Heat flows by three mechanisms: conduction, convection, and radiation.
Conduction is the transfer of heat through a solid object. When one part of an object is heated, the molecules within it begin to move faster and more vigorously, when these molecules hit other molecules within the object they cause heat to be transferred through the entire object. The handle on a cast iron skillet gets hot as heat is transferred from the bottom by means of conduction.
Convection is the transfer of heat by the movement of a fluid (water, air, etc.) Hold your hand above the stove and you feel the heat as the hot air rises by means of Conduction. Inside of a wall air removes heat from a hot exterior wall, then circulates to the colder interior wall where it loses the heat. Forced-air heating systems work by moving hot air from one place to another.
Radiation is a direct transfer of heat from one object to another, without heating the air in between, the same process in which the Earth receives heat from the Sun or a wood stove supplies heat to its surroundings.
With buildings, we refer to heat flow in a
number of different ways. The most common reference is
"R-value," or resistance to heat flow. The higher the
R-value of a material, the better it is at resisting heat loss (or heat
gain). U-factor (or "U-value," as it is often called) is a
measure of the flow of heat--thermal transmittance--through a material,
given a difference in temperature on either side. In the inch-pound
(I-P) system, the U-factor is the number of Btus (British Thermal Units)
of energy passing through a square foot of the material in an hour for
every degree Fahrenheit difference in temperature across the material
In metric, it's usually given in watts per square meter per degree
R-values are measured by testing
laboratories, usually in something called a guarded hot box. Heat
flow through the layer of material can be calculated by keeping one side
of the material at a constant temperature, say 90°F
and measuring how much supplemental energy is required to keep the other
side of the material at a different constant temperature, say 50°F
this is defined in great detail in ASTM (American Society of Testing and
Materials) procedures. The result is a steady-state R-value
("steady-state" because the difference in temperature across
the material is kept steady). R-value and U-factor are the inverse of
one another: U = 1/R. Materials that are very good at resisting the flow
of heat (high R-value, low U-factor) can serve as insulation materials.
So far, so good.
Materials have another property that can
affect their energy performance in certain situations: heat capacity.
Heat capacity is a measure of how much heat a material can hold. The
property is most significant with heavy, high-thermal-mass materials
such as solid concrete. As
typically used in energy performance computer modelling, heat capacity is
determined per unit area of wall. For each layer in a wall system, the
heat capacity is found by multiplying the density of that material, by
its thickness, by its specific heat (specific heat is the amount of heat
a material can hold per unit of mass). Water has a heat capacity of 1
Btu/lb.°F (4.2 kJ/kg°K),
while most building materials are around 0.2 to 0.3 Btu/lb.°F (0.8 to
If there are various layers
in the wall, total heat capacity is found by adding up the heat
capacities for each layer (drywall, solid concrete, masonry block, and stucco, for
example). In the following section, we will examine how the heat
capacity of materials can affect the energy performance of buildings.
Thermal Mass is a property that enables building materials to absorb, store, and later release significant amounts of heat. Buildings constructed of concrete have a unique energy saving advantage because of their inherent thermal mass. These materials absorb energy slowly and hold it for much longer periods of time than do less massive materials. This delays and reduces heat transfer through a thermal mass building component, leading to three important results. First, there are fewer spikes in the heating and cooling requirements, since mass slows the response time and moderates indoor temperature fluctuations. Second a massive building uses less energy than a similar low mass building due to the reduced heat transfer through the massive elements. Third, thermal mass can shift energy demand to off peak time periods when utility rates are lower.
Thermal bridging is the transfer of heat across building elements, which have less thermal resistance than the added insulation. This decreases the overall R-value.
Wall frames and ceiling joists are examples of thermal bridges, having a lower R-value than the insulating material placed between them. Because of this the overall R-value of a typical construction element can be reduced. For example, adding R2.5 bulk insulation between ceiling joists will actually only achieve an overall R-value for the ceiling of R2.2.
The overall R Value or Whole Wall R Value is the total resistance of a building element such as a wall or ceiling. It takes into account resistance provided by construction materials, internal air spaces, insulation materials and air films adjacent to solid materials.
When people refer to the "mass
effect" or "effective R-value," they are generally
referring to the ability of high-mass materials, when used in certain
ways, to achieve better energy performance than would be
expected if only the commonly accepted (steady-state) R-value or
U-factor of that material were considered. Let's take a look at a
typical use of one of these high-mass materials in a wall system. When
one side of the wall is warmer than the other side, heat will conduct
from the warm side into the material and gradually move through it to
the colder side. If both sides are at constant temperatures--say the
inside surface at 75°F (42°C) and the outside
surface at 32°F
will carry heat out of the building at an easily predicted rate. As
described above, this steady-state heat flow is what most test
procedures for determining R-value measure.
In real-life situations, however, the
inside and outside temperatures are not constant. In fact, in many parts
of the country, the driving force for conductive heat flow (remember,
heat always moves from warmer to colder) can change dramatically or even
reverse during the course of a day. On a summer afternoon in
Albuquerque, New Mexico, for example, it might be 90°F (32°C)
outside, and the outside wall surface--because it has a dark
stucco--might be even hotter. It's cooler inside, so heat conducts from
the outside surface of the wall inward. As night falls, however, it
cools down outside. The air temperature may drop to 50°F (10°C). The
driving force for heat flow changes. As the temperature difference
across the wall is reversed, the heat flow is also reversed--drawing
heat back towards the outside of the building. As a result of this
modulating heat flow through a high-heat-capacity material, less heat
from outside the building makes its way inside. Under these conditions,
the wall has an
effective thermal performance that is higher than
the steady-state R-value listed in books (such as ASHRAE's Handbook
of Fundamentals). This dynamic process is what some people call the
Another common scenario is
when the outside temperature fluctuates but never crosses the indoor set point
temperature. In this case, the direction of heat flow never
changes, but the
thermal lag or
time delay in
heat flow can still be beneficial by delaying the peak heating or
cooling load. For example, if the outdoor temperature in Miami peaks at
95°F (35°C) at 5:00 on a summer afternoon, but it takes eight hours
for the heat to travel through the wall, the effect of that peak
temperature won't be felt inside the building until the middle of the
night. Because most cooling equipment operates at higher efficiency if
the outdoor air temperature is lower and because night time thermostat
settings may be higher (at least in commercial buildings), potentially
significant savings can result. Not only can total cooling energy be
reduced, but peak loads can also be reduced. This can lead to smaller
(and less costly) mechanical systems and lower demand charges for
electricity. This time lag effect can save energy and money, but note
that it does not affect the total amount of heat flowing through the
As noted above, the amount
of heat flow through a wall is reduced by the use of thermal mass when
the temperatures fluctuate above and below the desired indoor
temperature, so under these conditions a material might have a
"mass-enhanced" R-value that is greater than its steady-state
R-value. To estimate this mass-enhanced R-value for a given high-mass
material in a particular climate, researchers at Oak Ridge National
Laboratory measure the thermal performance of a high-mass wall under dynamic
conditions, in which the temperature on one side of the wall is kept
constant and the temperature on the other side is made to fluctuate up
or down. With this measured heat flow under dynamic conditions as a
basis, they then use computer modeling to arrive at steady-state wall
R-values that would be required to achieve comparable overall energy
performance under various climate conditions. Those results are what we
are calling the "mass-enhanced R-values" for the high-mass
material under the modeled conditions.
The mass effect is real.
High-mass walls really can significantly outperform low-mass walls of
comparable steady-state R-value--i.e., they can achieve a higher
"mass-enhanced R-value." BUT (and this is an important
"but"), this mass-enhanced R-value is only significant when
the outdoor temperatures cycle above and below indoor temperatures
within a 24-hour period. Thus, high-mass walls are most beneficial in
moderate climates that have high diurnal (daily) temperature swings
around the desired indoor setpoint.
Nearly all areas with
significant cooling loads can benefit from thermal mass in exterior
walls. The sunny Southwest, particularly high-elevation areas of
Arizona, New Mexico and Colorado, benefit the most from the mass effect
for heating. In northern climates, when the temperature during a 24-hour
period in winter is always well below the indoor temperature, the mass
effect offers almost no benefit, and the mass-enhanced R-value is nearly
identical to the steady-state R-value. The ASHRAE Handbook of Fundamentals
lists "mean daily temperature range" data for hundreds of U.S.
climates in the chapter on climate data. These values can be helpful in
figuring out how significant mass-enhanced R-value might be for a
particular climate, but they do not tell the whole story; also
significant is the percentage of days during the heating and cooling
seasons when the outdoor temperature cycles above and below the
Clearly, high-mass materials
used in exterior walls perform better than would be expected based
solely on their steady-state R-values. But the actual thermal
performance is highly dependent on where the building is located.
Manufacturers of these materials rightly want to take credit for this
improved performance, but how can that be done in a way that doesn't
exaggerate performance for parts of the country where the mass effect
benefit just isn't there? "Right now, we don't have a system that
forces people to deal with calculations in a constant way," says
Bruce Wilcox, P.E., of the Berkeley Solar Group, who has done extensive
modeling of mass effects for the Portland Cement Association and others.
All sorts of claims are
being made about mass-enhanced R-value (usually called "effective
R-value") with little standardization. The first step needs to be
consensus on how the mass effect should be accounted for in testing and
modeling. Jeffrey Christian at Oak Ridge National Laboratory has been
developing and refining the method of dynamic thermal analysis and
simulation described above. This is the most extensive
effort to date to quantify the mass effect. Christian's group, with the
help of Bruce Wilcox and others, also developed thermal mass tables for
the Model Energy Code in the late 1980s that can be used to account for
the thermal mass benefits of high-mass building materials in wall
The next step, suggests
Christian, might be to formalize the testing
and simulation procedures through development of ASTM standards.
Establishment of an ASHRAE committee to address the mass effect may also
be in the works. To ensure that such standards would be applied in a
consistent manner, Wilcox suggests that applicable industries might have
to set up some sort of council, perhaps modeled after the National
Fenestration Rating Council (NFRC), which enforces consistent reporting
of window energy performance. Such a "Thermal Mass Rating
Council" might oversee standards relating to how mass effect and
mass-enhanced R-value are reported. Wilcox remains leery of the whole
concept of mass-enhanced R-value--not that the effect exists, but
whether it can be used clearly with building materials. "I don't
know if there's any way to make it a property of the material," he
told EBN, "It's a property of the system." There are a
lot of questions to sort out, such as how many climates need to be
modeled: are six enough, as Oak Ridge researchers have used, or do we
need 20? Would such a system take credit for time delays in heat
transfer, or just actual reductions in the amount of heat that moves
through? Who will pay for all the
Heat naturally flows from warm areas to cooler areas,
regardless of direction. In winter, heat flows from the inside of a building to the outside and
in the summer high heat from roofs and walls travels from outside to
inside. This flow of heat can never be stopped completely, but the rate at which
it flows can be reduced by using materials which have a high resistance
to heat flow.
Obviously an important step in the creation of an
energy efficient house or building is to control heat loss or gain,
which accounts for 75% of the total energy loss of a home.
How does insulation work?
Choosing an insulation
Remember... R-value means "resistance", if a product resists, it does not stop radiant heat transfer. R-value material only deals with conductive heat transfer. Other factors to consider when choosing an insulation are the materials fire, mold, insect, vermin and moisture resistant properties, as well as its cost and ease of application.
The R values required by Standard 90.1 are based on equivalent energy performance. For example, in Tulsa, OK, Standard 90.1 requires an R 8.3 frame wall or an R 4.3 mass wall for some buildings. These requirements are based on the fact that an R 4.3 mass wall is as energy efficient on an annual basis in an occupied building as an R 8.3 frame wall, in this particular climate. The benefits of mass walls vary by climate and can be influenced by factors such as temperature "swings" (differences between the high and low outdoor temperatures during the day), by solar radiation and wind near the building, and by how the building is designed, operated, and maintained for comfort to be energy efficient.
systems with significant thermal mass, have the potential to reduce
building annual heating and cooling energy requirements, depending on
the climate, below that required by standard wood-frame construction
with similar steady-state R-value. High-mass building materials
can offer significant energy benefits in exterior walls. The benefit may
be primarily in the shifting of peak load conditions
or in an actual
reduction in overall heat gain or loss
thermal performance. These benefits are highly
dependent upon where the building is located, how it is designed, and
how it is operated. How we should give credit--in terms of energy
performance--for high-mass building materials is still very much open