Always Use a Moisture Barrier
The new urethane finishes common on today’s hardwood
floors put a moisture barrier on top of the wood. The last
thing you want is for the unfinished back to absorb moisture
from the subfloor — a common cause of cupping.
Hardwood always used to be installed over resin paper or
something similar. With a radiant floor this "cleavage
membrane" serves another function — to stop moisture from
migrating from the subfloor or Gyp-Crete into the flooring. A
moisture barrier won’t help a chronically wet house, but
it will buy a little time while a new house dries, and will
buffer seasonal moisture changes.
The one moisture barrier not to use under a radiant floor is
black asphalt-impregnated felt — that is unless you enjoy
the smell of asphalt every time the heat comes on. A sheet of
poly would probably do the job, but the installers we work with
prefer a moisture-retarding membrane such as Moistop
(Fortifiber Corp., 300 Industrial Dr., Fernley, NV 89408;
800/773-4777). This product is 12 mils thick, and consists of
Kraft paper and fiberglass reinforcing laminated between two
sheets of black poly (Figure 3).
3.For hardwood strip flooring over radiant
heating, always use a moisture barrier like Moistop (top), a
fiber-reinforced composite of poly and Kraft paper popular with
flooring contractors in the author’s area. Under floating
hardwood floors, a pad like AstroBarrier (above) provides a
foam cleavage layer and a poly moisture break.
Moistop has been used for years in the concrete industry,
primarily as a sub-slab vapor retarder and also as a radon
barrier. In the past few years, the product has found a niche
in the flooring industry. At around 5¢ per square foot,
it’s cheap insurance.
If you’re installing a floating laminated wood floor,
use a product like AstroBarrier (Innovative Energy, Inc., 10653
W. 181st Ave., Lowell, IN 46356; 800/776-3645), which is a
high-density foam pad bonded to a poly sheet. It comes in
you’ve read this far, it’s probably clear to you
that successful installation of a hardwood floor over radiant
heat will involve coordination and cooperation — between
the trades, the building designer, and the homeowner. I make it
a point to have a preconstruction meeting with all the parties
— the GC, the designer, the flooring contractor, and the
owner — as early as possible to discuss the issues. At
that point, hopefully it’s not too late to influence the
decisions that have to be made — about the different
types of radiant floors, the types of hardwood products and
finishes, the system controls, the need for supplemental heat
sources, and so forth. I try to get as much information out
there as possible, to make sure that all the parties are on the
Many failures of hardwood flooring are not failures at all,
but simply the owners’ dissatisfaction with the tendency
of wood to expand and contract. Even if the builder has done a
great job of moisture-proofing the house, the hvac system is
perfect, and the flooring is installed exactly as recommended,
there will still be some seasonal movement in any wood floor.
Let your clients know what to expect and how their choices will
impact the end result. If the homeowner expects hairline cracks
to show up in February, then it’s no longer a "failure"
when it happens.
Laminated wood flooring products are more dimensionally
stable than solid wood floors, so they tend to perform better
over radiant heat. Because solid wood behaves badly over a
poorly designed radiant floor, always check with the flooring
manufacturer for warranty details before committing to a
•Avoid solid wide-plank flooring of any species; large
gaps are guaranteed during the heating season. If the client
insists, use a laminated product with a plank look.
Flooring made from quartersawn lumber (left) will expand
and contract much less than flatsawn lumber flooring
•When using conventional nail-down hardwood strip
flooring, try to budget for a quartersawn product, which will
move much less than a flatsawn flooring. Use strip flooring no
wider than 3 inches; the narrower, the better.
•Although the scientific data might lead to a different
conclusion, expansion/contraction problems have been reported
with solid-sawn floors of very dense woods like hard maple,
possibly because the material is more difficult to
•Always follow manufacturer instructions exactly when
installing over radiant heat. Pay attention to pads, moisture
barriers, expansion clearances, and transitions.
•Prefinished urethane flooring is a good choice because
it is generally coated on all sides. Beware of wax finishes:
They are harder to care for.
•When applying a floor finish on site, use a sanding
sealer such as Color-Lok (Basic Coatings, 800/441-1934) first,
to reduce the chance of "panelization" — the tendency for
strip flooring to bond together at the edges and contract in
large sheets, opening up huge cracks in only a few spots.
•Prefinished flooring profiles with eased edges, or
"micro-bevels," will show expansion and contraction less than
•Be careful with glue-down flooring products. In some
cases, the adhesive may not perform well because of the
constant heat. Always check with the manufacturer.