Notes on the Suspended Slab

When radiant tubing is installed on an existing floor, it’s called a “suspended slab”. If you’ve decided that the suspended slab method is best for your application, but aren’t sure which type of suspended slab to use, the following explanation may help.

There are two primary methods of installation and each has advantages and disadvantages.

Pouring a Thin Slab Without Sleepers

Method one involves laying out the tubing directly on top of the existing floor, then pouring a layer of concrete much like you would with a standard slab on grade. This method is practical if the installer is comfortable with pouring concrete and can reasonably expect to end up with a nice, level floor once the pour is done.

With this version of the suspended slab, the tubing can be attached to an existing concrete floor with tubing straps by power nailing directly into the slab with a ram set. If possible, fasten the tubing every 2′ to 3′. Or, wire mesh can be anchored to the concrete in the same manner and the tubing can be attached to the mesh.

In the case of a suspended slab over an existing plywood floor, the tubing is held down with copper, plastic, or galvanized tubing straps, then the new layer of concrete is poured over it.

If this layer of new concrete is to become the finished floor, then wire re-enforcement should be laid over the tubing to minimize cracking. At least 1″ of concrete should cover the tubing.

If cracking and shrinking isn’t an issue and the new concrete serves primarily to anchor the tubing and act as thermal mass, then a thinner layer of concrete is okay. A floating wood floor, tile, stone, or carpet can then be installed over the new radiant slab.

Advantages: Self-leveling concrete can be used for a perfect, level floor and it’s done in one operation. Excellent thermal properties. Ideal surface for laying a finished floor of tile, marble, stone, or carpet. Easy to lay out tubing.

Disadvantages: Difficult or impossible for the do it yourselfer. Self-leveling concrete (i.e. Gypcrete, Thermafloor) is very expensive and normally requires factory trained installers with special equipment. Not practical to install ¾” hardwood floors upon. Could be structural issues due to increased weight.

The Suspended Slab With Sleepers

Method two involves laying down sleepers in order to raise the floor and create a channel for the radiant tubing.

Sleepers can take many forms depending upon which size of radiant tubing is used. If headroom is critical, then ½” PEX tubing on 8″ centers is commonly used with sleepers made from 4″ wide strips of ¾” plywood.

If ceilings are high, or if new construction allows you to compensate for an approximate 2″ rise in the floor, then a larger, higher output tubing like 7/8″ PEX can be used with common 2×4’s laying flat as sleepers. Pressure treated 2×4’s are used if the sleepers are installed over an existing concrete floor. Sleepers are normally spaced across the floor 16″ on center.

The beauty of 7/8″ PEX is twice the heat output compared to the ½” PEX. That means less money spent on tubing to do the same heating job. The outer diameter of the 7/8″ tubing is 1″, so even if headroom is critical the floor can still be kept low by using sleepers made from 5/4 board (1″ thick deck lumber).

Advantages: Easier for the do it yourselfer. If concrete is poured over the tubing, the pre-leveled sleepers can be used to screed across and a perfect level floor is the result.

Plywood or hardwood can be nailed to the sleepers after the radiant floor in installed. A suspended slab using sleepers offers maximum flexibility when it comes to final floor coverings.

Sand is lighter than concrete and offers good thermal properties. It can be poured between the sleepers and used as a thermal mass instead of concrete because ¾” hardwood flooring can span across the sleepers. The sleepers can also be decked with plywood when tile or carpet is used for the final floor.

Disadvantages: Slightly higher material and labor costs than the non-sleeper method, (unless the very expensive self-leveling concrete is used), less flexible when it comes to laying out the radiant tubing.

Insulation Under a Suspended Slab

When installing a suspended slab over an existing concrete floor, remember that the slab below, if left uninsulated, will act as a “heat sink” and eventually reach the same temperature as the new suspended slab. This can work to your advantage by significantly increasing the thermal mass in your heated space and most of this thermal energy will eventually end up in your living space. However, unless the existing slab is insulated, some thermal leakage will occur wherever the warm existing slab contacts any surface colder than it is (i.e. foundation walls, cold earth, etc.)

If headroom is at a premium, then this approach may be a viable option.

If, however, you wish to maximize the transfer of radiant energy to the living space and greatly restrict heat loss to the floor below, then a 1″ to 2″ layer of Extruded Polystyrene foam insulation should be applied to the existing floor prior to installing the radiant tubing.

Note: For more information on the “Suspended Slab Installation” see our Design and Installation Manual or the Suspended Slab section of this website.