When engaging with the design community and contractors, I often hear the terms “movement, compression, extension,” and “deflection” used interchangeably when discussing head-of-wall joint performance in fire-rated and acoustic-rated assemblies. But what is the best way to define and discuss “movement”? 

In third-party tested fire-resistive joint systems, “movement” is the standard term (see Figure 1). Systems describe movement as a percentage—prompting the common question: “A percentage of what?” The answer: it’s a percentage of the nominal joint width at installation. For example, a nominal joint width of 1 inch with 100 percent compression and 100 percent extension results in 2 inches of total deflection. 

Compression, extension, and deflection are used to describe the total movement required to meet the structural design intent. When choosing products, both fire performance and movement capabilities (compression and extension) are critical to achieving a code-compliant fire-resistive joint system. 

Photo: CEMCO

Standards and Testing

Movement capabilities are determined through prescriptive test standards ASTM E1966 and UL 2079, which share the same cycling criteria. Testing yields one of three movement classifications: Class I (thermal), Class II (wind and sway), or Class III (seismic). Each class requires a specific number of cycles—ranging from 100 to 500—at speeds from 1 to 30 cycles per minute. 

Once cycling is complete, the joint is locked in its fully extended (open) position and subjected to fire exposure, temperature, and often a hose stream test. Testing in the open position ensures performance under the most demanding conditions. 

Product Capabilities 

Diagram: CEMCO

Manufacturers determine which movement class their products meet based on intended use. Some products are static and not tested for movement. Dynamic products vary widely: 

• Wet-applied products typically allow 17 percent–50 percent movement, limited by their chemical composition
• Preformed products, such as CEMCO Fire Gasket, combine vinyl and intumescent strips, enabling 100 percent compression and 100 percent extension

For example, Fire Gasket 0.5 achieves 1/2 inch of deflection with only a 1/4-inch nominal joint width. By contrast, wet-applied sealants (limited to about 25 percent movement) would require a 1-inch joint to achieve the same deflection. 

In STC-rated assemblies, joint movement introduces another challenge. The ASTM E90 acoustic standard does not account for movement; tests are conducted in static joints, often within hours of installation. Wet-applied products may not have fully cured, raising the question: does curing status impact acoustic performance?  

Photo: CEMCO

Guidance for Designers and Contractors 

Given these complexities, the design community should first define movement requirements and clearly document them in construction drawings. Different structural elements, such as concrete floor/ceiling assemblies vs. TPO roofs, may demand different movement criteria. 

Contractors must then select systems that meet both fire and acoustic requirements by: 

1. Understanding the specified movement (compression and extension)
2. Evaluating product performance based on manufacturer data
3. Ensuring correct installation of framing and joint products

By aligning design intent, product capability, and proper installation, code-compliant and performance-driven solutions can be achieved for both fire-rated and acoustic applications.