Coloured scanning electron micrograph (SEM) of a block of wood
from an elm tree. At top is a transverse section and at bottom
is a longitudinal section. Xylem vessels (larger tubes)
transport water and mineral nutrients from the roots throughout
the plant. Its thick lignin walls also provide structural
support for the stem.
Due to woods cell structure a change in moisture content results
in a change in dimension.
Image credit: Andrew Syred/Science Photo Library
What do wood experts and material scientists
have to say...
problems with wood-based building materials are moisture
Wood Myths: Facts and Fictions About Wood, P. Fisette,
Building Materials and Wood Technology, University of
Massachusetts Amherst, 2005
moisture content of materials is almost solely dependant on
relative humidity and is largely unaffected by temperature."
Understanding Psychrometrics, Donald P. Gatley,
P.E., ASHRAE Publications
"Wooden floors over UFH can perform well, and
could even be argued that a wooden floor installed and operated
correctly will out-perform those with conventional heating
systems that cause hot-spots in the floor with extreme localised
dimensional change. A wooden floor with UFH operated with
moderate and gradual temperature changes will apply constant and
even heat to the floor avoiding problems associated with
The effects of heating systems on floor performance in cool
temperate climate, R. Farrell, S. Gadient, Centre for
Sustainable Architecture with Wood.
"Dimensional changes in wood are commonly
observed but are not always recognized as the inevitable
consequence of moisture changes in the material."
National Research Council Canada, Building
Science for a Cold Climate,
N.B. Hutcheon, G.O.P. Handegord
"The relative humidity in the ambient air
determines the equilibrium moisture content of materials; the
vapour pressure determines the movement of vapour."
National Research Council Canada, Building Science for a Cold
N.B. Hutcheon, G.O.P. Handegord
"...wood expands and contracts in direct
proportion to the amount of moisture contained in its cell
walls, changes in dimension occur only below the fibre
saturation point, when the cell walls begin to dry out."
Wood frame envelopes, Best practice guide,
Canada Mortgage and Housing Corporation.
My advice is don’t freak out about the heat
especially in modern high performance buildings: typically the
fluid temperatures in a
properly designed floor heating system when controlled
properly will be lower than the temperature of your blood with
the surface temperatures operating at or lower than the
temperature of your skin.
Courtesy of the Hardwood Manufacturers Association
Part I : Considering Radiant Heat?
to advances in the heated floor industry, you can install
hardwood floors over radiant heat - with confidence. That means
you can enjoy the natural beauty of oak, ash, cherry, maple,
hickory, walnut and other fine hardwoods and the comfort and
efficiency of radiant heating.
The York family chose cherry floors and radiant heat for the
recent 1,500-square-foot expansion of their 250-year-old
farmhouse in central New Jersey. "I like warmth under my feet,"
Susan York says. "If my feet are warm, I'm comfortable."
allergies and was eager to avoid some of the dust
blown about by a traditional forced-air heating system. The
cherry floors and radiant heating system were chosen for the new
great room, kitchen, dining room and office in the
colonial-style home in Asbury, NJ. York believes the choice will
boost her home's resale value.
"I think buyers today are more concerned about the
the air inside the house," she says.
As early as 60 AD, the Romans discovered one way to heat an
enclosed space is to introduce heat below the floor surface and
let it radiate upward into the structure
(see updated history).
Millennia later, radiant heating is more energy-efficient
than conventional forced-air heating systems. Some manufacturers
say their radiant heating systems will cut energy bills by 20 to
40 percent by avoiding the heat loss associated with forced-air
systems. In most buildings, heat loss is greatest in the top
half of the rooms and that's where heat is concentrated with
forced air heating systems.
(A note from RB on energy consumption:
Building efficiency is the determining factor in energy
consumption. Consumers are advised that efficiency saving in the
magnitude stated above are only possible when mechanical,
and building efficiency are addressed.)
How Radiant Heat Works
Experienced installers of
radiant heating systems and hardwood
floors work together to choose the system that best suits your
needs. Electric systems use thin electric mats that work much
like electric blankets. They often are used to heat small rooms
like kitchens, bathrooms and entryways. Larger rooms often
justify the more costly initial expense of installing a hydronic
system, which involves heating water that runs through plastic
tubes under the floor. Hydronic systems can be powered by gas,
oil, electricity or solar energy.
Radiant heat systems use a three-stage process to convey
heated water to its destination.
heating system - comprising a standard boiler, water heater,
heat pumps or geo and solar thermal - warms the water.
A series of
controls then pumps the heated water through a tubing
network that is installed in the subfloor.
As the warm
water moves through the tubing network, it releases its
energy and returns to the boiler system to be reheated. This
makes for smooth and gentle temperature variations.
Hydronic radiant heat systems can be installed in just one
room or throughout a
new or existing home. A plumbing and
heating contractor typically performs the work in conjunction
flooring installer. Most radiant heat system
manufacturers will provide the names of contractors in a given
The recommended maximum surface temperature for any floor is 85
degrees. Make sure your installer chooses a control strategy
that assures this limit will not be exceeded, and gradually
takes the floor through temperature changes.
Natural hardwoods warm quickly and are cozy for bare feet
because they conduct heat more efficiently than thick, padded
carpets. Solid hardwood or engineered wood floors work fine with
today’s high-tech radiant heat systems – whether on subfloors or
concrete slabs. You can add inlays to new or existing wood
floors over radiant heat and you also can paint and stencil the
What about tropical woods?
The Radiant Panel Association, a trade organization for the
radiant heating industry, warns against pioneering the use of a
wood with little information on its dimensional stability. If
you’re importing a tropical or exotic wood, you must pay close
attention to the source and age of the wood as well as the
method used in drying it. Quick drying creates stresses that can
affect the wood later as it expands and contracts.
Which hardwood floors work best?
Extensive laboratory testing by Launstein Hardwood Floors in
Mason, Mich., found that American hardwoods – including cherry,
oak, ash, maple, hickory and walnut – are good choices for
For best results, use narrow boards, preferably not wider
than 3 inches. Narrow boards will better accommodate wood’s
expansion and contraction across a floor.
Consider using quarter-sawn wood for planks wider than 3
inches, regardless of species, for enhanced dimensional
The Launstein testing found that quarter-sawn planks up to 7
inches across (when properly installed) can work well with
radiant heat systems.
The testing also found that hardwood flooring that is
three-eighths of an inch thick conducts heat better than thicker
floors and resists gapping.
(click here for illustrations of other heated floors)
Wood naturally expands and contracts to reach balance with
the relative humidity of its surroundings. That’s why it’s
important to avoid installing wood flooring when moisture levels
typically rise sharply, especially during painting or the
installation of sheet rock. When radiant heat is installed in
concrete, mortar beds or gypsum cement, operate the heating
system until these are completely dry before topping with wood
floors. This can take several weeks.
Before hardwood floor installation, operate the heating
system until the relative humidity in the space stabilizes to
the average level expected for seasonal conditions in the area
in which the wood floor will be installed. For example, if the
space is expected to average 30 percent relative humidity in the
winter and 50 percent in the summer, the average would be 40
percent. In especially dry regions like the American Southwest,
a humidifier may be recommended, according to Robert Rohr, owner
of Maxrohr, Inc., a radiant heat system installer in
Rogersville, MO. Likewise, a dehumidifier may be called for in
especially humid climates.
Experienced flooring installers will ensure that the wood
floor’s moisture content is appropriate for your area. A 6 to 8
percent moisture content is average in most parts of the
Care and maintenance
Caring for a hardwood floor with radiant heat is no different
than caring for any other hardwood floor. Radiant heat systems
also typically have no effect on refinishing guidelines. (It is,
however, always best to check with the finish manufacturer.)
hardwood floors over radiant heat
Republished with permission(c)2001-2005
Hardwood Manufacturers Association
Washington State University Civil and Environmental Engineering
department has some good animations
showing how wood changes with moisture content.
100% of all hardwood floor complaints in homes heated
exclusively with forced air did not have radiant heating
Learn more about
designing radiant heating systems
Learn more about using finite element analysis (FEA)
radiant heating and cooling designs.
Suggested reading for consumers and office
How to "ball
park" your budget for indoor climate control.
Built to code: What does it mean for
consumer thermal comfort?
Indoor environments: Self assessment
Comfort: A 40 grit perspective for consumers
Comfort System - The "Un-minimum" System
Comfort: A Condition of Mind
Do-It-Yourself HVAC - Should you do it?
The Cost of HVAC Systems -
Are You Paying Too Much for Downgrades?
Radiant Installations - The Good, Bad and Ugly
Thermal Comfort Surveys - Post Occupancy, Part 1
Thermal Comfort Surveys - Post Occupancy, Part II
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