I am pouring a slab for radiant heat next week and I have been given a variety of advice on whether or not to use rebar. The slab will be 3" thick with 2.5" of XPS foam board underneath, 1150 square feet, 7 feet below ground. I just learned about a fiberglass additive that will add strength to the mix, anyone have experience with this? I have also heard of, and never used, wire mesh. Also, any suggestions on a good heat loss calculator. I have used the one at warmly yours and came up with a heat loss of 9000 btu/hour in one of the units (there will be two units in the basement) and 6000 btu/hr heat loss in the second. However, the boilers I have been recommended is the Electro boiler (30,000 btu/hr) for the larger unit with the higher heat loss which seems too large given the heat losses and the 9000 btu/hr boiler for the smaller unit which seems about right. I would like to use another calculator (and am willing to pay for it) to get another calculation. Regards, Trent.

I first used this one:

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

Then used rescheck 4.3.0, which is the more professional calculator (I downloaded it for free). I got similar numbers from both.

Good luck.
-m

Regarding the rebar, what will your floor finish be? You'll get very differant answers for polished concrete vs carpet.

Will the space be finished or unfinished?

Regardless of whether or not you use rebar (frankly, that sounds like a question for an engineer), the fiberglass mesh of which you speak does not add appreciable strength to the concrete, at least as far as rebar-type strength goes. I used it on our exposed concrete floors, and what it does do is aid in the prevention of plastic or hairline cracks in the finished product, between the control joints that you etch or sketch into the concrete.

BTW, I am certainly not an engineer, but I would think that for a slab of that size, you'd want rebar running in both directions to keep the concrete from developing large cracks when put under unbalanced pressure. Concrete is very good at compression type forces, but it is quite poor at resisting shear type forces - that is what the rebar does. In our 4.5" thick concrete slabs, the largest of the three being about 500 square feet, our engineer specified #4's at 16 inches on center.

If you are laying the slab between footings that are already poured, you might also consider a method of tying the slab to the footings. We did not epoxy or rigidly fix the slab in there, but we did use some #3 rebar dowels (not epoxied) at 12" intervals to keep the slab level. While the engineer suggested this, it was not required for the building. Rather, it was to help stabilize the slab during the minor expansions and contractions it would experience as the slab was heated and cooled.

Again, I'd suggest that you talk to an engineer with experience with this type of concrete, or a sophisticated concrete contractor. You can only pour concrete once, after all.

Jeff

Thanks for the input. I'll find an engineer to get his two cents worth.

I'm planning on stained concrete floors in my new home's basement, but it won't have radiant heat in the slab. I will be using 6" x 6" #6 deformed welded wire mesh in the slab held 1" above the bottom with plastic mesh chairs. Around the perimeter I will install a continuous #4 rebar band tied to the wire mesh. I'm also considering having the polypropylene fibers added to the concrete prior to the pour. I wouldn't use fiberglass fibers in the slab unless it will have some type of covering, i.e. carpet, tile, etc.

Don't use fiber reinforcement in lieu of steel for a radiant slab. Fiber reinforcement is about minor crack control, not strength. When it cracks (and it will) the PEX will become your rebar. Any settlement stress will be applied directly to the PEX and could shear it. I believe 6" mesh is ideal. It also facilitates PEX layout and fastening. You should try to obtain sheets (typically 10' square) because they are much easier to work with than rolls (more often used on commercial jobs). John

The welded wire mesh that I am using is 6x6-6/6 or also known as 6x6 W2.9/2.9 (6" x 6" spacing of #6 wire running both ways). This typically comes in flat panels 8' wide x 20' long. The rolls of wire mesh at the home centers is typically the lighter gage 6" x 6" #10 wire. It is a pain to unroll it and get it to lay flat.

The #6 6x6 with 2'' foam is also what we see in our area. Slabs are usually 4'' though (more of a storage for heat). As Arkie said it also facilitates tubing fastening and a whole lot less labour then rebar(if it's in sheets and not rolls). Regarding cracks, after the floor's been poured and has hardened, one can flood to slow down curring and help consolidation strenght to achieve it's maximum performance.