Architectural Record BE - Building Enclosure

Masonry Wall Systems Well Positioned to Meet Evolving Codes

New technology ups masonry’s high-performance quotient
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IECC Iterations

The constant in all of the iterations of the IECC code is that greater energy efficiency is required. The 2012 code called for buildings that use 30 percent less energy than that required by the 2006 IECC edition. The 2015 targets of the IECC, in turn, are 20 percent above the 2012 edition.

Most of the changes in the 2015 IECC code involve HVAC, lighting, and other systems. In terms of wall design, the changes are primarily directed toward steel stud and wood-framed walls. The IECC 2015 made only minor changes regarding above-grade walls in the cavity insulation or continuous insulation requirements on prescriptive table. Masonry walls were essentially left alone, with no mandated R-value increases. This may reflect the inherent thermal value of masonry’s mass wall. The benefit of thermal mass, which is a measure of a material’s capacity to store heat for future distribution, is that its slow rate of heat transfer keeps interiors warm in winters and cool in summers. When used with complementary products or systems, concrete masonry units (CMUs) are particularly energy efficient. Mass walls can also prevent easy sound transmission, reducing noise pollution, a feature much sought after in public buildings with numerous users.

Mass walls make sense in light of the fact that ever more insulation is required as each three-year code adjustment is made. If the AIA 2030 Challenge is the timeline, the resulting template will require ever thicker amounts of insulation. Thicker insulation takes inches away from “living space,” which can be undesirable from an economic and user standpoint. In view of these constraints, mass walls present a compelling alternative to meeting increasing R-value requirements.

Compliance

Complying with R-Value requirements for building envelope components is determined by climate zone and is also impacted by building occupancy types, wall types, and the compliance path chosen.

Climate Zone: It is first necessary to determine in what climate zone the project is located. Today, there are eight climate zones for the entire United States, a vast improvement over 1989 when there were 38.

US map depicting climate zones.

Source: U.S. Department of Energy

Building Type: The code contains one set of provisions for commercial buildings and one for residential buildings. The commercial provisions apply to all buildings with the exception of residential structures of three stories or less.

Wall Type: While the IECC has reduced the number of climate zones, R-value tables have become more complex. The commercial table lists multiple variations within each envelope component, except for walls below grade, where only one type is listed.

Compliance Path: The main compliance paths are the prescriptive and performance paths. In the 2015 IECC, there were no important changes in the prescriptive requirements. A new performance method was the Energy Rating Index, which allows builders the option of meeting a target ERI score via several performance options. This compliance method is for the residential market only, however.

Building size and complexity may determine which path to use. Smaller commercial buildings with one HVAC, hot water, and lighting system lend themselves more to a prescriptive approach, while larger commercial buildings with multiple systems, uses, and loads are more suitable to the tradeoffs found in a performance-based code.

In the prescriptive path, building design and components must meet R-values listed in the tables. Minimum mandatory requirements must be met for mechanical equipment, including lighting, HVAC, water heating systems, and electrical power systems.

While architects have traditionally used the prescriptive path, it has recently fallen out of favor because of the tedious use of tables and lack of tradeoffs. It can curtail design freedom and reflect the notion that the building is composed of separate, unrelated systems.

Prescriptive building envelope paths include R-Value and U-Factor options. Historically, every material is R-value rated. In order to meet the prescriptive requirements using the R-value method (table C402.1.3 of 2015 IECC), every wall type (mass wall, wood framing, metal framing, and metal building) is described. This prescriptive option specifies the minimum continuous insulation to be added at each assembly to meet the code.

The second, the U-factor method, allows the designer to configure the wall so that the wall assembly configured is permissible, provided its overall thermal transmittance (U-factor) is less than that specified in the code (Table C402.1.4 of 2015 IECC). Values used in the calculation of the overall U-factor are provided by the ASHRAE 90.1.

Both options aim at the same total heat loss target, but the second option (U-factor method) is less restrictive. This approach is more flexible than the R-value method.

In the IECC 2015, compliance with the first path requires compliance with ASHRAE Standard 90.1-2013, which is more stringent than ASHRAE 2010.3

Designers can use COMcheck software to see how to comply and to demonstrate compliance with ASHRAE 2007 and 2010, where allowed by the jurisdiction. It is applicable for demonstrating compliance with the three prescriptive options. Inputs include compliance the project is seeking, as well as details on the building envelope and various systems, including HVAC and lighting. A distinct advantage of using this software is that it produces not only a project summary but a certificate of code compliance for the specified code. Because of its clarity in defining a project, many states now require the use of COMcheck. The program is available for free download at www.energycodes.gov/comcheck.

The performance path offers more flexibility but involves more complex energy simulations and tradeoffs between systems; COMcheck is insufficient here for compliance. It is necessary to use ASHRAE 90.1 Section 11: Energy Modeling, which is much like LEED-required energy modeling, and takes into account actual environmental conditions and includes input on all major building systems, with mechanical and lighting systems factored into the building envelope equation.


Characteristics of a High-Performance, Code-Compliant Wall

High-performance walls have a great many elements in common. They are energy efficient, with a high R-value, high air tightness, and little or no thermal bridging. They also score high marks in moisture management, with no leaks, the ability to control condensation, and the ability to dry should moisture infiltrate the wall cavity. Other qualities include sound and fire resistance.

To promote these characteristics, a variety of code-compliant components have been developed.

Continuous Insulation

ASHRAE has required continuous insulation (CI) for many years. CI is defined as insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. The amount of insulation is climate-zone dependent and listed by ASHRAE in its 2004, 2007, 2010, and 2013 editions. There are prescriptive insulation requirements for wood-framed walls, steel-framed walls, and mass (concrete, stone, or CMU) walls for each of the eight United States climate zones.

Code requires a layer of CI in most U.S. climate zones for the purpose of limiting or eliminating thermal bridges that compromise a wall’s energy efficiency. Exterior continuous insulation has a variety of benefits. First, it reduces the condensation in the wall frame and inside the building, and the dew point of the wall is displaced toward the exterior where condensation will not cause damage.

Moisture management is critical to a successful wall and, in addition to creating a high-performance building, can head off the potential for health, legal, and financial concerns. Some moisture is likely to infiltrate even the best constructed walls, whether it is in the form of bulk moisture, capillary action, water vapor, or groundwater. Walls that control any water that has breached the cavity and enable it to drain quickly and the wall assembly to dry are the ideal.

It is important to mention that exterior CI also contributes to reducing the air leakage through the wall. For example, a rigid EPS foam continuously covering an exterior opaque wall is a contributor for reducing air leakage through the wall. It seals tears and other accidental defects occurred in the weather-resistive barrier (WRB).

Due to its mass, a masonry wall is a barrier against sound transmission. Moreover, a multilayer concrete wall like a CMU wall in addition to a veneer with insulation in the cavity gives excellent result.

The 2015 IECC, Section 402.2.1 adds language regarding multiple layers of continuous insulation and describes the installation methods for multiple CI layers, effectively upping the stringency of the 2012 IECC.3

Air Barriers

Of prime importance to designers of masonry systems are the air-barrier requirements. Air-barrier systems are comprised of a number of materials that are assembled together to provide a complete barrier to air leakage through the building enclosure. They control the unintended movement of air into and out of a building enclosure—an important consideration in reducing energy costs, as air leakage from a building can result in an increased use in energy costs of up to 30–40 percent in heating climates and 10–15 percent in cooling costs.

The 2012 version of the IECC required air barriers in both commercial and residential construction, and the directive has remained essentially the same in 2015.

According to the U.S. DOE, in the 2015 IECC, “Continuous air barriers are required except in Climate Zone 2B:

  • Air barrier placement allowed inside of building envelope, outside of building envelope, located within assemblies composing envelope, or any combination thereof
  • Continuous for all assemblies that are part of the thermal envelope and across joints and assemblies
  • Joints and seams sealed, including sealing transitions in places and changes in materials, securely installed in or on the joint for its entire length to not dislodge, loosen or otherwise impair its ability to resist positive and negative pressure from wind, stack effect, and mechanical ventilation.”

There are two ways to comply with air- barrier requirements: Materials – C402.5.1.2.1 or Assemblies – C402.5.1.2.2.4

Fire Safety

In multistory projects where the wall assembly contains a combustible material, the IBC code requires NFPA 285 test compliancy to verify vertical and lateral fire propagation. The IBC also establishes minimum requirements for hourly fire rating based on building types, fire separation distance, and occupancy group. The fire rating of a specific wall configuration is evaluated using ASTM E119 test method.

 

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Originally published in Architectural Record

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