Types of concrete slabs with radiant floor heating
Thick slabs
Thick slabs are concrete slabs with an overall thickness of 4-6" or greater and can be either grade level (slab on grade) or below grade (i.e. basement foundations). All thick slabs can be categorized into:
Reinforced slabs – where welded wire mesh or rebar is used to reinforce the slab.
Non-reinforced slabs – where no reinforcement is added.
While reinforcement itself does not affect the radiant floor heating system, it determines placement of the PEX tubing in the slab, which in itself is an important factor. Unless special design considerations require otherwise, the tubing should always be positioned on top of the reinforcement, in order to stay closer to the surface of the slab.
If using welded wire mesh, you may want opt for sheets rather than rolls whenever possible. They are noticeably easier to install and provide a flatter surface. The main downside is that the sheets have to be tied together.
Optimal depth of PEX tubing in the thick slab is considered to be in the 1-2" range and, whenever possible, should not be deeper than 4" for the following reasons:
Placing tubing too deep in the slab will increase response time, which means it will take longer for the floor to reach the desired temperature, will result in increased BTU load, require more energy and possibly will require larger tubing diameter.
Concrete height above PEX adds additional R value, and while it’s minimal in most instances, more energy would be required to heat the topmost surface.
Since in non-reinforced slabs tubing is generally positioned at the bottom (secured with foamboard staples or PEX rails), their thickness should not exceed 4-5". Otherwise, the system will not run efficiently. The only remedy for a deep slab is to install reinforcement and position PEX tubing on top, closer to the surface.
