January 12, 2018
by Ed Weinmann
Many conflate R-value with good thermal performance or insulating value. The belief is the higher the R-value, the better the thermal performance. However, as a blanket statement, this could not be any further from the truth. (An earlier version of this article appeared in a 2015 issue of SMART|Dynamics of Masonry [vol. 2, no. 4] with co-author Brendan Quinn. Visit www.dynamicsofmasonry.com.)
The International Energy Conservation Code (IECC) defines R-value as:
the inverse of the time rate of heat flow through a body from one of its bounding surfaces to the other for a unit temperature difference between the two surfaces, under steady-state conditions, per unit area.
R-value is determined through a test performed in a guarded hot box where a steady-state condition is established. The environment is completely controlled with no variances except the initial passive absorption of heat into one side of the tested materials and the release of said heat on the other side of the materials until a steady state (constant heat at 38 C [100 F]) exists on both sides of the material prior to testing.
Guarded hot-box R-value testing and assessments do not account for dynamic conditions like fluctuations in sunlight, temperature, humidity, pressure, or wind. The test bypasses thermal mass, thermal lag, response to solar heat, and thermal flywheel effect. Since we do not live in a steady state and the test bypasses several real-world physics principles of heat transfer, it lacks accuracy and provides a false view of thermal performance. Simply put, it is the wrong test for masonry or high-thermal-mass material.
Would one measure air volume with a liquid measuring cup? Would one design a rocket to go to the moon without accounting for gravity? So why use R-value when it comes to designing buildings in the dynamic world? Why use R-value to assess the performance of masonry when thermal mass and thermal lag are bypassed?
According to the R-value testing, glass has about double the R-value of masonry per inch of thickness. If higher R-values alone equated to better insulation performance (i.e. resistance to heat loss or gain), then glass should be a better insulator than masonry. However, anyone who has gotten into a car on a sunny summer day knows how hot it gets. The greenhouse effect is what occurs through car windshields and through every southwest-facing window on an office building when the sunlight hits. The glass, with a higher R-value than masonry, is doing a wonderful job of amplifying the heat and a terrible job of insulating against heat intrusion (i.e. it is neither lowering energy usage nor improving the indoor environment).
Masonry, on the other hand, has high thermal mass. Although it has a low R-value, high-mass materials properly placed and protected tend to decrease both heating and cooling loads in a given building, thus saving energy. (For more, see National Concrete Masonry Association [NCMA] TEK Note 6-16A, Heat Capacity [HC] Values for Concrete Masonry Walls.) Mass can store, slow, and dissipate heat. It will not amplify heat with the greenhouse effect of glass.
The National Concrete Masonry Association (NCMA) and other code-contributing groups may argue thermal lag and thermal mass benefits are taken into account by requiring lower R-values for masonry in the code. However, this author sees R-value as a standalone narrative to be an incomplete method of setting the design parameters for a building’s thermal performance. It is clearly a piece of the energy efficiency puzzle, but there are other pieces that may have greater effect and even greater importance to complete the picture.
The myth surrounding continuous insulation
That continuous insulation (ci) is a mandatory requirement remains a longstanding misunderstanding of IECC. Unfortunately, while clarifications and explanations are issued repetitively, many architects and designers remain under the impression the energy code requires masonry or ‘mass’ walls to have ‘continuous insulation’ at all times. The confusion regarding the code is generally that of the readers rather than IECC itself—continuous insulation for mass walls is only a requirement of prescriptive design methodology.
As covered in NCMA FAQ 12-14, the energy code allows three different methods to be used to show compliance with minimum energy efficiency requirements: prescriptive, trade-off or system performance, and whole-building energy analysis. A project need only comply with one of these methods, not all three. (For more, visit multibriefs.com/briefs/ncmaorg/073114_FAQ_Continuous_Insulation.pdf.)
The idea of having continuous insulation over every inch of masonry seems to be a good idea in order to prevent any unwanted heat loss or gain through the walls. However, glass windows and doors are big holes in the continuous insulation. Buildings with glass envelopes or floor-to-ceiling windows do not require continuous insulation even though they have very low R-value due to their thickness. (Glazing also adds other detrimental effects on thermal performance like the aforementioned greenhouse effect, amplifying heat in a room or losing heat from a room.) Why does glass not require continuous insulation?
Current design solutions
Many inventors and designers over the years have devised solutions that improve the performance of masonry. Exterior insulating finishing systems (EIFS) offer an acceptable single-wythe wall solution to the increasing R-values with insulation to the exterior of a mass wall. Exterior insulation can be preferable over interior insulation as it performs better thermally by protecting the thermal mass from the solar heat. Mass, properly placed and protected, makes for a highly energy-efficient building.
However, traditional EIFS does not always offer the great variety of textures, shapes, colors, and aesthetics desired by owners or architects. These systems can also potentially add significant costs in labor and materials. In other words, EIFS offers one of many solutions to single-wythe masonry design, but it is not the singular system of choice for everyone or every project.
Another solution involves hiding away the exterior insulation with a veneer (such as concrete masonry units [CMU], brick, or stone) over a block structural backup. Common practice for quite some time, these walls are generally known as double-wythe or cavity walls.
Prescriptive design methodology allows one to simply add the R-values of layered materials to reach a total R-value. For example, placing R-4 foam board over R-19 batt insulation gives one an R-23 insulating layer. Applying that same prescriptive principle to the average 200-mm (8-in.) CMU (with an R-value of 2.0) would require laying a dozen 200-mm blocks side by side to reach an R-24. Based on R-value and prescriptive design methods, a mason would have to build a block wall that is 2.4 m (8 ft) thick to match the R-value of 25-mm (1-in.) foam board covering a 100-mm (4-in.) thick batt insulation. If a building has a 2.4-m thick concrete-block wall, not much heat is ever going to make it through due to the incredible amount of thermal lag and thermal mass.
Figure 1 illustrates another point. The R-6 wood stud is performing better than the R-23 center of cavity with batt insulation and foam board. If higher R-value equates to better thermal performance and insulation, then why is the lower R-value wood stud performing better than the higher R-value insulation? Thermal mass.
Reviewing the infrared photography, one can easily see an R-23 frame wall does not operate better than an integrally insulated single-wythe CMU wall. Prescriptive design with R-value for thermal performance simply makes no sense. There are limitations to return on R-value and, as mentioned, that metric alone is not a good measure of thermal performance.
A 2013 article in SMART|Dynamics of Masonry further clarifies things, addressing the three paths to compliance for meeting the energy code. (See the 2013 issue of SMART [vol.1, no. 3] for the article, “Meeting Energy Codes with Single-wythe Masonry: COMcheck Provides Tradeoff to Compliance,” by Dan Zechmeister, PE, and Elizabeth Young.) It has taken quite a bit of time to educate architects and designers IECC does not require continuous insulation over mass walls unless a prescriptive design methodology is chosen. This process of educating and clarifying prescriptive design versus system performance versus whole-building energy analysis is still ongoing with the latter two options preferred.
Concrete masonry product innovations
The negative attributes of masonry when utilized in single-wythe design are fairly obvious. Cross-webs of CMUs will act as thermal bridges for heat to conduct and traverse. The air space in the cavity may be easily affected by face-shell temperature changes or air flow. Additionally, mass improperly positioned or used in a masonry wall design may adversely store heat. Overcoming these deficiencies has led to ongoing innovations in wall design and product design.
Foam-filled standard CMU block cores, whether by liquid-filled expandable insulating foam or preformed inserts, have been a block-insulating process for many years. A claim by this methodology’s supporters is it may raise the R-value of a standard 200-mm (8-in.) CMU from an R-2 to better than R-14. Regardless of the claims, thermal bridging through the cross-webs would bypass any insulation in the cores, reducing the effectiveness of the insulation.
According to NCMA’s Thermal Catalog of Concrete Masonry Assemblies (second edition), a CMU wall with foam-filled cores does perform better thermally than a CMU wall without foam filling. Insulating the cores of standard CMU increases thermal performance by insulating the air pockets and reducing airflow through the wall.
Knowing cross-webs serve as thermal bridges that bypass foam-filled cores, industry professionals and inventors sought to decrease thermal bridging by reducing or eliminating these components. A typical CMU has three cross-webs. Manufacturers have developed CMU with either one (i.e. H-blocks) or two (i.e. A-blocks) cross-webs rather than three to improve thermal performance. (These products are referred to as one- or two-web assemblies by NCMA.)
NCMA’s Thermal Catalog of Concrete Masonry Assemblies offers tables for expandable-foam-filled cores. In this catalog, a 200-mm block wall, lightly reinforced and varying by block density, will have a range of R-values from R-3.27 to as much as an R-6.73. A 200-mm CMU two-web assembly lightly reinforced will vary by block density from an R-4.34 to as much as an R-7.94. In the same table, a 200-mm CMU single-web assembly lightly reinforced can vary by block density from about an R-5.4 to as much as an R-8.95.
Then, product design became more interesting. Rather than just reducing the cross-webs, several manufacturers varied their proprietary approaches to reducing thermal bridging by eliminating the cross-webs or lowering the height. Additional product designs attempt to:
For these products, some manufacturers have completely removed their thermal bridges. Two types have a face shell and core divided by a foam insert, while another has plastic ‘arms’ connecting the two face shells. This separation of mass by insulation or non-conducting materials is the thermal break giving these systems more effectiveness by removing thermal bridging.
Integrally insulated concrete masonry
Employing the principles of thermal mass and thermal lag, another design has offset cross-webs constricted in their heights that do not reach the top of the block’s face shells. To keep the block intact as one unit, a middle lineal wall (acting as a heat sink) on the interior connects the offset cross-webs in a 200-mm CMU and two middle lineal walls connect the offset cross-webs in a 300-mm (12-in.) design. These alterations disrupt the thermal bridge by significantly increasing the length of the thermal pathway and positively position additional mass for heating or cooling benefits. This increases the block’s thermal performance in any climate or condition.
The addition of offset webs and middle lineal wall(s) create cells for specially shaped non-mortar interfering insulating inserts. These inserts fill the air spaces, creating an airtight single-wythe wall with alternating layers of mass and insulation, along with minor, but heavily disrupted, thermal bridging.
For this type of CMU, one manufacturer performed a guarded hot-box test (then known as ASTM C236, Standard Test Method for Steady-state Thermal Performance of Building Assemblies by Means of a Guarded Hot Box) and was rated an R-13.57. However, third-party guaranteed utility programs have rated the wall system to operate at an R-24.6. Calculated methods of analysis have rated 200-mm CMUs to have an R-18 to R-20 depending on the block density, and 300-mm CMUs to operate between an R-28 and R-30.
This mortar-set block system is very similar in its construction methodology of standard CMU walls. Its closed-cell expanded polystyrene (EPS) inserts are made from recycled post-industrial contents, are non-combustible, repel moisture, and cross the head and bed joints to add insulation throughout the height and length of the blocks, as well as between neighboring blocks for thorough insulating effects.
Nevertheless, this type of product is considered to be an integrally insulated block and has some thermal bridging—therefore, it cannot be considered to have continuous insulation. Despite the minor bridging, its performance over 25 years, in four countries, and seven climate zones has been stellar.
As with all integrally insulated block, one must use the U-factor alternative tables to evaluate whether it offers the correct U-factor (inverse of R-value) per climate zone regulation.
Clearly, there are many viable options for modern single-wythe masonry designs and construction. Any of the product and system types discussed in this article may be used in a single-wythe design based on the current energy code. As whole-building energy analysis is a better way to design a building for its energy use, the limitations of prescriptive design may be overridden and people can perhaps enjoy the many benefits of single-wythe masonry.
Ed Weinmann, LEED GA, is an architectural specialist for Ernest Maier, a concrete masonry unit (CMU) producer serving the Washington, D.C., area. He is a board director for the CSI Baltimore Chapter and a member of the U.S. Green Building Council (USGBC) National Capital Region, having served on the 2015 GreenBuild host committee. Weinmann is a member of the Baltimore chapter of the American Institute of Architects (AIA) and the USGBC Maryland Chapter. He is an independent consultant for Omni Block and a subject matter expert on the thermal performance characteristics of masonry. Weinmann can be reached via e-mail at email@example.com.
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