- Q.Instead of tile, my clients
want to install a wood-finish floor over the insulated
concrete slab of a planned passive solar addition.
They've also asked us to install tubing in the slab to
give them the option of radiantly heating the space.
But won't wood flooring have a significant insulating
effect that will diminish both solar heat gain and
radiant heat transfer?
A.John Siegenthaler, a
consulting engineer who specializes in
hydronic-heating-system design in Holland Patent,
N.Y., responds: For an in-floor radiant
heating system, engineered-wood flooring can be a
reasonable alternative to harder stone and tile,
offering better dimensional stability and lower
thermal resistance than solid-sawn lumber flooring.
I usually specify prefinished 3/8-inch
engineered-wood flooring that's surface-glued to
the heated slab, with unglued (rather than
edge-glued) tongue-and-groove joints on all four
edges. This kind of floor provides good heat
transfer while allowing for very minor shrinkage
without laterally stressing the wood.
But if the space is designed to be used as a
passive solar collector, keep in mind that even a
relatively thin engineered-wood floor over the slab
will significantly interfere with transfer of solar
heat gains to the underlying concrete. A
dark-colored tile, stone, or finished-concrete
floor would be your best choice here.
In any case, radiant slab heating is not
necessarily a good choice for a space that also
experiences significant solar heat gain. If the
heated floor keeps the space at a comfortable
temperature at night, its thermal mass will be
fully "charged" with heat when solar radiation
comes through the window the following morning. The
likely result will be overheating, because the
floor slab simply can't accept additional heat
input while also allowing the room to remain
Better choices for heating passive solar spaces
include low-mass radiant wall or ceiling panels and
steel-panel radiators, which respond quickly and
won't interfere with solar heat gain to the floor