Details Are Critical For Successful One-Hour Fire and Sound Assemblies

I-joists continue to be a popular floor framing option in multi-family structures. Floor/ceiling assemblies in these buildings typically must meet the requirement of a one-hour fire-resistance rating. Working with the I-joist manufacturer, the architect designs and details assemblies which provide the necessary protection. However, the work cannot stop here. Sound performance of the floor/ceiling assembly must meet minimum standards or possibly more restrictive requirements necessitated by client expectations.

Floor/ceiling assemblies are not universal

The fire performance of floor/ceiling assemblies have been tested by I-joist manufacturers for decades—and the results of those tests have provided numerous assemblies that are listed in code evaluation reports or third party product listings (ICC-ES, Intertek, PFS, etc.). In more recent years, model building codes have included several generic assemblies. Typically the assemblies found in these sources do not allow significant modifications. Although the Component Additive Method (CAM) allows for individual components to be added together for a calculated fire-resistance rating, many design professionals prefer a prescriptive or tested assembly listed in a code evaluation report over CAM.

In addition to following assembly-specific details like joist depth/spacing, size and type of insulation and subfloor type/thickness, a design professional must also be conscious of joist specific properties such as the I-joist manufacturer and flange size. For example, I-joists with a flange smaller than 1.5” x 3.5” will require 2-layers of gypsum to achieve a one-hour fire-rated assembly…with one exception, TJI® Joists with Flak Jacket® Protection.

Flak Jacket® Protection is a proprietary, patent-pending coating that enhances the fire resistance of TJI® Joists. By specifying a Flak Jacket assembly, an architect can achieve a one-hour floor/ceiling assembly with only one layer of gypsum while using an appropriately sized (smaller flanged) I-joist. This eliminates either the additional cost and weight of a 1.5” x 3.5” flanged TJI (large flanges are often unnecessary for the shorter spans common in multi-family housing) or the extra cost and labor time of adding a second layer of gypsum.

Sounds like a good assembly

After selecting a floor/ceiling assembly and assuring the I-joist selected is appropriate for the assembly, a final consideration still needs to be addressed: sound performance. Achieving the necessary Sound Transmission Class (STC) for airborne sound and Impact Insulation Class (IIC) for contact noises is done through careful detailing, including the specification of sound isolating and insulating components within a floor/ceiling assembly.

Weyerhaeuser provides several sound-specific options for common floor/ceiling assemblies.  In addition to the information presented with the one-hour fire-rated assemblies in ESR-1153 and the Weyerhaeuser Fire-Rated Assemblies and Sprinkler Systems Fire Guide, extensive sound information is detailed in Weyerhaeuser’s Technical Resource Sheet titled Sound Performance of Trus Joist® TJI® Joist Fire-Rated Floor Assemblies (TJ-4035).

For good sound performance, joist floor assemblies must include resilient channels between the joists and the drywall ceiling and a small amount of insulation (typically 3.5 in. fiberglass batt) in the cavities.  In addition, code-conforming sound performance (IIC ≥ 50, STC ≥ 50) typically requires lightweight concrete or gypsum concrete topping.  Specially designed, proprietary acoustic mats may also be required either above or below the topping for acceptable impact sound performance.

Significantly different sound performance can be achieved by varying components including finish flooring, acoustic mats, underlayment and insulation.  Hard finished flooring materials, such as tile, vinyl, and hardwood transmit more impact noise than soft flooring materials, such as carpet.  For example, testing demonstrates that a gypcrete-topped assembly with cushioned vinyl floor covering may achieve an IIC rating of 51, while the same assembly with carpet and pad achieved an IIC rating of 74.  With either flooring, the assembly would have an STC rating of 57. A similar assembly with cushioned vinyl flooring, but without gypcrete topping scored only IIC = 43 and STC = 53.  Additional comparisons like this and information regarding IIC and STC testing is included in the Technical Resource Sheet (TJ-4035) mentioned earlier.

A designer must ensure compatibility between a selected one-hour fire-resistance rated assembly and the selected I-joists.  This life safety consideration provides a necessary level of protection that will hopefully never be needed for the life of the structure.  At the same time, it is important for the designer to consider sound control as part of the assembly, as this performance factor will be tested daily throughout the life of the structure.  If you have questions about Trus Joist products in floor/ceiling assemblies, please contact your Trus Joist representative or submit an e-mail at techsupport@weyerhaeuser.com.

Jason Shumaker, PE
Jason Shumaker, PE
Jason Shumaker, PE is a Product Support Engineer at Weyerhaeuser. For 15 years, he has been providing engineering support for Trus Joist and Weyerhaeuser products to homeowners, builders, dealers, architects, engineers, and code officials.