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Radiant heating with post-tensioned slabs

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Has anyone any comment on the idea of installing hydronic radiant heating/cooling (say 5/8" I.D. PEX tubing at 8" O.C., water temperature 150 F maximum, 90 F more common for heating, temperature for cooling unknown to me as yet) directly in a post-tensioned suspended slab?  Initial thinking is for 20-foot by 30-foot bays, with a 7½-inch, 5000 psi, normal-weight, two-way, post-tensioned concrete flat plate with the tubing located in the upper 1/3 of the slab's thickness.  There would be (adhered?) foam sheet insulation on the bottom of the slab.  This is for fully-enclosed office/light storage/light workshop occupancy in a mild climate (Pacific Northwest Willamette Valley).  Has anyone ever done a similar program?  Are there any problems to report?  Naturally, I am very interested in the expected thermal expansion effects on the P-T slab.  If you say "use a topping for the tubing," also say why you think that helps.  The structural slab still gets heated/cooled, doesn't it?  The objection to insulation seems to be that if insulation separates the slab from the topping, the benefit of the thermal mass of the slab is lost in moderating the daily thermal cycle.

Thanks for your help!


Thomas B. Higgins, P.E., S.E.

Group Mackenzie
0690 S.W. Bancroft Street
Portland, OR 97239-0039
Phone (503) 224-9560
Fax (503) 228-1285
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