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in floor heating

Brad WhiteBrad White Member Posts: 105
If you are asking "should you use insulation below a radiant slab", the answer is a resounding "yes". The ground is a heat sink which, below frost-line typically is the annual average temperature for your area; in our case, Boston, that is about 55 degrees. Although it eventually warms up, that is on your dime. At least R-10 around the perimeter and at least R-5, if not R-10 also, underneath it all.

Your area and local codes will dictate what the thicknesses are, regardless. But even if they say R-5 on the perimeter, I would go R-10 if you need heat at all.

As for the sequence of construction, depending on your area and soil conditions, pea gravel, a vapor barrier sheeting, sometimes more pea gravel as ballast, insulation, welded wire mesh (to which the tubing often is tied), then the pour. The R-10 perimeter insulation acts as an expansion joint too.

There are systems of plastic screw chairs which allow you to clip the tubing to the insulation directly, not to mention that there are a good half-dozen variations of what was mentioned.

Might I suggest the RPA's "Radiant Basics" and "Radiant Precision" books (written by John Siegenthaler) as a good place to start? I refer to those books regularly among others.


  • TimTim Member Posts: 37
    in floor heat

    When installing radiant tube under a concrete floor is it necessary to lay a layer of extruded polystyrene insulation on top of the pea gravel then laying the tube on the insulation prior to the concrete being poured.
  • TimTim Member Posts: 37
    in f;loor heat

    Thanks for the feed back Brad
  • mark ransleymark ransley Member Posts: 155

    Yes it will save you energy, the best is not the blue or pink board but Polyisocyanurate, it has a foil covering its at all stores, its R value is 7.2 vs 5 for pink or blue and will do better because the foil is a radiant barrier, I used 2" for 14.4R, but even that is not up to modern codes, without it the dirt sucks out the heat. evemn without radiant heat in cold areas foam should be mandated, but we have no energy saving policy that pushes it.
  • Brad WhiteBrad White Member Posts: 105
    I have to disagree

    on using Polyisocyanurate versus blue or pink (extruded polystyrene). The polyiso does not hold up well when buried and has higher water absorption.

    Yes, it does have a higher R value, by nearly 50% on a per-inch basis, but performance below ground suffers. Others may disagree and I do not have specific data to back that up, but have been told that by a lot of people whom I respect and value their opinions.

    My $0.02
  • klausklaus Member Posts: 183
    The Pink Stuff

    I agree with Brad. I only use the pink stuff, R-10. I question the benefit of the foil if you're in direct contact with it.

    Mark you're right on the money with the thought that it should be required everywhere, radiant or not. It has to make a difference on heat loss through a floor.

    BTW my preferred method of installing is smooth gravel, vapor barrier, 2" pink foam and foam staples. Note my little sweetheart's matching shoes! (She's so fashionable!)
  • Andrew HagenAndrew Hagen Member Posts: 236

    I also agree with Brad and Rich.

    One benefit of pink over blue board is that the pink board doesn't cause reflective sunburns as quickly. I've been burned to a crisp from blue board.
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928

    The foil radiant barrier has no effect when covered by a solid on both sides. By very definition, there is little or no radiant energy to block.
  • TimTim Member Posts: 37

    I have heard of Double Bubble that i have been told has a higher R value than the extruded board with a white poly wrap on one side to contact the cement and foil to face towards the ground, i have been told when the foil faces the concrete it will react with the caustics in the portland which will desolve the foil face. Has anyone had this experience
  • kpckpc Member Posts: 62
    bubble foil...

    under slab is a waste of money....both now and the energy that later.
    Don't belive me look here... 55/Page_55_o_bldg_sys.htm
  • TimTim Member Posts: 37

    Thanks kpc, i will most likely go with a extruded insulation with a vapor barrier on top of the pea gravel.
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