. However they are all necessarily wide approximations which do not take into account your specific site conditions.

    I have seen the passivehouse energy modeller... it's very different. and I am no longer convinced that 2" of rigid foam under slabs is adequate, based partly on that and partly on my observations in my extremely well insulated shop and cool slab level in the summer even though it's mostly exposed, has two big garage doors, and is R20 to any dirt it touches except under the slab which is only R10. I suspect I lose significant heat to the ground below down there. Nice in the summer, but in the winter... blah.

    If I built this place again, I'd use at least 4" under the slab.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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