[Strawbale] Sub Floor Radiant Heating

[Shody Ryon]

Radiant-Floor Heating   



The following article was written for the
Environmental Building News by Alex Wilson. Copyright
© 2002 by BuildingGreen, Inc. All rights reserved;
reprinted with permission. www.BuildingGreen.com

Radiant-Floor Heating: When It Does—and Doesn’t—Make
Sense
During judging of the Northeast Green Building Design
Competition last spring, I was struck by the number of
residential entries with really stellar passive solar
design and super-high-performance building envelopes.
Clearly, I thought as I began reviewing the features,
we’ve come a long way in high-performance residential
green building since my first experience with passive
solar in the mid-1970s. But something also seemed odd.
A majority of these entries had sophisticated
radiant-floor heating systems. After going to all the
effort and expense to superinsulate the envelopes of
these houses and provide passive solar design, did
they still need $10,000 heating systems? And did those
systems really make sense from a performance
standpoint? I wasn’t sure, and decided to dig into
these questions.
I’ve long been a fan of the comfort delivered by
radiant-floor heat, and strong arguments are often
made about energy savings and indoor air quality
advantages. But is this really the best match for
high-performance green homes? In the most
energy-efficient buildings, the answer seems to be
“no,” though radiant-floor heating can offer both
comfort and IAQ benefits. This article provides a
quick overview of radiant-floor heating, reviews the
benefits of this heat-delivery approach, and reviews
when these systems do—and do not—make sense in homes
and small commercial buildings.
 
Radiant-Floor Heating Overview
Radiant-floor heating has its origin in ancient Rome,
where fires were built beneath the floors of villas.
Early Korean buildings were similarly heated by
channeling flue gases beneath floors before venting
those gases up through chimneys. Frank Lloyd Wright
piped hot water, rather than air, through the floors
of many of his buildings in the 1930s—a practice that
has become common in custom homes today.
Radiant-floor heating turns a floor into a large-area,
low-temperature radiator. In most modern radiant-floor
heating systems, warm water circulates through plastic
tubing either embedded in a floor slab or attached to
the underside of subflooring. With slab systems, one
can use either a standard concrete slab-on-grade, or a
thinner, lightweight gypsum-concrete slab poured on a
subfloor or over an existing finished floor. In either
case, the thermal mass of the slab holds heat and
radiates it slowly to the living space above.
In addition to hot water as the heat source, radiant
floors can also use electricity or hot air. Due to the
high cost of electricity in most areas,
radiant-electric floor heating usually makes the most
sense when off-peak electricity is available for
charging a slab at night and during other off-peak
hours. Production of electromagnetic fields (EMFs) is
also a potential concern with radiant electric heating
(see EBN Vol. 3, No. 2). Radiant-air floors are
occasionally used in commercial buildings but are
generally impractical and too expensive for
residential applications.
For hydronic radiant-floor systems, copper piping has
been used in the past, but most systems today use
either rubber or cross-linked polyethylene (PEX)
tubing—the latter being by far the most common. Design
of radiant-floor heating systems is quite complex and
should be done by someone with adequate training or
experience. Various design manuals,
manufacturer-specific installation guides, and
software tools are available for use in designing and
sizing radiant-floor heating systems. The length of
tubing required per square foot of floor depends on
such variables as tubing diameter, type of
radiant-floor system (thick slab, thin slab, no slab),
climate, heat load of the building, and type of boiler
and controls used. Manufacturers have done a great job
in recent years in packaging the various components to
simplify the design of radiant-floor systems.
A key requirement for most radiant-floor heating
systems is adequate insulation beneath the heated slab
or beneath the tubing (when tubing is attached to the
underside of a subfloor). Most manufacturers recommend
a minimum of 1” (25 mm) of extruded polystyrene (XPS)
for concrete slab-on-grade radiant heating systems,
but significantly higher levels are justified in cold
climates.
Zoning of radiant floors is usually done with advanced
manifold modules that allow the water temperature to
be varied in different zones. This provides
flexibility for maintaining different temperatures in
different rooms and for allowing differential heat
delivery to spaces with and without solar gain.
Finally, sophisticated controls are often installed to
ensure optimal comfort and to maximize energy
performance. Some radiant-floor systems rely on
separate temperature sensors outdoors, within the
floor slab, and in the living space—with
microprocessor control to regulate just when and where
hot water should be delivered. Because of the long
lag-time with concrete-slab radiant-floor heating
systems, standard set-back thermostats usually are not
effective, though set-back thermostats that have a
built-in anticipation feature may work well for this
application, says building consultant Andy Shapiro, of
Montpelier, Vermont.
 
Benefits of Radiant-Floor Heating
Radiant-floor heating offers a number of significant
benefits:

Comfort. By far, the biggest selling point for
radiant-floor heating is comfort. The large radiant
surface means that most of the heat will be delivered
by radiation—heating occupants directly—rather than by
convection (the primary mechanism of heat delivery
from conventional hydronic baseboard “radiators”).
Warmer surfaces in a living space result in a higher
mean radiant temperature, a measure of surface
temperatures in a space that influences the rate of
radiant heat loss from occupants). With higher mean
radiant temperatures, most people are comfortable even
at lower air temperatures. Delivery of the heat at
floor level with a warm floor surface also allows
occupants to walk around barefoot even in winter—a
very popular feature. Enhanced comfort should be a big
selling point in any green home, so a strong case can
be made for this heating approach.
“Until you’re lived with this form of heat,” says
Radiant Panel Association executive director Larry
Drake (who got involved with radiant heating after
years of working with solar houses), “it’s hard to
understand how comfortable it is.” He argues that with
green homes in particular, after going to all the
effort and expense to incorporate healthy and
sustainable materials, ensuring high levels of comfort
with radiant heat should be a top priority.
 
Energy savings. There is potential for saving energy
with radiant-floor heating through several mechanisms,
including lower thermostat settings, lower-temperature
boiler settings, and reduced infiltration. Homeowners
with radiant-floor heating are likely to be
comfortable at lower air temperatures because of the
elevated mean radiant temperature in such homes, the
lack of significant airflow (as occurs with convective
hydronic heating and forced-air heating systems), and
the delivery of heat at floor level. Proponents of
radiant-floor heating argue that someone normally
comfortable at 72°F (22°C) will be comfortable in a
building with radiant-floor heating kept at 68°F
(20°C). If this is true, one would expect people with
radiant-floor heating to keep their thermostats lower
and thus realize significant energy savings. (See page
13 for further discussion.)
The second opportunity for energy savings with
radiant-floor heating is through keeping the boiler
temperature lower than is necessary with conventional
baseboard hot water distribution. The typical European
approach with radiant-floor heating is to circulate
fairly low-temperature water on an almost-continuous
basis, varying the water temperature as needed to
satisfy the load. This practice might reduce heat loss
into unconditioned space if boiler and piping are
located in an unheated basement, but experts EBN spoke
with suggest that the savings would be very small at
best—especially because of the additional electricity
consumption to operate pumps for long hours. Green
building consultant Marc Rosenbaum, P.E., of Meriden,
New Hampshire, suggests using a low-mass boiler that
is fired on-demand, rather than a high-mass boiler
operated almost continuously.
The third opportunity for energy savings (over
forced-air heat) is that radiant-floor systems do not
increase the rate of air infiltration. Standard
forced-air heating systems can significantly increase
or decrease air pressure in different parts of a
building, which in turn can increase air
infiltration/exfiltration rates—at least in a
conventional, leaky building. With radiant-floor
heating, as with baseboard hydronic heating, this will
not happen. (A well-designed, properly balanced
forced-air system should not increase infiltration.)
Potential for use of solar energy. The relatively low
temperature required for circulation water in a
radiant-floor heating system provides an opportunity
to utilize solar hot water. This approach works best
with concrete-slab systems; higher-temperature water
is generally required when the tubing is attached to
the underside of wooden floors. While such systems are
fairly complex and expensive, radiant slabs offer one
of the best ways to make use of solar energy for
heating portions of a building without direct access
to sunlight. Most practical are systems in which solar
energy heats water in a storage tank that can then be
circulated through the slab. According to an EREN
Consumer Energy Information Brief (www.eren.doe.gov)
titled “Solar Radiant Floor Heating,” such systems
typically cost at least $14,000. Backup heat is still
required and can be provided with a wood stove,
through-the-wall-vented gas heater, electric
resistance heat, or backup heating element in the
solar storage tank.
 
Increased boiler life. By operating a boiler at a
lower temperature, its life can be extended.
Radiant-floor heating systems typically use water
temperatures of 85–140°F (30–60°C), compared with
baseboard hydronic systems that typically operate at
130–160°F (55–70°C). At these operating temperatures,
boiler life can exceed 45 years, according to
information from DOE. (Shapiro is skeptical of this
claim, however, pointing out that newer boilers are
made for cold-start operation and should hold up well
with this temperature cycling.)
 
Quiet operation. Radiant hydronic floor heating is
extremely quiet. Unlike forced-air heat, there is no
noise from a fan or airflow through ducts; and unlike
hydronic baseboard heat, there is usually no gurgle of
water through baseboard radiators or creaking from
expansion and contraction. The primary noise will be
the sound of circulating pumps and the fan used in
power-venting the boiler. With radiant-floor systems
that have tubing attached to the underside of wood
flooring, there may also be some creaking from
expansion and contraction.
 
Flexible room layout. Because there are no baseboard
radiators or air registers with radiant-floor heating,
there is much greater freedom as to where furniture
can be placed. Radiant-floor heating systems are
“invisible.”
 
Improved indoor air quality. An argument can be made
for improved indoor air quality in houses with
radiant-floor heat. Compared with a conventional
forced-air distribution system, there is likely to be
less dust circulated around the house. And unlike
electric baseboard or forced-air heat, there will be
no surfaces hot enough to burn dust particles—which
could introduce volatile chemicals or toxic
particulates into house air (even passing through
filters). This concern would be greatest for people
with acute chemical sensitivities. In fact, veteran
builder Max Strickland, of Burkholder Construction in
Travers City, Michigan, first became interested in
radiant-floor heating several years ago after his wife
became chemically sensitive. He’s worried about
“frying the air” with conventional heating systems and
feels that conventional filters on forced-air systems
are not effective. Strickland went on to build an
American Lung Association (ALA) Health House in
Travers City three years ago, and he now incorporates
radiant-floor heating into all of his homes (typically
4 to 6 high-end custom houses per year).

So What’s Wrong with Radiant-Floor Heating?
In the right application, radiant-floor heating is a
superb heat-delivery system—in fact, perhaps the very
best. You usually pay more for it, but the enhanced
comfort, potential energy savings, and other benefits
can easily justify the extra cost. That said, however,
super-energy-efficient green buildings may not be as
well-suited to radiant-floor heating. Here’s why:

Economics
It can be reasonably argued that a green home in a
moderate-to-cold climate should have very high levels
of insulation (at least R-25 walls and R-40
ceiling/roof), extremely low infiltration rates,
high-performance glazings (unit U-factors below 0.3),
and at least some passive solar gain or suntempering.
We’re not talking about conventional houses, mind you,
but high-performance green homes. Such a house will
use very little heating energy—probably less than 2.0
Btu/ft2 · degree-day (41 kJ/m2 · °C), which would
translate into very low heating costs. To achieve that
level of energy performance requires a significant
investment in the building envelope (for example,
double 2x4 walls). In such a house, putting in an
expensive heating system doesn’t make good economic
sense. As Rosenbaum notes, “It just doesn’t make sense
to put in a $10,000 heating system to provide $100
worth of heat per year.”
Investing so much money in the building envelope and
still putting in an expensive radiant-floor heating
system eliminates the potential for offsetting much of
the extra cost in building envelope improvements
through savings in the mechanical equipment—one of the
key principles of integrated, whole-systems building
design. In most highly energy-efficient houses, the
same high level of comfort provided by a radiant-floor
heating should be achievable simply by installing one
or two small, quiet, high-efficiency through-the-wall
gas heaters (such as those produced by Rinnai) or a
few short sections of electric baseboard heat. At
$1,000 to $2,000 apiece for Rinnai heaters (installed)
or a few hundred dollars for electric baseboard vs.
$10,000 for a typical radiant-floor heating system,
savings of $6,000 to over $9,000 would be possible—and
that savings could pay for most of the envelope
improvements required to bring the heating load so far
down that space heating (instead of distributed heat)
becomes a viable option.
Even Larry Drake, a strong proponent of radiant-floor
heating systems as executive director of the Radiant
Panel Association in Loveland, Colorado, admits that
radiant heat is more difficult to justify in
high-performance buildings. “The tighter the envelope,
the less the amount of savings of a radiant system,”
he told EBN.
 
Heating performance with micro-loads
Along with the economic questions about the wisdom of
radiant-floor heating systems for high-performance
green homes, there are building science reasons why
this may not be a great fit. Heat is transferred from
an exposed slab to the space at a rate of about 2
Btu/ft2 · hr · °F (11 w/m2 · °C), according to
Rosenbaum. In a well-insulated house, this rate of
heatflow means that even when it is very cold outside,
the slab can only be a few degrees warmer than the
rest of the room or the room will keep heating up. For
a concrete slab to feel warm, however, it needs to be
about 80°F (27°C). Thus, for most of the heating
season, the greatest feature of radiant-floor heat—a
warm floor—won’t occur. With moderate solar gain, heat
delivery from a floor slab will be even less. Because
the floor is insulated underneath, it will be more
comfortable to walk on than most slab floors, but the
benefit will be from the insulation, not the radiant
heat.
The time lag of heat movement through concrete can
also be a problem. In a very well-insulated house,
that lag time can result in overheating, particularly
if there are other sources of heat being delivered to
the space, such as passive solar. If a concrete slab
is “charged” with heat during the early morning hours
and the surface is warmed to the point where it cannot
readily absorb solar radiation striking it, that solar
heat will more directly heat the air, increasing the
risk of overheating. The same thing happens to a much
greater extent in high-performance passive solar homes
with masonry heaters because the surface of an
operating masonry heater is at a higher temperature.
In such houses, occupants usually need to check
weather forecasts—if they load up the masonry heater
firebox in the morning and it turns out to be a
bright, sunny day, the space will very likely
overheat. A radiant floor maintains a much lower
surface temperature than a masonry heater, so the
floor will effectively “turn off” as the room warms up
with solar gain. “If the floor temperature is 76°F,”
says Rosenbaum, “then the radiant system can’t heat
the place to hotter than that.” Therefore, this isn’t
a huge problem with radiant-floor heating systems, but
it may mean that homeowners will have to open windows
periodically in the winter and their overall energy
savings from solar energy will not be as great.
Shapiro counsels against the use of radiant slabs in
areas of houses with passive solar heat. “It’s a waste
of energy,” he says, though just how much waste occurs
is unclear.
The risk of overheating with concrete-slab
radiant-floor heating systems in very energy-efficient
buildings leads some designers to incorporate
sophisticated control systems. Rather than a simple
room thermostat, many radiant-floor designers install
control systems that also adjust the circulating water
temperature based on outside air temperature and the
temperature of the slab. It can also be important to
have different zones in a concrete-slab radiant-floor
heating system—so that less heat can be delivered, for
example, to portions of the slab that are warmed by
solar gain. However, according to Rosenbaum, a
radiant-floor slab is somewhat self-regulating when it
comes to solar gain. If the floor slab begins
absorbing solar heat and warms up, it will extract
less heat from the circulating water; that heat will
return to the boiler and can be circulated to nonsolar
zones.
 
Heat loss into the ground
With slab-on-grade radiant-floor heating systems,
there is potential for significant heat loss into the
ground. According to Paul Torcellini, Ph.D., P.E., of
the National Renewable Energy Laboratory, even with
insulation under the slab, 20% of the heat entering
the slab can be lost into the ground. This reduces the
overall efficiency of the radiant-slab system,
offsetting the potential savings described above.
Typical manufacturer recommendations for 1” (25 mm) of
XPS insulation beneath a radiant slab are clearly
inadequate; even 2” (50 mm) may not be enough. Shapiro
recommends up to 4” (100 mm) in cold climates. In
place of ozone-depleting XPS, one can use high-density
expanded polystyrene (minimum 1.5 pcf, 24 kg/m3 foam
recommended).
It is ironic that most people want radiant floor heat
because they don’t like a cold floor, yet there has
long been resistance to insulating beneath concrete
floor slabs—which would dramatically reduce the
cold-floor problem. They solve the problem with an
expensive radiant-floor heating system (including
rigid insulation under the slab) when the rigid
insulation alone would solve most of the problem. (To
be fair to radiant-floor heating proponents, the only
way to make a slab floor actually warm to the touch is
to provide radiant-floor heating—because the high
conductivity of concrete makes a slab feel cool even
when it is at or slightly above room temperature.)
 
Challenges with cooling
Most radiant-floor heating systems cannot provide
cooling, and most homes and small commercial buildings
are being built today to provide cooling—even in
relatively cool climates. This is why forced-air
systems are far more popular than hydronic heating
systems nationwide—the ducts used for forced-air
heating can also be used to deliver chilled air (see
further discussion under “Radiant-Floor Heating vs.
Forced-Air Heating” below). One of the problems in
turning a floor into a heat sink is the risk of
condensation on the cool surface. (Condensation occurs
when a surface temperature drops below the dew
point—which can be quite high in more humid parts of
the country.)
Radiant cooling (generally with ceiling panels) is
used quite commonly in Europe, where humidity levels
are generally not as high as in eastern North America
and where the comfort envelope of building occupants
(the temperature range at which they are comfortable)
is wider than here. That said, there is some
interesting research underway in the U.S. on radiant
cooling. This concept is being tried out, for example,
at an architecture school studio at Penn State
University. Chilled water is circulated through
ceiling panels to provide radiant cooling, with 100%
fresh air used for ventilation. The key is that the
ventilation air is dehumidified before delivery to the
conditioned space, thus eliminating the potential for
condensation on the radiant ceiling panels. This
system is saving energy in two ways: because pumping
water requires less energy than moving air, and
because the chilled water has to remove only the
sensible heat loads—not the latent loads. With the
100% outside-air supply, the total amount of
circulated air is reduced by about 80%, compared with
conventional recirculating systems.
 
Predicted vs. actual savings
The final concern with radiant-floor heating systems
is that much of the assumed energy savings may not be
occurring. There is very little hard data to back up
the common claim that radiant-floor heating systems
save a lot of energy because people with this form of
heat are comfortable at lower temperatures and thus
keep their thermostats lower. In fact, the only study
we could find shows this not to be the case.
Last winter, the Canada Mortgage and Housing
Corporation (CMHC) carried out a study of 75 houses in
Nova Scotia: 50 with radiant-floor heating and 25 with
other heat distribution systems—research that was
first reported in the December 2001 issue of the
Journal of Light Construction. These houses were
visited during daylight hours on weekends, and
thermostat settings were recorded. Thermostat settings
in the houses with radiant-floor heating averaged
68.7°F (20.4°C), while settings in the control houses
averaged only 67.6°F (19.8°C). Although the sample
size was small, this study shows no evidence that
homeowners with radiant-floor heating keep their
thermostat settings lower; in fact, it shows the
opposite. Don Fugler of CMHC, who managed the research
project, told EBN that they launched the study after a
radiant-floor heating product manufacturer contacted
CMHC asking for more detail on standard information
the agency had been giving out about the energy
savings from radiant-floor heat. He cautions that this
was a very superficial study, but that it points out
the need for additional research into the common claim
about energy savings.
Larry Drake of the Radiant Panel Association says that
the CMHC study was very interesting and the
conclusions being drawn from it are misleading. “To
assume that people don’t feel comfortable at lower
temperature is conjecture,” he said. He argues that
the relationship between comfort and mean radiant
temperature has been well established by ASHRAE for
decades. He speculates that if homeowners with radiant
heat have opted to keep their thermostats about where
they keep them without radiant heat, they have opted
to increase their level of comfort rather than going
for the energy savings. He also suggests that
homeowners may tend to set their thermostats
numerically, irrespective of comfort—so that if they
used to keep their thermostats at 70°F and then put in
radiant-floor heating, they may well still keep their
thermostats at 70° (and end up being more
comfortable).
Andy Shapiro prefers not to make claims about energy
savings with radiant-floor heat. “Radiant heat can be
a wonderful amenity in a house,” he says, “but to sell
it as an energy saver stretches the point.”
 
Radiant-Floor Heating vs. Forced-Air Heating
Many people who opt for radiant-floor heating do so
because they don’t like forced-air heat. There is a
common perception that forced-air heating systems dry
out air and generate dust. “Nothing could be farther
from the truth with a properly installed forced-air
system,” says Betsy Pettit, AIA, of Building Science
Corporation in Westford, Massachusetts. Forced-air
systems, she argues, offer the benefit of being “all
things to all systems.” A forced-air system can
provide heat, air conditioning, ventilation, and
filtration—all through a single system of ducts and
with shared fans. A radiant-floor heating system, on
the other hand, only does one thing, according to
Pettit, and it does it at a cost that is typically
higher than that of a forced-air system serving those
multiple functions. “For me it’s just a hard sell,”
she told EBN. “If you insulate the slab and if you
build your building envelope correctly—that is to say,
leak-free—you can be more comfortable for less money
with a ducted distribution system,” she says.
Pettit could think of no tract-home builders in the
U.S. who install radiant-floor heating, though there
are many custom and spec builders who are very happy
with radiant-floor heat. Max Strickland confirms that
cost is indeed higher for radiant-floor heat—typically
50% higher than for forced-air—but he notes that if
you provide the same level of zoning with forced-air,
the costs would be much closer. He deals with air
conditioning in houses that have radiant-floor heat by
putting in ductless mini-split air conditioners made
by Sanyo or Mitsubishi, which he says are very
efficient. Larry Drake adds that in addition to using
ductless mini-split systems, some builders of houses
with radiant-floor heat also often put in
high-velocity duct systems for air conditioning, in
which very small, 3” (75 mm) round ducts can be run
through wall cavities. Drake was unaware of any large
tract-home builders who have adopted radiant-floor
heating over forced-air systems.
 
When and Where Radiant-Floor Heating Makes Sense
It has been pointed out that radiant-floor heating
systems may not be the best choice for extremely
well-insulated, passive solar homes. So when do they
make sense? 
• In houses and small commercial buildings with
conventional levels of insulation and standard
insulated-glass windows—especially those in climates
with minimal cooling loads—where the extra comfort of
radiant heat is desired and the budget allows.
• In buildings with large open spaces and tall
ceilings.
• In buildings where air-flushing is common, such as
garages, fire stations, airplane hangars, and
industrial spaces (because the large-area radiant
floor allows quick recovery).
• When cost is not an issue and satisfying most or all
of the heating load with solar energy is a high
priority.
• When building occupants have acute chemical
sensitivity or allergies—in which case there may be
concern that dust could be distributed through a
forced-air system or that high surface temperatures
from a gas burner or electric heating element will
burn dust particles and cause health problems.
Final Thoughts
It’s hard to express doubts about something that’s
really popular. Like ground-source heat pumps,
radiant-floor heating has a loyal and zealous
following of builders, designers, and homeowners who
consider it to be the best heating option around—and
appropriate in almost any situation.
One of the reasons radiant-floor heating is so popular
is that it is so much more comfortable than what most
of us have experience with: older, drafty houses where
there is significant floor-to-ceiling temperature
stratification. If more people realized that the
same—or at least a similar—level of comfort could be
achieved simply by creating a really well-insulated,
tight building envelope, we could be keeping a lot of
people extremely comfortable while also saving a huge
amount of energy, without needing radiant-floor heat.
“A house with a good enough envelope to be called
green—well-insulated and tight—will have a very high
level of comfort no matter what type of heating system
is used,” says Shapiro, “as long as that heating
system is well designed.”
In homes with conventional levels of insulation and
typical glazings, radiant-floor heating is an
extremely comfortable heat-distribution option. It
does not contribute to IAQ problems, and it might well
even save a little energy if homeowners can be
convinced to turn down their thermostats to a level
that will provide the same level of comfort as a house
without radiant heat. But in an extremely
well-insulated, green home, radiant-floor heating
usually is not the best option. If you’ve gone to all
the effort and spent all the money to achieve a truly
stand-out energy-conserving envelope with passive
solar gain, why not offset that cost by dramatically
reducing the cost of the heating system?
– Alex Wilson

For more information:
Radiant Panel Association
P.O. Box 717
Loveland, CO 80539
800/660-7187, 970/613-0100
970/613-0098 (fax)
www.radiantpanelassociation.org
For subscription information contact: Environmental
Building News,
122 Birge St., Suite 30, Brattleboro, VT 05301 (USA).
E-mail: ebn@
BuildingGreen.com. Web site:www. BuildingGreen.com




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