Architectural Record BE - Building Enclosure

Use Cementitious Wood Fiber for Great Acoustical Design

Sustainable panels and roof decks help spaces sound better
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Sponsored by Armstrong Ceiling and Wall Solutions
By Jeanette Fitzgerald Pitts
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Structural Acoustical Roof Decks

Roof decking supports the roof elements but does not provide the weatherproofing layer. Before acoustical roof decks were developed, whenever a project space needed to be designed to deliver an impressive acoustical experience, a specifier would select a desirable roof deck and then add the necessary acoustical treatments to the space in the form ofwall or ceiling panels. Now specifiers can eliminate a step in the design and construction processes and help to ensure proper installation by selecting roof decks that arrive on-site as a composite of the acoustical panel and insulation.

Photo of Lexington Park Library.

At the Lexington Park Library in Lexington Park, Maryland, CWF was used to create beautiful interiors and eliminate a step in the construction process, providing a composite acoustical roof deck that includes an R-value of 44.

There are two types of acoustical roof decks on the market today. One is made from CWF and the other is steel. Roof decks made from CWF provide the same exceptional and even sound absorption that the CWF wall and ceiling panels provide with NRC values reaching up to 1.00. Beyond creating great acoustics in a space, the structural solution is able to offer additional features and benefits that enhance the energy performance of the project and the ease with which the CWF material is installed.

Insulated Solution for Sloped Roofs

There are CWF products designed for either flat or sloped roofs, but the mechanics of installing a flat roof deck and a sloped roof deck are very different. When working with a flat roof, the normal practice is to install the flat roof deck and then roll out the required layer of insulation, stapling or screwing the insulation in place. In sloped roof deck scenarios, laying a separate layer of insulation against a pitch of any grade can be both difficult and dangerous for everyone involved.

Photo of various product samples.   Photo of various product samples. Photo of various product samples.

Left: CWF structural, acoustic roof deck panels are available for sloped or low-sloped roof lines in spans of up to 12 feet.

Middle: Fabric-wrapped acoustical CWF panels provide additional aesthetic options.

Right: CWF interior products are available in a wide range of colors and surface treatments.

As an alternative solution for sloped applications, CWF roof decks are available in a composite product that includes both the CWF substrate for acoustical management and a layer of insulating foam that provides thermal regulation. With the insulation already attached to the roof deck panel, the need to apply a separate layer of insulation is eliminated. The foam layer can range from 11/2 inches to 8 inches thick depending upon the specific needs and location of the project.

Reduce Moisture Drive

The CWF-insulating foam composite can achieve more than thermal management. Pair the CWF material with a layer of a special foam insulation, extruded polystyrene (XPS), and effectively give the roof deck a water-tight seal that acts as a barrier against moisture drive. Moisture drive refers to the movement of moisture or water vapor from the interior of the building to the outside. This phenomenon is an important design consideration in specific applications such as indoor swimming pools, or ice rinks. Consult the manufacturer to ensure that both design and installation will work together for the long term.

Photo of the Lemberg Children's Center.

A CWF composite roof deck panel that offers spans of up to 12 feet was used in the Lemberg Children's Center at Brandeis University. CWF panels include a wide range of finishes, shapes, and colors.

Slip-Resistant, Nailable Surface

The CWF roof deck panel can also be equipped with a sheathing of oriented strand board (OSB). The CWF material will face the interior of the space with a layer of foam insulation above it and then a layer of OSB comprising the outermost layer. The OSB sheath provides a slip-resistant surface for installers to walk on during the installation of the roof and, simultaneously, provides a nailable surface making it easy to attach the roofing membrane directly to the CWF roof deck.

Comparing Steel and CWF Roof Decks

There are many similarities between the steel and CWF acoustical roof deck solutions available. Both CWF and steel acoustical roof decks can satisfy the basic requirements for acceptable structural integrity. These roof deck materials can meet specified design load criteria, which outlines the load that a deck must be able to carry per square foot, and diaphragm requirements, which define the amount of force that a roof deck must be able to withstand.

When it comes to acoustical performance, CWF and steel roof decks are distinct. The way each roof deck system manages sound, installation requirements, and the impact that variations in installation can have on the overall performance of these systems are all important considerations for a specifier. CWF roof decks are constructed from sound absorbing CWF material. Depending upon the needs of the application, layers of foam installation and an OSB sheathing may also be present, but the sound is absorbed by the CWF panel. Steel acoustical roof decks rely on special filler that will absorb the sound waves that travel through holes in the steel structure. Variations in the way steel roof decks are installed can effect whether the sound-absorbing filler is included in the roof deck, which dramatically affects the acoustical performance of the roof deck on the project. Without the sound-absorbing filler, the steel roof deck does not perform acoustically as tested. But much more critically, when the sound-absorbing filler is installed as frequently directed, the end result is exposed foam plastics inside the building—a code violation throughout the country. Code requires a 15-minute thermal barrier between the foam plastics and the inside of the building, but installation of this barrier reduces the acoustic efficacy of the installation to an NRC of approximately 0.35.

 

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