The building industry has evolved enormously in the past 20 years. While labor rates have increased, the amount of skilled labor available for many construction jobs is extremely limited. This is even more true for highly-skilled jobs, such as welding, carpentry, plumbing, and other trades. When this occurs, Architects and Contractors are forced to find new ways to hit their STC 50 Building Code targets.
CASE STUDY: STC Improvement for Modular Construction with Clayco
Summary: It is a straight-forward effort to hit STC 50 with light-gauge (25-gauge) studs, but becomes much more difficult once heavier gauges are used. The cost savings of modular construction should consider the need for specialty soundproofing materials in order to meet the building code.
General Contractors continue to look for ways to meet difficult schedules with this labor shortage, and many have turned to modular construction. By manufacturing the wall partition systems offsite, the amount of labor personnel needed on site is cut in half, and sometimes by more.
However, since these partitions are often load-bearing, it is much more difficult to hit minimal building codes (usually STC 50) for the walls. This is because stronger, stiffer studs allow more sound to pass through a wall, lowering the STC ratings.
How to Achieve an STC 50 in Modular Walls
There are several assemblies that achieve an STC 50+ in modular framing, but some may be much more expensive (and schedule-friendly) than others.
- Consider using a Soundproofing Membrane, such as Wall Blokker, that is independently lab- and field-tested on heavy-gauge studs. One layer of Wall Blokker on 16-gauge studs @ 24 inches on center, with 2 layers of drywall on each side, achieves an STC of 57. Similar STC tests are available for 12-gauge, 14-gauge, and 18-gauge studs.
- If necessary, consider using Resilient Channel. However, note that there are a number of Resilient Channel problems that arise in the field if not properly identified.
- Staggered Studs: Ultimately, if soundproofing materials are out of the budget and resilient channel is not possible (because the walls have cabinets, shelving, etc.), then staggering or doubling the studs may be the only option.
When considering pricing options, remember that Resilient Channel or Soundproofing Membranes have an installed cost in the $1/square foot range. If using resilient channel, strongly consider using a soundproofing consultant to ensure it is properly handled and tested.
Why Is STC 50 Critical for Architects?
Since STC 50 is required in the International Building Code section 1207.2, it is extremely important that this rating is met and exceeded between dwelling units and adjacent dwelling units or common spaces. This is true for demising walls, corridor walls, and floors and ceilings.
While there are many other, and often stricter, requirements, the IBC code is the most commonly employed. For instance, hotels often require STC ratings above 55, but this is often dependent on brand standard. There is also significant litigation when STC ratings fall below the code, primarily in the condo industry. As condo owners are long-term investors in the property, they are more likely to pursue litigation than a group of apartment renters that would instead need to gather as a class-action group. However, that does not mean that code is written or enforced differently between condos and apartments.
In some cases, judgment has been passed that condo owners had “commercially reasonable” expectations of higher than STC 50. that means that just meeting the building code does not absolve the builder of the expectations of the clients.
Ultimately, acoustic field testing by an independent firm is the most risk-averse solution to ensuring that STC 50 will be met and exceeded. When designing an STC 50 wall, ensure that there is a factor of safety involved, since workmanship variation may often fluctuate from job to job.
