Replies

1. Expert Member
   Dana Dorsett | Mar 01, 2019 06:29pm | #1

   There are various approaches, but zip-tying the tubing to the rebar or wire mesh are both commonly done, sometimes propping it off the foam with "chairs" to establish the right depth.

2. 
   rhl_ | Mar 02, 2019 07:03am | #2

   If you are trying to put the tubes 2” up with 8 or 9 “ spacing, you need to stand on the thing you put the pipes on, you either need a bunch (or a very little bit) of rebar, but certainly not the wire stuff..

3. Expert Member
   Akos | Mar 02, 2019 09:19am | #3

   Ryan,

   The way I've seen this done is to put down your wire mesh tie it together, run the pex and zip tie then finally raise the whole thing up to height with chairs.

   If you don't need reinforcement in your slab, the best is to staple the pex to your foam. With a proper amount of insulation underneath, there is no benefit to having the tube in the middle of the concrete. If anything it is slightly worse as the pipe is closer to the surface so you get a bit more heat striping.

   These folks did some analysis to see the effects.

   https://www.supplyht.com/articles/98799-tubing-depth-does-affect-the-performance-of-heated-slabs

   The problem with their setup is they only used 1" of underslab and ridiculously high 30btu/sqft heat output. With any decent floor heat setup (3-4" concrete, 2-3" of insulation), there is no difference where you put your pex.

4. 
   jberks | Mar 02, 2019 10:47am | #4

   I personally prefer to have the tubing in the upper half of the slab cross section. It can be argued it doesn't matter, and in terms of total btu output, it doesn't. but in my personal preference, I imagine the tubes closer to the top of slab will have less reaction time.

   In my current build, with the basement slab I did 5/8" rebar in a 1'x1' grid. I had them on rebar chairs to keep them elevated at the proper height. I didn't need this rebar specifically for the concrete design, but I used it anyway to make the radiant install go smoother. Rebar, wireties, and chairs are relatively cheap, so it was worth the cost for extra reinforcement, aid in crack mitigation, and ease of radiant tubing installation.

   Rebar on chairs is a lot easier to walk on than wiremesh on chairs. And don't even bother with trying to lift the wiremesh to the correct height during the pour. Concrete guys still do this and I personally think it's horrendous.

   However, on my veranda, where I installed a radiant snowmelt system. I had steel pan decking where the concrete thickness on top is only 2.5". So in this case I used mesh because rebar would eat up most of the limited cross section height I had. I raised the mesh by 1/4" on makeshift chairs and ziptied the tubing to it paying careful attention so the zip ties loose ends we're cut and pointed down so they don't stick up and mess with the concrete finishers. So walking on the mesh was quite easy in that circumstance since it didn't have much room to flex. BTW, I expect Wiremesh in rolls are horrible to work with due to it wanting to curl, I've always bought flat sheets to not have to spend extra energy tying them down.

   So to answer your question. It depends on the size of the area you are doing to decide which path to struggle with. I personally think a good way to do it would do 5/8" rebar in a cross grid on 1" chairs. 2 layers of rebar equal 1.25" in height. So the 1/2" tubing which is 0.75 in OD would sit between 2.25"-3" in the slab cross section. I assume you're not driving cars or heavy machinery on this slab, I'm no concrete engineer, but I personally think 1" of fiber reinforced concrete above the tubing is adequate in avoiding any issues like cracks or spalling above the tubing. However, you could drop it by 5/8" by having the chairs sit on the higher rebar in the grid, which would put your tubing at 1.625"-2.375".

   Hope that helps and doesn't confuse you further!

   Jamie