If copper tubing is to be installed in a slab, certain precautions must be taken. If it
is slab on grade, soil compaction is crucial. Flexing or cracks in the concrete can result
in split tubing and leaks. The copper can be fixed in place with wire or tie straps to the
re-enforcing wire or rods, but the steel and the copper must be grounded. The system must
be a closed loop. Fresh water (hot water from a domestic hot water heater) will cause an
electrolytic reaction between the chemicals in the concrete and the copper, eventually corroding
and pitting the copper; creating plenty of leaks. If the slab is on top of wood joists,
a double layer of 3/4" plywood is needed to keep the deck as rigid as possible.
Slab on Grade
After soil compaction, two inches of rigid insulation is recommended. The concrete re-enforcing
mesh becomes a grid to fasten the tubing to with plastic tie straps or wire. At least 3/4'
of concrete should cover the tubing to prevent weak spots that could collapse under heavy
objects placed on the finished floor. If extremely heavy objects are to roll or sit on the
floor, the deeper and more even the cover the better.
Slab on Slab
When pouring a new slab over an old one, rigid insulation is recommended even if the old
slab has insulation under it. Efficiency and recovery time will be improved. Tubing can
be attached to the mesh or to tracks designed to clip the tubing into. At least 3/4' of
concrete should cover the tubing for floor strength.
Slab over wood
There are two ways to insulate the wood deck before pouring the slab over the tubing.
Foil faced fiberglass insulation can be fitted into the floor joists directly under the
deck, allowing a 2'' air space. The tubing is then stapled to the deck or fitted in the
tracks manufactured specifically for the tubing. An alternate method is to lay rigid insulation
on top of the deck. The fastening devices must reach through the insulation and into the
wood to prevent the tubing from floating up as the concrete is poured.
Wood over Slab
If a wood floor is to be placed over a slab, sleepers are used to create a space between
the wood and the slab deep enough to fit the tubing on top of rigid insulation. At least
one inch of rigid is needed, and two is better. Wood has nearly the same R-value as insulation,
and if the tubing is sandwiched between two materials of equal insulation values, it will
lose heat equally in all directions, including the slab and into the soil below it.