Hardwood Flooring over Radiant Heat
I’ve been installing radiant floors around New York
state for about ten years. From the very beginning, I heard
horror stories about hardwood floors shrinking, heaving,
cracking, and swelling — all blamed on radiant heating. I
wondered how all these problems could be caused by exposing the
wood to such low, even temperatures?
From the outset, I wanted to make radiant floor heat a
viable option for anyone who wanted it, and I knew that many
clients would choose hardwood as their finish floor surface. I
was determined to solve the hardwood dilemma.
By following up on reports of wood flooring failures, I came
to realize that many of the problems blamed on radiant heating
would actually have occurred anyway — usually because of
excessive job-site or household moisture. Where the radiant
heating was a factor, in many cases simple modifications in the
design and installation of the system could have prevented the
Consequently, my "program" for success with hardwood
flooring has two main points: careful control of job-site
moisture levels and good hvac design. Following that advice,
I’ve found that with the possible exception of wide plank
floors, almost any type of hardwood floor — be it solid
or laminated, nail-down, glue-down, or floating — can
work with radiant floor heat, though some choices may perform
better than others (see "").
Good Hvac Design Is Key
perfect hardwood installation will fail if the radiant floor is
not properly designed. Two enemies of hardwood are excessive
floor temperatures and heat that is not evenly distributed, so
I design the system accordingly. Although I wouldn’t
expect you to do your own hvac design, here are four guidelines
to keep in mind when you talk to your radiant system designer
about protecting the hardwood floors.
1. Use the lowest possible fluid design temperature (the
temperature of the water or glycol solution in the heating
2. Modify the tubing or insulation design instead of cranking
up the fluid temperature.
3. Use responsive controls to keep the heat as constant as
4. Use supplemental heating when necessary.
Low Water Temperature
Radiant floors are designed to work at lower supply
temperatures than hydronic baseboards or panel radiators. This
is because floors warmer than about 85°F are uncomfortable
underfoot. This is also the maximum temperature that a wood
floor should be exposed to, according to the flooring industry.
In fact, I’ve found that lower temperatures —
around 80°F — are healthier for wood floors.
The fluid temperature required to produce that 80°F
surface temperature will vary greatly, depending on floor
construction and finishes and the type of radiant system used
(see ""). A bare concrete radiant slab, for example, offers
little thermal resistance; 100°F water in tubes spaced 12
inches on-center may do the job. But lay down 2x4 sleepers, a
plywood subfloor, and hardwood strip flooring, and now
you’ve added an insulation layer that the heat output of
the tubing has to overcome (see Figure 1).
Vinyl sheet flooring
3/8-inch hardwood flooring
3/4-inch hardwood flooring
1/2-inch acrylic plush carpet
7/8-inch nylon plush carpet
1/2-inch wool plush carpet
1/4-inch waffled rubber pad
3/8-inch urethane pad
1.Finished flooring acts as an insulator on
top of a radiant floor, and must be accounted for in the design
of the system. Ceramic tile provides little resistance to heat
flow, so it works well with the low fluid temperatures used in
radiant heating. A conventional hardwood floor with 3/4-inch
subfloor has an R-value near 2. If the homeowners plan to add a
rug on top, it becomes trickier for the designer to meet the
heating needs of the room without overheating the hardwood
The temptation is to crank up the supply temperature to
compensate. When hardwood floors are involved, fluid
temperatures typically range from 105°F to 125°F for
Gyp-Crete type floors, and could be as high as 145°F for
staple-up under-floor applications. It’s hard to make
assumptions about design because of the variables, but if
I’m having a problem reaching a comfortable surface
temperature, I’ll add more insulation under the tubing,
space the tubes closer together, increase the diameter of the
tubing, or suggest a flooring with a lower R-value —
anything except jack up the fluid temperatures.
too far apart will create hot and cold streaks — a bad
thing for a wood floor, which should expand and contract
uniformly (Figure 2).
2.The floor illustrated here has a
20 Btu/hr. output. Note that with 12-inch on-center
tube spacing, the "striping" effect is
A tubing design that works great in a warehouse with a bare
concrete floor is probably not suitable for a living room. Many
radiant floor designs are based on tubing layouts 12 inches
on-center or more. For hardwood flooring, though, the spacing
should be less, ideally 6 to 8 inches. The closer the spacing,
the more even the heat will be, and the happier the wood
flooring will be.
In some cases it may be necessary to decrease the diameter
of the tubing as the spacing grows closer. Be careful not to
exceed the maximum length of each loop, which, depending on the
pump size, could be as little as 100 feet for some types of
3/8-inch tubing. Closer tubing spacing means more individual
loops. More loops mean more manifolds, which in turn means more
Controls for radiant floors have come a long way since the
first systems were put in. The simplest control is the basic
indoor wall thermostat. Setback thermostats, which are standard
nowadays in conventional hvac systems, are of little value with
radiant floor heat because of the slow response time of the
floor. I encourage clients to find a temperature they’re
happy with and leave the thermostat alone.
When wood floors enter the picture, I encourage the use of
more advanced controls and components that help to gradually
modulate the heat output of the floor in response to outdoor
temperature. I often install an outdoor reset control, which
varies the temperature of the water in the floor depending on
the temperature outside. In great rooms with walls of windows,
thermal gain can be a problem as the sun beats on the floor
during the day, while at night the glass siphons the heat back
out. Since wood floors don’t like sudden changes in
temperature, to minimize these solar swings, we may install
sensors in the slab or thin-slab that signal the heating system
to shut down a zone until the sun stops "loading" the floor.
Another option we might consider is one of the new "modulating"
boilers, which regulate fluid temperature by adjusting the gas
flow to the burner based on different heating demands.
I also encourage the extra expense of injection pumps, which
provide more responsive control of fluid temperature than the
more conventional four-way mixing valves. When used in
combination with the outdoor reset control, injection pumps
help keep the fluid at the lowest possible temperature to
satisfy the heating load for the room. This minimizes problems
with the wood floor.
properly designed radiant floor will produce around 20 to 25
Btu/hr./sq.ft. If that’s not enough heat to satisfy the
room, the common mistake is to jack up the floor temperature
— again putting the hardwood at risk. A better solution
is to supplement the floor with radiant panels in the walls or
ceiling, or add some conventional hot-water baseboard or a
cabinet unit to the room.
Another common mistake is not to allow for area rugs that
the owner plans to put down on the floor. An 18x20-foot great
room may have a wool rug and pad in the middle that covers more
than half of the floor area. This is basically a big insulating
blanket that has to be factored into the design.
Moisture Control Is
Because radiant floor heating is often
the scapegoat for any wood floor failure, I take a proactive
position when it comes to job-site moisture. Perfectly
acclimated hardwood flooring laid over a perfectly cured slab
will still fail if the basement or ground below is constantly
wet. If we see something going on that could impact the final
floor — for instance, a basement slab going down with no
vapor barrier or a crawlspace with no groundcover — we
step in and try to educate the builder as to why he needs to
take the extra steps. Usually builders and homeowners are happy
to oblige — they don’t want problems with the
hardwood floors any more than I do.
Another place I stick my nose in is making sure there is
enough time in the schedule. For example, a radiant Gyp-Crete
floor can’t be turned on for 4 to 5 days after it’s
poured so that the Gyp-Crete develops proper strength before
being subjected to rapid heat. It might then take another two
to three weeks with the heat on for the floor to reach its
final moisture content so the hardwood can be installed.
That’s close to a month in the schedule that has to be
accounted for. Since most of the homeowners we work with are
prepared to spend more for a radiant heating system, it’s
not hard to make them realize that rushing things to save a
couple of bucks on their construction loan is not in their best
Checking for Moisture
usual product literature that tells you to "let the flooring
adjust to the room for 72 hours before installation" is in a
word — bull. It’s important to keep track of
moisture levels in the products being installed, but it is far
more important that the house reach its "normal" moisture level
before any finished flooring is even brought on site. A
perfectly dry floor will swell as it soaks up job-site moisture
— from a wet slab or subfloor or even the air inside the
Moisture levels should be monitored in both the substrate
and the flooring before installation. The moisture content in
the substrate should be no more than 11-12%, and the flooring
should be about 4% less, or 8% max. Be sure to test all
Moisture meters. Anyone
involved with wood flooring — or any kind of woodwork,
for that matter — should have a hand-held moisture meter.
For around $300, you can get a meter that works on wood,
concrete, and Gyp-Crete. Also, if you expect your moisture
tests to stand up in court, in some states the meter must be
capable of lab calibration. For a variety of moisture meters,
check out the Professional Equipment catalog
Concrete slabs can start being checked for moisture content
30 days after placement, and Gyp-Crete after about a week.
However, don’t make the fatal mistake of just assuming a
slab is okay because of its age — use a meter.
The polyfilm test. If
you don’t have a moisture meter, an easy way to check
moisture in a slab is to tape 24-inch-square pieces of
polyethylene at several points on the floor. After 24 hours, if
any water is present or even if the concrete is darker under
the plastic, then there is too much moisture in the slab. The
test can be accelerated by placing a 60-watt light bulb 18
inches above the plastic. Keep performing the test until the
moisture has dried up.
Acclimating the House
the radiant floor is ready to be turned on, we bring up the
temperature to let it start drying the house. If the house also
has central air-conditioning, we run that at the same time to
further dehumidify and control the indoor temperature. If no
a/c is available, large drying fans, available at rental yards,
can be used.
Once the temperature and moisture levels stabilize, then the
hardwood can be brought to the job site. If conditions are
right where the flooring is stored and it is handled properly,
there is no reason the hardwood can’t be acclimated off
site, which may shave a few days off the schedule.