Tag Archives: Thermal mass

Designing Masonry Buildings to the 2012 Energy Code: Thermal Mass Basics

A material’s thermal mass denotes its ability to store heat within a cycle of time. K-values, generally calculated on a 24-hour cycle, are important because they give general references to a material’s capabilities for storing heat. All materials may be considered for use in a thermal mass calculation, but steel, aluminum, and other metal claddings tend to cycle too quickly, while wood tends to cycle too slowly to offer desirable design values.

Masonry—such as concrete masonry unit (CMU), stone, and brick—offers a good blend of characteristics for the thermal mass design based on several values. Storing heat well, the dense material can be designed with wall thicknesses that allow for normal window and door jamb details with reasonable per-area costs to construct.

In most cases, thermal mass should be measured on a cycle representative of both a typical heating and cooling cycle or a variable daily winter cold temperature cycle. While this is done for either season with the same principals, external factors contribute to the winter wall calculations in a more direct way. Building orientation, ceiling heights, lighting, solar heating, soffits, wall finishes, number of occupants, and usage round out a general list for design.

In colder climates, thermal mass is based on the function of interior heating cycling through the core of the wall. As the evening temperatures fall and the interior begins to feel cooler, warmth that was gained and stored during the daylight hours can then reverse the heat path and move back to the interior space of the building.

Summer cycles seem a bit clearer when explained, as the heat of the day penetrates toward the core of the wall. The term ‘decrement property’ takes into account the wall’s material density (e.g. concrete mix), final façade finishes, and exposure. The decrement factor dictates the speed at which the heat can be absorbed into the building. The design should stop the absorption of the heat before it alters the interior of the building’s cooler temperature and cycles the heat to the exterior of the structure as the afternoon temperatures begin to fall.

‘R-value’ has become a term familiar to even consumers, as it is listed on every insulation package in the home improvement stores. The general thought often reduces this metric’s significance to ‘the higher the R-value, the better the product when placed in the wall.’ However, as a unit of thermal resistance, R-value is the conduction rate of heat flow through a combination of materials comprising a wall. Mass-enhanced R-value walls are a combination of thermal mass walls and use of materials that offer high resistance to heat flow. They are extremely useful in climates where the external building temperatures rise well above and fall well below the interior space daily temperatures.

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Designing Masonry Buildings to the 2012 Energy Code

All images courtesy Mortar Net Solutions

All images courtesy Mortar Net Solutions

by Steven Fechino

The 2012 International Energy Conservation Code (IECC) will bring tremendous change to the way buildings are designed, constructed, and renovated. Several of the code’s changes have already been implemented throughout the industry, with many of the currently specified systems and products meeting these new codes. However, there are also materials and assemblies that will need to evolve to remain compliant.

For instance, HVAC systems will require improvements to the mechanical systems and ductwork. Window glazing is becoming more energy-efficient to meet ever-tightening performance criteria. Further, there is this author’s focus—the insulation requirements for masonry construction have been written to higher performance levels. There are many rumors about how the changes will limit the available products with which masonry structures can be built and designed. This article will address some of those rumors by providing simple explanations of the code and some helpful insight into how the industry is dealing with the changes on a positive level.

This diagram shows the elements of a cavity wall This particular assembly includes an insect barrier and mortar-dropping-collection device.

This diagram shows the elements of a cavity wall This particular assembly includes an insect barrier and mortar-dropping-collection device.

The 2012 energy code has been adopted state by state, and jurisdiction by jurisdiction, so the changes have not been applied uniformly across the country.1 Nevertheless, the updated IECC is important to all design/construction professionals, because it is a positive step toward reducing the country’s energy consumption through the design, construction, and operation of more efficient structures. It is important to adapt to these changes as soon as possible, since all regions will likely be affected by the code sooner or later. Preparing now will make the eventual transition to the new energy standards much easier.

CMUs and continuous insulation
The prescriptive energy code for the masonry industry is based primarily on the requirement for continuous insulation (ci) within the wall envelope. This becomes an issue when one looks at the standard concrete masonry unit (CMU)—the cross-webs prevent continuous insulation within the block because they allow thermal bridging. By reducing the cross-web dimension, thermal bridging is reduced and the thermal efficiency of the unit is increased. However, this, in itself, is not the solution to code compliance. It is important to look at all the compliance criteria.

In some cases, a CMU assembly’s mass and resistance to heat transfer (i.e. R-value) are all that is necessary to meet the code, but only in warmer climates. Differing temperature conditions means various types of insulation are used in designing the many single-wythe and cavity wall systems specified across the country. Rigid insulation, foam inserts, dry loose fill, injected foam, spray-on foam, and proprietary block design round out the field of techniques for increasing R-value, with typical gains of 5 to 25.

An important factor for determining energy efficiency of a CMU wall is the envelope’s design, specifications, and the materials making up the assembly—various concrete masonry manufacturers will have similar, but ultimately different, mixes. This is one factor that can change a CMU wall’s R-value and the thermal mass performance of otherwise similar envelopes. (For more, see “Thermal Mass Basics.”)

Other factors include geographical climate history, insulation specifications (within either the CMU or the cavity), and the actual cross-section of the masonry units comprising the wall design. For assistance with this, the National Concrete Masonry Association’s (NCMA’s) TEK 6-2B, R-Values and U-Factors of Single-wythe Concrete Masonry Walls, discusses thermal performance of a CMU wall and its thermal properties based on material properties.2

This church was built with masonry cavity walls and brick veneer. An important factor for determining such a wall’s energy effi ciency is the envelope’s design, specifi cations, and the materials making up the assembly.

This church was built with masonry cavity
walls and brick veneer. An important factor
for determining such a wall’s energy efficiency
is the envelope’s design, specifications, and the materials making up the assembly.

The three paths to compliance
It is important to clear up the rumor that all masonry walls will require continuous insulation in order to meet the new standards. There are many ways a designer can achieve compliance using complete building systems to meet the new IECC requirements rather than relying solely on continuous insulation.

Right now, there are three methods available for determining code compliance, and many of the current masonry designs will show acceptable numbers in at least one of them. These methods are:

  • prescriptive compliance;
  • compliant software (also known as ‘performance method’); and
  • whole building analysis.

Prescriptive method
The prescriptive method uses a series of material or assembly requirements to meet compliance. For example, designers can employ tabulated values for mass walls that specify requirements for continuous insulation to determine compliance. This is the method most manufacturers and designers use today. Many of the products and systems on the market gain compliance through this path.

However, this prescriptive method may not be part of the next energy code in 2015, so it is important to keep an open mind to developing newer technologies and improvements to existing systems for future compliance to the code. Using the prescriptive tables is easy and straightforward, but this method also limits design flexibility and makes some masonry wall types difficult or impractical to build.

Performance method
The performance or compliant software method uses computer programs developed specifically to determine whether an assembly meets the code. There are two popular programs: the American Society of Heating, Refrigerating, and Air-conditioning Engineers’ (ASHRAE’s) EnvStd and the U.S. Department of Energy’s (DOE’s) COMcheck.3 Though the programs differ in their capabilities, they can both offer the designer thermal property constants for various masonry wall configurations. Depending on which part of the energy code the designer needs to meet, these programs can offer wall configurations that meet prevailing codes and which also comply with IECC in many cases.

COMcheck is a bit more complex to use than EnvStd, but it offers options to modify many components within the structure that can then be compiled to achieve compliance, offering the design community the ability to use the products they know how to bid, construct, and sell in energy-efficient buildings. If the designer compiles all the information about the project and compliance is not achieved, he or she can adjust various individual properties of the building envelope to meet the code requirements. This method allows more design flexibility because the designer can test how multiple building components interact to achieve compliance.

Whole building analysis
Whole building analysis is not yet widely used. However, it will likely be the prevailing method in the future because it takes into account everything about the building, and can produce accurate guidelines for the most energy-efficient sources.

This method can analyze annual total energy use rather than individual component compliance. It demonstrates when new design methods can reduce energy costs as compared to standard building methods. The whole building method not only takes into account the various wall types, but also includes entire building envelope information, plus mechanical and lighting specifications to determine compliance.

At left, a mason ‘butters’ a brick with mortar for installation in a masonry cavity wall. In the photo on the right, one can see the detail of a masonry cavity wall comprising a concrete-unit structural wall and brick veneer.

At left, a mason ‘butters’ a brick with mortar for installation in a masonry cavity wall. In the photo on the right, one can see the detail of a masonry cavity wall comprising a concrete-unit structural wall and brick veneer.

iStock_000013773064Medium

Other changes
Beyond IECC, there are a couple of other standards that have recently undergone changes of which those working with masonry design should be aware. ASTM C90-11b, Hollow Load-bearing Concrete Masonry Units, for example, allows cross-web configurations to regulate by cross-sectional area, not by web thickness. The reasons for paying close attention to this change include:

  • R-values may be increased,4 while structural characteristics and performance will not change;
  • a reduction in cost may be achieved (i.e. using less material in each block); and
  • less demand for materials to produce the units, which reduces energy costs for manufacture and transportation.

Another important change is associated with National Fire Protection Association (NFPA) 285-12, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components. Language in the next energy code will exempt this NFPA test when:

  • material flame indexes are met to published standards (cited below); and
  • air spaces that contain insulation are kept to 25 mm (1 in.) or less.

The new language approved for inclusion in the code that permits exclusion refers to:

Envelopes where rigid or spray-applied insulation is encased by at least one inch of masonry, and there is no gap between the insulation and the masonry; or the insulation and the CMU are not separated by an air space greater than one inch, and the insulation has an index for flame rate meeting requirements of ASTM E84 [Standard Test Method for Surface Burning Characteristics of Building Materials] or [Underwriters Laboratories] UL 723 [Test for Surface Burning Characteristics of Building Materials].

Conclusion
Change is coming to the building industry, driven by a need for far more efficient energy use in the built environment. Masonry has many qualities that make it an ideal building material for energy-efficient construction, including its thermal mass, sustainability, high level of availability, and design flexibility. A combination of new building materials, a better understanding of building dynamics, and improved design software is making it possible for designers to create masonry buildings that meet the new energy codes; skilled masons will be key to making these energy-efficient buildings a reality.

Notes
1 To determine how the new IECC will affect a particular project, visit the U.S. Department of Energy (DOE) website at www.energycodes.gov. (back to top)
2 Visit www.ncma.org/etek/Pages/Manualviewer.aspx?filename=TEK%2006-02B.pdf. (back to top)
3 Visit www.ashrae.org/resources–publications/publication-updates/standard-90-1-users-manual-software-envstd-4-0 and www.energycodes.gov/comcheck, respectively. (back to top)
4 R-values may be increased because of a reduction in thermal bridging via the cross-webs and additional space for insulation. (back to top)

Steven Fechino is the engineering and construction manager for Mortar Net Solutions. He provides engineering support services and product training. Fechino has a bachelor’s of science degree in civil engineering technology and two associate degrees in civil engineering and drafting and drafting and design specializing in building construction. He can be contacted at sfechino@mortarnet.com.

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