Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Radiant over precast

jhansen Member Posts: 59
I am installing radiant tubing to serve garage area on top of precast. Can I use the radiant reflective shield with air bubbles before under the tubing and top 4 inches of concrete? Thanks for any input.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    edited April 2011
    I wouldn't...

    Bubble foil bubble has an actual R value similar to that of a 1/2" piece of sheet rock. Plus, when you start piling concrete on top of this stuff, the bubbles pop and go away, along with whatever meager R value it had in the first place.

    I'd prefer to see foam sprayed on the bottom side of the slab, but if that can't be done, then I'd place a minimum of 1" of high density XPS insulation on top of the existing concrete. Make sure that the foam can handle the dead load of the placed concrete, as well as the live loads of moving automobiles. It comes in varying densities. I used 25 PSI foam beneath a 6" truck ramp that had loaded 18 wheelers backing over it. I questioned whether it was a good choice, and the projects structural engineer did some quick numbers crunching and said the insulation had more crush resistance than the ground upon which it was being placed (river bottom pit run gravel). Been back many times (11 year old system) and no signs of collapse.

    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
    To add

     Bubble foil would definitely be a structural problem with the 4" concrete topping.

    As Mark pointed out its R value is not much to speak of compared to XPS, and coupled with the fact its structural integrity is much less than 25psi xps.  I believe your 4" of concrete would not last under load for very long not to mention the bubble foil would soon be plain foil.
  • Brad White
    Brad White Member Posts: 2,393
    Sorta like what Mark said,

    spraying the underside of the deck with foam to capture the mass makes sense, but watch the edges! Truly, if you forget the edges, your losses will increase and your fuel with it. I would go for at least a perimeter of 2" XPS and carry that across on top of the slab, an insulation bathtub if you will.

    Bubble foil is not a recommended product. Great entertainment for bored kids that is about it.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • TMorg
    TMorg Member Posts: 23

    You better check if the concrete topping needs to be bonded to the precast for structural integrity of the precast system. The insulation layer may present a problem. We installed a 3000 sq. foot  precast panel house with radiant tubes power nailed to the precast and then overpoured. Spray insulation applied under planks. The real big issue is to be very careful where you drill. If you break a tension cable we were told it is a real problem.
  • Gordy
    Gordy Member Posts: 9,514
    Precast topping

     Never heard of an over pour used to actually strengthen precast designs. Usually the precast is designed to accommodate the additional dead load of the topping, and live loads though.

     Yes drilling through tension cables can create load deficiencies in pre stressed concrete. But it would be hard pressed to drill through the steel used for tension cables with a carbide drill bit say 3/16" used to wire nail accessories to the concrete. You will burn the bit up before causing damage.

     Precast, Flexicore, Double T designs are all very resilient. I have demoed a lot of bridges that were rotten to the core exposed rusted cables rotten concrete, and had cranes, backhoes on them even after they were deemed deficient to carry traffic rated loads..
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!