I have a slab design that has two layers of rebar. One is 3/4" under the surface, and the other is 3/4 above the bottom. The slab is 5 1/2" thick.

We're scratching our heads trying to figure out how to lay this tubing. One idea is to have it on top of the lower rebar. But that puts the tubing 3 inches below the slab surface.

I've seen a chart that shows the heating characteristics with respect to tubing distance below slab surface. I can't find it. That's why I'm asking here what you all think.

My questions are two-

-Is 3" below the surface a big deal?

-If so, how can we fasten this tubing so that it's a reasonable distance below the surface, when we have no rebar there?


Thanks.



Well, I did find, after considerable searching, this article on the subject. It doesn't answer my question to my satisfaction. I'd sure like to hear from someone who has real experience. But I'll post it for reference-

http://findarticles.com/p/articles/mi_m0BPR/is_6_20/ai_102862289/

3 inches below the surface seems reasonable, I would be more concerned with the insulation, 2 inch rigid?
Dan

Siggy is the authority in our industry.

that said, the effect of tubing placement at the bottom of the slab is small. The effect on ease of installation and piping safety is large. so we usually recommend it on the bottom. with thicker slabs like yours, a little higher up is fine. I would go with the lower rebar, and I would share Dan's concern for proper insulation. don't forget the vertical slab edges.

Thanks you guys! I really appreciate the reassurance.

Yes, I have Foamular 250 going under the slab. 2" thick.

Performance can be enhanced by raising the tube, but is entirely unnecessary in most applications, 5.5" is not that thick. Proper design would require a proper heat load analysis, the load dynamics and control considerations♦.

Two layers of bar is new to me and too close the the surface.

Who is designing this, or is it design by internet?

We did a project not to long back with 2 layers of bar in an 8 inch slab in the basement. The slab spanned a large sewer pipe (much deeper) and in essence was a bridge. That was one pipe you did not want to break. the home was several million dollars, on a golf course and all in an exclusive Seattle area, just the unfortunate placement of a pipe 75 years ago.
oh well,
Dan

This is a slab being designed by a qualified local engineer. I've used him before. The foundation is not trivial. It's piers and grade beams. The slab is floating.

The only thing I am having issues with is construction joints with hydronics. How does that work? I should post it in another thread. I would like construction joints, but with several joints, that makes running the tubing, etc. much more complicated than just laying it out and pouring. Unless there are construction joints that are part of one single pour. And there probably are.

Just out of curiousity, how large is your slab? I have a floating 5" thick slab here in CA that's 600 square feet but with only one layer of rebar - in the middle.

Jeff

It's a complicated shape. Lots of zigs and zags. 2100 square feet.

It is essentially a house up on 23 piers. In the air.

I assume the engineer designed it knowing that radiant tubes would be in the slab. Where did he plan for them to be? If not, better run that by the engineer to make sure puting in the tubes doesn't throw off his slab design.

Posted By gregj on 11 Oct 2010 02:38 PM
I assume the engineer designed it knowing that radiant tubes would be in the slab. Where did he plan for them to be? If not, better run that by the engineer to make sure puting in the tubes doesn't throw off his slab design.

Yes. It's all accounted for.

I have put the tubing directly on the EPS foam.  Then the rebar mats using a 2 inch dobie so there would be no way that someone could crush the pex.

Posted By Roundeye on 11 Oct 2010 06:32 PM
I have put the tubing directly on the EPS foam.  Then the rebar mats using a 2 inch dobie so there would be no way that someone could crush the pex.

I like that concept. That would make my installation really nice, since my rebar is 3/4" off the bottom of the slab.

If having the hydronic tubing at the bottom of the slab is an issue, I would prefer to have it at least on top of the rebar. So that really begs the question all over again.  What would be the heating characteristics at 5 1/2" from the top of the slab?

Use plastic snap lock rebar ties that will accept the pex pipe.

1.Lay down the insulation and first mat on chairs.
2. Rough out radiant.
3. Lay upper mat on top of radiant pipe.
4. Use plastic snap locks to hold the pipe and upper mat in final position.
5. Seperate mats using tied dobies or spreaaders.

CL of pipe will be approximately 1.75 - 2.25 off the finished surface (depends on the rebar# and pipe size)
Finally

Bag tie pex to lower course of re-bar, lift re-bar with chairs (dobe block). Run second course of Bar as needed at hight using bricks. done