by Catherine Howlett | January 6, 2013 11:41 am
by Kari Moosmann
For many years, designers and contractors have espoused tilt-up construction for providing a more airtight building envelope than competing wall assemblies, such as wood and metal stud wall, metal siding, and concrete and brick masonry. Now, code language may also be a supporting factor.
Air barrier requirements have become part of the 2012 version of the International Energy Conservation Code (IECC), which demands performance testing of airtightness when approved construction assemblies or materials cannot be demonstrated. Once adopted by state and local code officials, all commercial buildings will need to comply with these requirements, including the ability to meet air leakage testing and verification parameters.
Jim Baty, technical director for the Tilt-up Concrete Association (TCA), says the new energy standards will have significant impact.
“At first glance, this new layer of performance seems a herculean task to deliver in the complex world of building construction,” he explains. “However, building technologies such as tilt-up concrete construction have decades of performance track records demonstrating the successful combination of envelope performance consistency and complex form and aesthetics. This is now essential to bring the elusive resolution to higher energy standards into reality for a culture seeking more complex and visually appealing building façades.”
To understand the possibilities of using tilt-up to deliver heightened energy and air-barrier performance, one must look no further than the basics of the construction method. (For more on the topic, see TCA’s resource volume, The Construction of Tilt-up. Visit www.tilt-up.org.) The term itself was coined in the late 1940s to describe a method for constructing concrete walls rapidly and economically without the formwork necessary for poured-in-place walls.
Tilt-up (also known as ‘tilt-wall’ or, in many specifications, ‘site-cast precast concrete walls’) is
a two-step process. First, slabs of concrete that will comprise wall sections are cast horizontally on the building floor slab or separate casting slab.
Then, after attaining sufficient strength, they are lifted (tilted) with a crane and set on prepared foundations to form the exterior walls. The erected panels are temporarily braced, but not typically connected to one another. The roof structure is then attached to the walls, the braces are removed, joints are caulked, and the wall finishes are applied to complete the building shell.
Several features make the tilt-up construction method unique. The panels are generally handled only once; they are lifted or tilted from the casting slab and erected in their final position in one continuous operation. Tilt-up panels tend to be of such large size and weight they can only be constructed onsite and in close proximity to their final location in the structure. Panel gravity loads are supported directly by the foundation instead of being supported by a structural frame. Typically, the panels are erected before the structural frame. They are usually load-bearing for gravity loads and lateral loads.
Tilt-up concrete is a unique form of precast construction and, as such, has its own specialized set of design parameters and construction techniques.
Looking at the language
A close examination of the 2012 IECC reveals the significant changes for both air barriers (C402.4–“Air Leakage Mandatory”) and thermal envelopes (Sections C402.4.1 through C402.4.8). The key components found in each of these sections that have significantly strengthened the code’s impact also deliver the final step of an initiative to reduce building energy performance in the United States by 30 percent over previous energy standards.
Section C402.4.1, “Air Barriers,” states:
A continuous air barrier shall be provided throughout the building thermal envelope … [and] … the air barrier shall comply with Sections C402.4.1.1 and C402.4.1.2.
This section is applied for all buildings under the jurisdiction of the code located in climate zones 4 and higher. As such, the air barriers are permitted to be located on the inside or outside of the building envelope, as well as within the assemblies composing the envelope (or any combination thereof). Therefore, selection of the building envelope construction may largely be determined by the effectiveness of achieving the continuous air barrier—otherwise, the structure may incur significantly higher costs of construction to detail for the required performance.
Details on delivering the air barrier are established in the sub-sections C402.4.1.1 (“Air Barrier Construction”) and C402.1.2 (“Air Barrier Compliance Options”). The air barrier must be continuous for all assemblies that are the thermal envelope of the building (and across the joints and assemblies). Further, air barrier joints and seams must be sealed, including sealing transitions in places and changes in materials.
Air barrier penetrations have to be sealed in accordance with Section C402.4.2. The joints and seals shall be securely installed in or on the joint for its entire length so as not to dislodge, loosen, or otherwise impair its ability to resist positive and negative pressure from wind, stack effect, and mechanical ventilation.
“The significance of this section for tilt-up construction,” says Baty, “is tilt-up as a mature process already delivers both of these components 100 percent of the time. The panels themselves are constructed of a material component qualified by C402.4.1.2 as a rated air barrier—concrete—and the individual panels are sealed to form a continuous building envelope at every joint with a high-performance caulking system on both the interior and exterior. Additionally, details incorporating the wall systems with the roof assembly maintain continuity from concrete to roof membrane with several standard details and approved methods.”
Referring to C402.4.1.2.1 for the air barrier compliance options, Baty points out the code states:
Materials with an air permeability no greater than [0.02 L/s • m2] 0.004 cfm/sf under a pressure differential of [75 Pa] 0.3 inches water gauge (w.g.) when tested in accordance with ASTM E2178 [Standard Test Method for Air Permeance of Building Materials] shall comply with this section.
Cast-in-place and precast concrete—the embodiment of tilt-up—are both listed as “acceptable” or “approved” materials under this section. Additionally, many modern tilt-up wall systems are energy-efficient insulated sandwich panels consisting of two layers of such concrete and a continuous layer of rigid insulation, also listed as an “acceptable” or “approved” material.
There are many options to consider and, as the air barrier section continues, more complex envelope assemblies can deliver the required performance. However, as Baty notes, this also means higher considerations for construction quality supervision and more extensive material selection and integration.
Section C402.4.1.2.2 of the 2012 IECC states:
Assemblies of materials and components with an average air leakage not to exceed [0.2 L/s • m2] 0.04 cfm/sf under a pressure differential of [75 Pa] 0.3 inches of water gauge (w.g.), when tested in accordance with ASTM E2357 [Standard Test Method for Determining Air Leakage of Air Barrier Assemblies], ASTM E1677 [Standard Specification for Air Barrier Material or System for Low-rise Framed Building Walls], or ASTM E283 [Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen], shall comply with this section. Assemblies listed in Items 1 and 2 shall be deemed to comply provided joints are sealed and requirements of Section C402.4.1.1 are met.
Other examples given in this section include:
These systems add significant cost to the traditional methods. Also, the code does not limit itself to the building envelope assembly. Ensuring the designed performance is a requirement, as IECC states in C402.4.1.2.3:
The completed building shall be tested and the air leakage rate of the building envelope shall not exceed 0.40 cfm/sf at a pressure differential of [2.0 L/s • m2 at 75 Pa] 0.3 inches water gauge in accordance with ASTM E779 [Standard Test Method for Determining Air Leakage Rate by Fan Pressurization] or an equivalent method approved by the code official.
The advantage in this knowledge is, according to the code, even a tilt-up building is not just an opaque building envelope, but rather an assembly including windows and doors. This means tests will likely still be required for the composite envelope assembly. However, with the simplicity of satisfying core requirements for materials and assemblies for air barrier performance, the project team can feel much more at ease with the pending outcome of the test.
The importance of air barriers
Lee Durston is the director of building science at BCRA, a design firm for the integration of building science and innovative technology. In “Achieving Good Airtightness in New and Retrofitted U.S. Army Buildings,” his white paper for the March 2012 U.S. Army Corps of Engineers (USACE) High-performance Building Envelope Workshop he states:
Effective, continuous air barrier systems can reduce air leakage by up to 83 percent, save on gas bills by more than 40 percent, and cut down on electrical consumption by as much as 25 percent.
This research focused on the behavior of multiple barrack facilities with varying construction methods and materials to determine effectiveness of USACE building tightening methods that have taken place ahead of IECC developments.
David Tomasula, a principal with LJB Inc., a design firm that employs tilt-up concrete strategies, concurs.
“With the advent of the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) in 1998, a large emphasis was placed on the interior guts of the building—the mechanical systems, the electrical systems, the plumbing systems, etc.,” he explains. “Meanwhile, the Department of Energy (DOE) has known the most cost-effective energy efficiency is achieved by building a well-insulated envelope. The building codes are catching up by adopting new criteria such as continuous insulation envelopes and air barriers. This will not only aid in the conservation of natural energy resources, but it will also provide financial savings to building owners, since energy costs account for more than a quarter of the lifecycle cost of owning a building.”
USACE leads the way in the trend toward incorporating air barrier requirements as part of the exterior wall construction. At the aforementioned workshop last year, the focus was on building the ‘perfect wall.’ According to Joseph Lstiburek, a principal with Building Science Corporation and speaker at the conference, this entails four principal layers:
The research from USACE confirms the air control layer can take the form of a supported flexible barrier, such as self-adhered modified bituminous (mod-bit) membrane sheets over concrete masonry or polyethylene sheets over stud walls. Alternatively, the air control layer can be achieved by the concrete tilt-up panel itself—without the need to introduce the cost of an additional material layer.
To evaluate the performance of various air barrier systems and wall construction methods, USACE commissioned whole-building pressurization testing on 200 buildings over a period of 29 months. The testing represented buildings from 34 Department of Defense (DOD) installations and all climate zones in the United States. Building sizes ranged from one to eight stories, and building envelope areas were from 93 to 34,375 m2 (1000 to 370,000 sf). Construction types included wood- or metal-framed walls, pre-engineered metal buildings (PEMBs), tilt-up concrete, and CMUs.
The overall results of the evaluation were positive, showing an average air leakage rate of 0.17 cfm/sf @ 75 Pa was achieved across the entire survey. More impressively, the tightest building tested was constructed of tilt-up construction, achieving an airtightness of 0.04 cfm/sf @ 75 Pa—more than four times tighter than the average leakage rate.
Another example of the implementation for the 2012 IECC is California’s 2013 Building Energy Efficiency Standard, with effectiveness established for January 2014.
Baty interprets the standard to suggest all new non-residential buildings in the state will require a test prior to occupancy certification in accordance with ASTM E770. This test must return a minimum building tightness of 0.4 cfm/sf. From the data evidenced by the USACE study, the tilt-up industry on average provides tightness 10 times greater than the minimum requirements. This translates directly to an energy efficiency advantage.
With increasing emphasis on high-performance building envelopes in commercial construction—and their role in reducing mechanical heating and cooling costs—tilt-up concrete is well-poised to be recognized for its superior performance.
Kari Moosmann is the editorial manager for Constructive Communications, a public relations and marketing firm for architecture, engineering, and construction (AEC) industries. She has specialized in the construction industry, particularly the concrete segment, for almost 25 years. Moosmann has also launched luncheons, books, and social networking sites for the concrete industry. Additionally, she co-chairs Women in Concrete Alliance (WICA)—an online networking organization for women in the concrete industry. Moosmann can be reached at firstname.lastname@example.org.
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