When installing a radiant heating system, the type of floor you are using will determine the method used to install it. Let’s look at the differences between a radiant concrete slab and a suspended floor.
For a radiant concrete slab, the first item we install is the vapor barrier. Ground water can wipe BTUs away from the heated slab. A vapor barrier works to prevent the water from reaching the bottom of the slab.
The next item is the slab insulation. State energy code dictates the r-value of the under slab and edge insulation. Heat comes off of the tubing in all directions, so proper insulation helps drive the heat up and into the conditioned space. Extruded polystyrene foam board with a 25 psi minimum compressive strength is typically used for floors. Expanded polystyrene “bead board” is to be avoided as it is susceptible to insect damage.
Tubing can be stapled to the foam board with plastic foam board staples or tied to re-rod or wire mesh. Care should be taken to insure that the tubing is clear of any sharp edges to avoid damage.
If you are using a very low temperature heat source, such as a geothermal heat pump, then you should be using the mesh or re-rod method and pulling the grid to the center of the slab during the pour. This will allow maximum BTU transfer to the surface with the lower temperature water.
Another consideration with a radiant concrete slab is the floor coverings. If you are using a hard surface, such as ceramic, stone or slate, you should have a crack isolation membrane applied by the flooring contractor to help prevent concrete cracks from transfering through to the surface. Also, be aware that hardwood, carpet and padding all have r-values and can significantly reduce the heat output.
For suspended floors, three methods can be used: 1-1/2” concrete/gyp-crete thin slabs, 5/8” above floor wood and aluminum plate systems, or below the floor staple-up systems. Each of these methods have advantages and disadvantages.
The 1-1/2” thin slab requires all interior and exterior walls to be built on additional plates to accommodate the 1-1/2” pour. Also, sleepers will have to be installed if a nail-down hardwood floor is desired. However, it is a relatively easy method of installation. The thermal mass makes for a quieter room as well.
A 5/8” wood and aluminum plate system is more labor intensive, since the panels require cutting and fitting a predesigned layout. On the upside, since it is a thin floor system, the response time is generally quicker.
Finally, staple-up systems are generally the least costly but poorest performing of the three, primarily because the subfloor and finished flooring creates more insulation above the heated tube and aluminum panel. After installing the staple-up system, be sure to add bubble foil reflective insulation to the bottom of the floor joists to help drive the heat back up to the backside of the floor.
Staple-up systems without metal plates are troublesome. Since there are no metal plates to assist in drawing heat from the tubing to the floor, higher temperatures are required to deliver the BTUs. The higher temperatures cause the tubing to expand and contract at greater rates than lower temperatures, which in turn causes rubbing noises in the tubing grid that will be transferred to the living spaces.
When installing a radiant system, be sure to do your research, as there are many different techniques available. The basic rules apply to each: air around the tubing is an insulator, and the BTUs need to get from the water to the surface of the floor as efficiently as possible.