There are several excellent ways to make acoustically isolated walls with cold-formed steel (CFS) framing: including double walls, multiple layers of sheathing, resilient channels, and sound isolation clips. Putting 3-5/8” studs in 6” tracks, and snaking insulation between the studs in a serpentine fashion, is NOT a good way to acoustically isolate walls.
This “serpentine acoustic wall”, or SAW, is something that has been successfully done in wood framed walls where no fire blocking is required. The solid wood studs in nonstructural applications less than 10’ tall can be adequately braced against the moderate interior 5 pounds per square foot (PSF) load by the sheathing on one flange and by attaching to the top and bottom plates. The steel framing typically used for interior applications (nonstructural drywall studs less than 33 mils thick) will require torsional bracing: at the top, bottom, and at mid-height. With wood framing, this top and bottom bracing can be accomplished by toe-nailing the studs to the top and bottom plates. For CFS framing, clips are required to attach the framing to the top and bottom tracks, and architects usually don’t know they have to show these clips on their wall framing details. In addition, bracing is usually required near mid-height. Standard 1-1/2” cold-rolled channel (CRC) will be too wide to bypass the 3-5/8” studs from the other side of the wall (see detail 1). Strap bracing can be used to brace the unsheathed flanges (detail 2), but the strap gets in the way of the insulation, and short-circuits the sound isolation that the architect is trying to achieve. So, what are the solutions?
- Use a double wall. With CFS framing, members are available in 1-1/2” and 2-1/2” depths, so double walls can easily fit in a 6” cavity and bracing in each individual wall will not interfere with the adjacent walls.
- Use a single wall with sound isolation clips or acoustically-rated sheathing. There are many great products on the market today that provide acoustic isolation: clips for attaching furring channel, several styles of resilient channel, and sheathing with special acoustic properties all perform well. Also make sure that gaps and joints around door and window frames, as well as along the top and bottom tracks, are appropriately finished with acoustical sealant to prevent sound leakage around the edges of the wall.
- Use multiple layers of gypsum board to increase wall mass. On fire rated walls, this is often already a requirement: so meeting proper finishing requirements and adding batt insulation can increase acoustical performance. Note that increasing the thickness of batt insulation really doesn’t help much with respect to acoustic performance. Just making sure that the insulation is there and fitted tightly in all stud cavities with no gaps, can give better performance than thicker insulation.
- Use studs designed for better acoustical performance. The EDGE™ from Super Stud Building Products has a unique ribbed design, which disrupts sound waves passing through the steel, as well as isolating the stud from sheathing attached to the flanges. Using the EDGE™ coupled with properly installed batt insulation and acoustical sealant can provide sound transmission class (STC) values up to 51 WITHOUT the use of resilient channel.
If you are installing framing, and it’s too late to get the architect to change the steel-framed SAW, here’s how you do it without compromising the structural integrity of the wall:
- Use clip angles at the top and bottom tracks to prevent the studs from rotating. This can be a fairly light clip: no more than 33 mils thick, with one or two screws to the stud and one or two shots to the concrete above or below. Note that if only one screw or shot is used, then the outside flange of the stud needs to be screwed to the flange of the track.
- If there is a slip connection at the top track, then slip clips will be required at the top of each stud. There are several good clips on the market that work well with drywall studs, including the ITTC 450 from Super Stud and the “Three-Legged-Dog” from Flex-Ability concepts. These clips will require at least two shots to the underside of the structure above, to prevent rotation.
- At mid-height of the studs, use ¾” CRC for bracing. This can be clipped to each stud using 1-1/2” x 1-1/2” clip angles with two screws to the stud and two screws to the CRC. Or, a specialty clip made for the ¾” CRC can be used: something like the DBC2.5 from Simpson Strong-tie.
If you are an architect and need assistance designing acoustical walls, visit www.buysuperstud.com and check out our acoustical wall details. In addition, the Steel Framing Alliance (SFA) has a fire and acoustic directory here, with hundreds of details and assemblies for walls, floors, ceilings, and roofs.