Sealant on Acoustically Rated Steel Partitions
Robert Grupe / September 2016
Q: Please explain the use of sealant on acoustically rated steel framing partitions. Can you provide me with some tips on how to apply this sealant?
A: There are four factors that help in the design of an acoustically performing partition. Conversely, there is only one factor that can eliminate all the good design and installation practices. That single biggest negative aspect is termed “flanking paths.” Flanking paths allow sound to circumvent or pass through an otherwise or potentially high performance wall assembly. As mentioned in an earlier article, Sound Transmission Classification is a single number rating system that indexes and predicts the performance of assemblies. The higher the STC, the better the wall is in impeding sound from passing through the wall.
The threshold for a good performing acoustical wall is an STC of 50. It is obtained by using three of the four factors that contribute to sound performance: mass, isolation, absorption and decoupling. The mass of the wall can be increased. The mass can then be divided and isolated from each other by framing members. The third factor is to add an absorptive material or insulation between the two masses. The isolation can be further enhanced by decoupling the mechanism, or framing, that holds the two halves together. This is accomplished with either a specially designed resilient channel or a clip system.
Tests run some time ago indicate that what would have been a 53 STC wall was reduced to a 29 simply by not using acoustical sealant around the perimeter of the test wall. To put this in comparison, the lowest performing wood-framed partition can achieve a 35 STC, and what is considered a high-performance wall is in the mid-60s. The 24-point drop renders the wall as acoustically transparent and ultimately makes for an extremely anxious owner. That amount of difference will be heard.
Assuming we are working with cold-formed steel framing, one potential solution could be to set the track in a bed of sealant and do nothing else. Unfortunately, that doesn’t work. The sound will travel under the gypsum panels on one side and also over the track and down under the gypsum panels on the other side. An analogy for sound control is the concept of a bucket designed to hold water. The bucket is seen as the partition and sound is the water. As water will find every hole in the bucket, so will sound find every hole in a wall. Another way of looking at the importance of sealing the perimeter is to acknowledge that a one square inch hole in a 100 square foot 50 STC partition will drop that STC 10 points to a 40. That amount of drop will be easily heard. It should be fairly easy to visually spot that one square inch hole, but what if it is 1/16-inch-high by 16 inches long?
The most efficient and cost-effective solution is a single bead of sealant around the perimeter of the partition. Ideally, that sealant should be in contact with the gypsum panel, the concrete floor and the leg of the track. The Gypsum Association specifies that the bottom edge of the wall panels be held a minimum of a quarter-inch above the floor. This is called out in Section 4.6.11 of their literature entitled “GA-216-2016, Application and Finishing of Gypsum Panel Products.” This is done to accommodate uneven floors and to obstruct the capillary action within the gypsum core if it comes in contact with standing water. This is the perfect location for the bead of sealant. The same is true for the head of the wall and the vertical ends of the partition runs. In other words, run a bead of sealant around the perimeter of the wall and on both sides of the partition.
The question of which should be installed first, the gypsum panels or the sealant, is not an uncommon question. Is it better to install the panel into a bed of sealant, or can the sealant be post-applied? It is the opinion of this author that is best left as a contractor option. Either sequence works assuming that the sealant be in continuous contact with all the materials.
Sound “leaking” around the perimeter of a wall is only one example of flanking noise. Unfortunately, there are many ways in which an intended high performance acoustic wall will not meet its expected and specified performance levels. Unintended flanking paths can occur from both design and installation practices. Duct work, location of door frames, even electrical outlets can and will have a negative effect on the overall performance of the wall. Another example of flanking paths is where gypsum panels are continuous over the head of a proposed sound-rated partition or along the vertical ends of sound-rated walls.
The contractor is well advised to learn the common installation details and practices that negatively influence sound performance and to exercise due caution during construction.
Robert Grupe is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.