Need Primer on PEX Tubing in Concrete
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I’m looking at a project here in cold New England where the homeowner wants to put PEX heating tubing in a 10′ x 11′ x 6″ concrete slab on grade under a bath/kitchenette area that will be finished with ceramic tile. He has already installed 3″ of white rigid foam board with no vapor barrier. There are drain and supply pipes in the floor. The current plan is to clip the tubing to wire reinforcing mesh and pour regular (4000 PSI?) concrete keeping the tubing near the surface.
Any suggestions? Is the lack of vapor barrier a big problem? Does the thickness of the slab have a big impact on heating efficiency? Should a special concrete be used? Would it be better to put the tubing in a bed of sand under the concrete? Anything else?
Thanks!
Al Mollitor, Sharon MA
Replies
Hi Al,
Some people do like to place the PEX in sand under the crete to cushion it but the stuff is incredibly tough.
The foam should satisfy the need for VB, unless there is a tremendous amt of subsurface water.
don't try to get the PEX too near the surface of the crete. The idea is to use the whole thermal mass to radiate. placing the PEX in roughly center of amss is the better wy to do this. also, it is less likely to initiate cracking of the concrete. We use 3000# crete with regular agregate. I had similar questions at first about whether peasstone or sand should be used.
For the layout, tie to the rewire at approx 18" OC and design the runs to have no more than 200 - 250 LF each. This small slab might be only one run of tubing. Lay it out so the supply side follows along exterior (heat loss higher) walls at perimeter first and the cooler water will be running in the center then, keeping the average of the slab close to equal throughout. For the first few times, it is a good idea to draw the room on graph paper and play with a pencil to figure where the loops will run. Try to aoid getting loops closer together than 6" and do not cross over. Post a drawing here and somebody will play with the layout. Maybe even me.
The water gets run through a mixing valve to about 111°-114°F. Too hot and it can be uncomfortable, demand too much BTU from boiler, and overheat slab, causing thermal stress in the crete. Theoretically, running the tubing close to surface and overheating the water is a fantastic way to crack the crete.
Excellence is its own reward!
When you say "white rigid board" are you speaking of the beaded board? I think that you should be using the extruded (either pink or blue). Saw another contractor pour on the beaded board, when the first concrete hit the white board it blew apart and the job ended up being 6" of concrete with white beads mixed in. Also make sure that you insulate down the side of the footings. Just to restate what has already been said, it heats the mass don't worry about getting it near the surface.
Poured my slab on 2" beadboard with no problems.
How high was the boom when the concrete hit the foam?
Idiots can screw up anything, the problem isn't/wasn't the beadboard.
Joe H
I was told that the beady foam did not have the PSI rating to handle concrete. It also has more water infiltration than that of extruded foam. So maybe a vapor barrier would not be a bad idea.
Don, it comes in different densities. Type II is what they carry at the yards and boxes. Type IX is what you need, but NOBODY stocks it. You need to contact a R-Control manufacturing plant and order it.
Even the type II stuff will support the concrete, it's the loads on the slab that will crush it if you are using it under a garage or shop. I think for residential it would be no problem. Although the concrete guys might step through it, not much being careful with most of them I've seen. I think (been awhile) the light stuff will support 15 PSI, so that's quite a bit of weight per square foot.
They are all over the country in places that are not close to you. Freight is expensive, but the board itself is waaaaaaaaaaay cheaper than pink or blue board.
After much screwing around I found that it actually comes in blocks 4X8' by 32" thick I think. They slice it to whatever you need in however many sheets you get from a block.
You do not need a vapor barrier under it, it is a VB.
These numbers might be off a bit, it's been awhile.
Joe H
I was in Contour Products, an R-control fabricator, and I forgot what the large largest block that I was but it was much larger than 4x8x32". I can't even guess at the size it was too long ago. But I remeber looking WAY UP at it. Mayve 4x8x12ft.
Bill, You're right, it comes in huge. They use it under highways in big chunks too.
But, I think if you or I need some in sheet form, we will have to buy the block. Or all the sheets in a block. They don't seem to stock it in sheet form, it is cut to order.
Not to say some other manufacturer doesn't,I just couldn't find one close.
Joe H
you should get pex wrap. basically its a 4 mil plastic sleeve that fits over the pex. It comes in blue and red to denote hot cold. Its used so that the PEX can expand and contract w/o rubbing the concrete. The sleeve just lets the plastic give and take while not abrading the PEX line in the concrete.