Thermal Mass and R-value: Making Sense of a Confusing Issue
The advertisement promises R-30 from a lightweight masonry block wall system. Log home product literature claims that log walls insulate as well as fiberglass because of the thermal mass. Salesmen at a trade show argue that a new fiber-cement building system achieves R-28 even though the “tested” R-value comes in at only R-16.
What’s going on here? Do these claims of “effective R-values” that greatly exceed the widely published R-values for high-mass materials hold up? Just what effect does thermal mass have on the energy performance of an exterior wall system? The issue of thermal mass and its effect on the energy performance of buildings is one of the most confusing issues facing designers, builders, and buyers of buildings today. This article tries to sort out these mysteries, providing enough background on the physics of heat transfer to understand the relationship between thermal storage and heat flow, and then explaining when this information is relevant and how it should be used in building design. This article does not address the use of thermal mass inside a building, where it can store heat (or coolth) and even out temperature fluctuations.
Understanding Heat Transfer
Heat flows by three mechanisms: conduction, convection, and radiation. Conduction is the molecule-to-molecule transfer of kinetic energy (one molecule becomes energized and, in turn, energizes adjacent molecules). A cast-iron skillet handle heats up because of conduction through the metal. Convection is the transfer of heat by physically moving the molecules from one place to another. Hot air rises; heated water thermosiphons; our forced-air heating systems work by moving hot air from one place to another. Radiation is the transfer of heat through space via electromagnetic waves (radiant energy). A campfire can warm you even if there is wind between you and the fire, because radiation is not affected by air.
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 (Btu/ft2hr°F). In metric, it’s usually given in watts per square meter per degree Celsius (w/m2°C).
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 (32°C), and measuring how much supplemental energy is required to keep the other side of the material at a different constant temperature, say 50°F (10°C)—all 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. As typically used in energy performance computer modeling, 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 specific heat of 1 Btu/lb.°F (1.9 kJ/kg°C), while most building materials are around 0.2 to 0.3 Btu/lb.°F (0.4 to 0.6 kJ/kg°C).
If there are various layers in the wall, total heat capacity is found by adding up the heat capacities for each layer (drywall, 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.
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 (24°C) and the outside surface at 32°F (0°C)—conductivity 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 “mass effect.”
Another common scenario is when the outside temperature fluctuates but never crosses the indoor setpoint 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 nighttime 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 wall.
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. (Others refer to this as the effective R-value, a term that can be misleading.) The multiplier obtained by dividing the mass-enhanced R-value of a material in a given climate by its steady-state R-value is referred to by Oak Ridge researchers as the Dynamic Benefit for Massive Systems (DBMS).
When is Mass-Enhanced R-Value Significant?
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 indoor temperature.
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 systems.
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 research to make this happen? Are the industries that sell high-mass materials large enough to support a Thermal Mass Rating Council and the additional research needed on these issues?
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. 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 for debate. Until standardized procedures for determining the regional significance of the mass effect are widely applied, there will likely be continued confusion and continued exaggeration regarding the energy benefits of thermal mass. Oak Ridge researchers and companies such as Agriboard Industries are helping to bring these issues into public awareness, but a great deal of work remains to be done.
For more information:
Oak Ridge National Laboratory
Bldg 3147, P.O. Box 2008
Oak Ridge, TN 37831-6070
423/574-5207, 423-574-9338 (fax)
Agriboard Industries, L.C.
P.O. Box 645
Fairfield, IA 52556
515/472-0363, 515/472-0018 (fax)
Portland Cement Association
5420 Old Orchard Road
Skokie, IL 60077
Bruce Wilcox, P.E.
Berkeley Solar Group
58 Calvert Court
Piedmont, CA 94611
510/601-7475, 510/601-7415 (fax)