One of the best win-win situations I've experienced was in collaborating with a self-employed contractor named Harvey Youker. Harvey is truly a "general practitioner" among contractors. Not only can he professionally install a complex multiload hydronic heating system, he's also a darn good carpenter. Harvey and I have worked on many projects together over the last 16 years or so. I'll typically draw up plans, give them to Harvey, and rest assured his artistry will make the completed job look even better than I imagined.

A few years ago Harvey and I were sitting in McDonald's eating lunch. We starting discussing a lost opportunity to install floor heating in a house because the budget couldn't support what we both felt was a fairly priced system using a gypsum thin slab. Both being strong advocates of floor heating, we started brainstorming for alternatives that would keep the benefits but lower the costs.

Harvey began describing his idea for how a heated thin concrete slab could be efficiently installed in new wood-framed construction projects. We kicked the idea back and forth both knowing we were on to something. After some research we came up with the necessary details. By the next year an ideal project came along to try it on. That system is now been in place for about five years and the slab is performing great. To give credit where it's due, I've dubbed this approach "The Youker System." As you read about it, glance over at the installation sequence in Figure 1.

A Giant Drawing Board

The Youker system is "optimized" for new construction applications. It begins right after plywood subflooring has been installed on the floor deck, before any walls or partitions are framed. After sweeping the deck clean, reference the floor plans and snap chalk lines on the subfloor for all walls, partitions and built-in cabinets. In effect you're copying the floor plan onto the floor deck. Next use the tubing layout plan, (which of course you've already prepared), along with a lumber crayon to mark out key features of the tube circuits like return bends and offsets. There's really no need to draw every line of the tubing layout unless you've got time to kill, or just need some quality time with crayons.

Now cover the area that gets the thin-slab with transparent 6-mill polyethylene film, overlapping it a couple of feet at the seams. The poly acts as a bondbreaker between the plywood and slab allowing slippage between them as the floor deck shrinks and expands in response to changes in temperature and humidity. Without it, cracks caused by tensile stresses in the concrete are much more likely. The poly also prevents the plywood from absorbing water from the fresh concrete. Wet plywood under a hardening slab sets the stage for serious stability problems when things finally dry out. And dry out they will! One final point regarding the bond breaker layer. Never use asphalt-saturated roofing felt instead of the polyethylene. When heated it creates an aroma even a chimney sweep can't appreciate.

The next step is to fasten down 2 by 4 or 2 by 6 "base plates" wherever walls or partitions will be located. Just follow the lines you drew earlier. They're easy to see through the transparent poly film. You should also block off areas under cabinets, islands or other places where tubing will not be installed. Don't forget to block out for toilet flanges. A piece of 1-1/2-inch styrofoam works well and it's easy to remove when it's time to install the flange. Also be sure to block off the edges around stairwell openings. You've now created shallow pans 1-1/2 inches deep within each room. The base plates will serve as screed guides for the concrete, and nailers for the walls and partitions to follow.

Install your manifold station and tubing. Just follow those layout lines you drew on the subfloor. Now is when you're really glad you made an accurate tubing layout plan. No worrying about tubing coils being too short, or circuits crossing over each other near the manifold. Now it's just a matter of how fast you can uncoil and fasten tubing in place. Payback for the time spent on the CAD system back at the office. After all the tubing is down, be sure to perform the usual pressure test.

When it comes to a thin concrete slab on wood-framed floors you have a choice: The slab will automatically crack itself where it feels like it, or, within reason, you can force the cracks to occur where they present the least problems. Either way there will be some cracks in the slab.

One way to minimize random cracks is to divide the slab into a "mosaic" of smaller pieces that can each flex independently. It's the same concept used with thicker on-grade slabs. But with at best 3/4-inch concrete coverage over the tubes. I suggest leaving the concrete saw in the truck. Instead use 1-inch by 1-inch plastic or metal angle strips fastened down with one leg pointing up. Youker and I found that 1-inch by 1-inch drywall trim strips about 1/16 inches thick work well. Here are the locations where random cracks are likely to form, and hence where a control joint strip should be installed.

  • Any interior doorway or other "bottleneck" through which the slab narrows down and then widens out.

  • Any outside corner of a room, (or other object such as a kitchen island), that creates a change in direction of the slab edge. The control joint strip should run away from the outside corner of the object so the (inside) corners of the resulting "slabetts" are not less than 90 degrees.

  • Control joint strips should be used to divide larger open floor areas into rectangles 8 to 10 feet on each side.

  • If rigid flooring like ceramic tile will be used, make every reasonable effort to locate control joint strips directly under where the joint lines between the tiles will be.

  • Transition lines between different types of finish flooring are also good locations for control joints.

For these details to work, there obviously has to be some discussion of finish flooring plans with the owner or builder beforehand. I suggest you draw the control joint locations on the tubing layout plan before placing the tubing circuits.

Sometimes control joint strips have to run perpendicular to the tubing. Just lay the strip over the fastened-down tubing, mark the crossover locations, and notch out the strip with a pair of snips before stapling it down. I've also seen the strips cut into segments and fastened down between the tubes. It seems to work, but I feel the joint will remain straighter if the strips are kept intact. In either case use a chalk line or laser to keep the joints straight.

One final pre-pour detail is to coat the sides of the 2 by 4 or 2 by 6 baseplates with mineral oil wherever concrete will contact them. Again the reason is to prevent bonding between wood and concrete as the latter shrinks during curing. PEX suppliers assured me mineral oil would not harm the tubing if spilled. Applying with a paintbrush is neat and fast.

The Recipe

Ingredients for the concrete mix we used are given in Figure 2. The superplastizer and water-reducing agent increase the flowability of the concrete, allowing it to fully encase the tubing for good heat transfer, without adding more water. They're available from most ready-mix concrete suppliers. The FibermeshR forms a three-dimensional high strength web throughout the slab to resist shrinkage cracking. It also eliminates the need for wire reinforcing. For a 1-1/2-inch slab, you'll need 1 cubic yard of concrete for each 200 sq. ft. of floor area.

When all the above steps are completed it's time to pour. If possible have the ready-mix truck parked near the floor deck so the concrete can flow down the chute right into a waiting wheelbarrow. I've had no problems running wheelbarrows full of concrete over tubing, but be careful when the wheelbarrow is dumped. Put a 2-foot by 2-foot scrap of plywood over the tubing where the nose bar will hit. This mix screeds easily, using the baseplates as guides. The best part is that no studs or walls get in the way of the screed bar.

Once placed, the concrete can be floated and trowelled as usual. Power trowels can glide right over the baseplates when necessary. Slab surfaces to be covered by carpet or ceramic tile don't have to be glass smooth as long as they're flat. Areas to be covered with vinyl flooring should be trowelled as smoothly as possible.

Framing can continue the day after the pour. The carpenters can stand those walls up and nail them right onto the baseplates. A day or two after the pour you'll see hairline cracks developing directly over the control joint strips. You'll also notice very slight gaps between the slab edge and the baseplates. These cracks and gaps are exactly what's supposed to happen.

For extra insurance against cracks in hard-surfaced flooring (such as ceramic tile) I suggest using an anti-fracture membrane between the top of the slab and the thin-set mortar layer. This membrane provides some elasticity between the slab and flooring to stop any hairline cracks in the slab from moving up into the tile. Some manufacturers of poured gypsum underlayments also recommend anti-fracture membranes under tile flooring. Most tile installation companies should have them available and know how to install them.

Plan Early

Thin-slab systems bring their own peculiarities to a building project. Coordination between building trades is essential. Talk to the building designer as early as possible about the following:

  • A 1-1/2-inch concrete slab adds about 18 pounds per sq. ft. (psf) to the floor's "dead loading." By comparison, a gypsum slab of the same thickness adds about 14.5 psf. For either system the building designer should select floor framing so maximum deflection under full loading is limited to 1/600 of the joist span. This is significantly stiffer than a "code-minimum" L/360 deflection rating. It assures your clients won't be walking around on a concrete-coated trampoline.

  • The minimum insulation to be installed under all heated thin-slab floor areas should be as follows:

    R-11 for floors above other heated spaces.

    R-19 for floors above partially heated spaces/basements.

    R-30 for floors above unheated spaces.

  • Be sure the slab is tested for dryness before installing finish flooring. One industry suggestion is to tape down all edges of a 2 sq. ft. piece of clear plastic film, and see if any moisture appears after a couple of days. If it does, the slab is still too damp for finish flooring. I also recommend operating the heated slab for at least a week to drive off any residual moisture prior to installing the flooring. The simple fact is that dry substrates make for stable finished floors.

Figure 3 shows a cross section of the completed Youker system floor with details of a control joint. Notice that the joint between the tiles is filled with a color-matched elastomer caulk rather than rigid grouting to allow for flexing without cracks.

You should consider the Youker system as part of your radiant floor offering. Work out who does what part of it with the project G.C. Maybe you'll even find ways of improving the system. Remember, however, that it is a system. You can't just leave out certain details and expect it to work correctly.

Oh and by the way, if you happen to run in to Harvey Youker, offer to take him to lunch at McDonald's. Who knows what new installation techniques might be discovered once both your brains are fueled by quarter pounders with cheese.