- Q.A client wants a
portion of a basement slab to have radiant heat to take
the chill off the floor of a planned playroom (there
will be supplemental heat). The hvac contractor wants
the radiant tubing, which will be attached to wire
mesh, to be lifted into the middle of the slab during
the pour to put the heat closer to the surface. The
concrete contractor doesn’t want to do this
because he insists that cracks will show up along the
tubing. He wants to leave the tubing at the bottom, and
says the insulation board will drive the heat up
anyway. He recommends at least 3 inches of
concrete-above the tubing. Which is
correct?
A.Hydronic heating
contractor Bill Clinton responds: We typically
try to keep at least an inch of concrete above our
tube. If the slab is 4 inches thick and the mesh is
in the middle, a 5/8-inch o.d. tube would have
about 1 1/4 inches above it. Pulling mesh up during
the pour is by nature somewhat inaccurate, so
it’s probably best to err on the side of
keeping the mesh low in the slab. I don’t
believe the height of the tube in the slab is going
to make a significant difference in system
performance even if it’s on the bottom
— the entire slab will be heated in any
case. My advice is to let the concrete contractor
have it his way.
One point about laying out the tube: Tie it
perpendicular to the wire as much as possible, away
from parallel strands of wire. The reinforcing
value of the wire depends on concrete bonding all
the way around it, so it’s best to keep
the tube out of the way.