Slab on Grade Radiant Heat Systems
Radiant Ready Basement Packages
As the past has
demonstrated, concrete slab-on-grade floors are ideal for radiant floor
heating. The number of buildings with this type of floor construction is
huge. It includes a significant percentage of single family houses as
well as a large percentage of commercial buildings. Some of the best
floor heating opportunities are in “garage facilities” such as
automotive service centers, town highway garages, fire stations and
aircraft hangers. These buildings almost always have uncovered concrete
floors, and benefit tremendously from the warm, dry floors that radiant
hydronic floor heating can provide.
The installation of a
heated floor slab begins by verifying the sub grade has been properly
leveled and compacted. Although the heating system installer is probably
not responsible for this aspect of construction, failing to check for
proper sub grade preparation could eventually compromise the embedded
circuits. It could also leave the installer having to defend why the
floor heating system isn’t at least partly responsible for cracks in the
slab or other defects.
After the sub grade has been
prepared, the soil vapor barrier and underslab insulation should be
installed. Some building specifications may not call for an
underslab vapor barrier
insulation. However, its ability to resist
moisture migration from the underlying soils can be indispensable,
especially when wood products are used as the finish flooring.
Heat loss from the edge and
underside of a heated slab on grade can be substantial, especially in
areas with high water tables or where the slab rests on bedrock. Edge
and underslab insulation are essential in reducing these losses. They
are a necessary part of any quality floor heating system. Not taking
steps to mitigate such heat loss is like leaving the windows open
throughout the winter.
Realistically there’s only
one opportunity to install underslab insulation—before the slab is
poured. Discovering high downward heat loss after the system is in
operation is a situation that’s virtually impossible to correct. It
makes little sense to attempt the installation of a high quality heating
system while omitting crucial and relatively low cost details. Do it
right the first time.
The most commonly used
material for slab edge and underside insulation is extruded polystyrene.
It’s sold in 2 by 8 foot and 4 by 8 foot sheets in several thicknesses.
It’s also available in several densities to handle different floor
loading. Extruded polystyrene panels are highly resistant to moisture
absorption, and have a well-established record in ground contact
insulation applications. We recommend
The Barrier Insulation.
The next step on most
installations is to locate and temporarily mount the manifold
station(s). If one more of the manifold stations will be located within stud cavity, it’s imperative to make
accurate measurements when fixing the manifold’s location. The manifolds can be temporarily bracketed to plywood panel supported on
wooden or steel stakes driven into the sub grade.
insulation is in place, the steel reinforcement for the slab is
installed. Most concrete slab on grade floors use welded wire fabric (WWF)
for reinforcement and crack control. WWF comes in sheets or rolls. It
should be placed directly on top of the underslab insulation. Edges
should be overlapped approximately 6” and tied together.
installation takes place one circuit at a time. Begin by securing one
end of the circuit to the supply manifold. Roll out the coil like
rolling a “tire” following the layout pattern. The composite pipe,
because of the metal content, allows laying the pipe roughly without
tying down immediately. This allows it to run the full loop and get the
end out to the manifold. Make sure the end reaches the manifold and then
tie the piping to the wire mesh. The main difference to laying
tubing is that the pipe stays in place and does not want to go back to
the coil shape.
This is why there is no
need to use an uncoiler. If the uncoiler is available, it is also
possible to lay the pipe using it. In this case place the tubing coil on
an uncoiler and pull the tubing from the coil as needed. Keep plenty of
slack ahead of you as the tubing is fastened in place.
Pex-AL-Pex should be secured to the WWF using either twisted wire ties
or nylon pull ties. The tubing should be tied to the WWF reinforcing
every 60 to 72” on straight runs, and two ties at the bend on each side.
When all circuits have been
installed, prepare the manifold(s) for pressure testing. Install a
pressure gauge in one end of either the supply or return manifold and a
Schrader air valve in the other end. Plug the unused manifold ends.
Use an air compressor to
increase the pressure in the circuits to about 100 psi. Use a soap
bubble solution to check for leaks at the manifold connections. Leave
the circuits pressurized for at least 24 hours. If the air pressure
drops double check all manifold
connections for possible leaks before inspecting the tubing. Aside from
the possibility of extreme damage from other construction activity, it’s
very unlikely that the tubing
is the source of the air leak. Still, a pressure test is mandatory on
any radiant tubing installation. If the WWF has to be positioned in the
concrete slab, be sure the concrete slab placement crew knows to lift
the tubing and WWF prior to starting the pour. If the WWF has to be
positioned within the slab, it has to be lifted or “chaired” up to the
final position before the concrete slab is poured. The WWF and attached
tubing should be lifted up so the pipe center is 2” below the slab
surface. This allows the concrete slab to respond faster when warm water
circulates through the tubing.
Radiant Heat Information