The Glitch

     Take at look at the photo showing placement of a 6-in.-thick concrete slab containing 5/8-in. PEX tubing at 12-in. spacing. A nearly complete layer of extruded polystyrene insulation is under the slab.

     Do you see any details the might compromise the structural or thermal performance of this slab?

The Fix

     These masons are dumping and leveling the concrete on top of the tubing and welded wire reinforcing while making no attempt to lift the steel or tubing above the bottom of the slab. Maybe they think that the reinforcing is just there to hold the tubing in place during the pour.

     Kind of makes you wonder what they would do when pouring a slab that doesn’t contain any tubing. “If WWR is just for holding tubing in place, why is it even present in a slab without tubing?”  Perhaps their collective answer is, “Don’t know and don’t care.”

     The guideline specifications from the Wire Reinforcing Institutecall for a single layer of welded wire reinforcing to be installed between 1/3and 1/2 the depth of the slab, measured down from the slab surface. Leaving the reinforcing at the bottom of the slab does not allow proper “engagement” with the concrete, and provides little if any resistance to shrinkage cracking, which is usually the main intended function of WWR.

     Thus, even in non-heated slabs, the WWR should be lifted or otherwise supported so that it is embedded at the proper depth in the slab.

     Now mix in the heat transfer issues. The Fix figure on shows the variation in upward heat output from a 4-inch thick bare concrete slab based on tubing spaced 12” apart, installed at two depths, and operating at an average water temperature of 100 ºF.

     There is a 26% drop in heat output for tubing installed at the bottom, rather than the mid-height of the slab. If the slab needs to release heat at a rate of 15 Btu/hr/ft², the average circuit water temperature needs to increase from 95° to 102º for tubing left at the bottom of the slab.

     If the slab needs to release heat at 30 Btu/hr/ft², the average circuit water temperature needs to increase from 120° to 134º if the tubing isn’t placed near the middle of the slab. These temperature increases will lower the thermal efficiency of modern hydronic heat sources such as condensing boilers, heat pumps and solar collectors. 

     Leaving tubing at the bottom of the slab also increases downward heat loss and lengthens the slab’s response to changes in water temperature.  The latter can lead to pronounced temperature overshoot, especially if there are significant and unpredictable internal heat gains from the sun, people or equipment.

     The bottom line is simple: The depth of the tubing within a heated slab does make a significant difference in performance. When the tubing is tied to welded wire reinforcing, both the WWR and the tubing should be placed at approximately half-slab depth. 

     Why compromise both the structure and thermal performance of a heated slab based on careless workmanship? Make sure your written specifications and construction drawings show and describe how to do it right. If the masons or the general contractor disagree, perhaps you can show them the Fix figure.

     If they still disagree, perhaps they would be willing to sign a release stating that they are now responsible for the performance of the slab.

     Yeah, right!