Field note
STC vs fire rating on the same metal-stud partition
The corridor partition is often asked to be two things at once: fire-rated and STC-rated. Those are not the same stamp. Fire is a time. STC is a number from a sound test. The same stud, board, and batt serve both only when the listed design and the acoustical design are the same wall, built as drawn.
Mixing them is how a shop hangs an extra layer “for sound,” screws through resilient channel, or caulks the head with the wrong tube. Inspector fails the listing; acoustical punch fails the seal. This page is takeoff language, not a substitute for the UL / GA-600 design and the acoustical schedule.
Two tests, two documents
Fire resistance. Hourly rating from ASTM E119 / UL 263. The design is a named assembly: UL number, Gypsum Association GA-600 design, or a mill system that cites one of those. Layer count, core (Type X or Type C), orientation, fastener class and spacing, stud depth and spacing, and any insulation the listing names — that is the wall. Hang still follows ASTM C840 / GA-216 unless the listing is tighter.
Sound (STC). Sound Transmission Class from ASTM E90 (lab) classified per ASTM E413. Field tests use ASTM E336 and a different number; IBC dwelling-unit language commonly allows a field result a step below the lab STC. Mass, decoupling, cavity absorption, and a sealed perimeter are the four levers. None of them is “add Type X and hope.”
A wall can be 1-hour and mediocre STC, or high STC and unrated. Both at once only if the design was tested or calculated for both. Do not copy an STC from one mill onto another mill’s fire listing because the stud depth matched.
Fire lives in the IBC fire chapters. Airborne sound for dwelling and sleeping units lives in the IBC sound section — commonly an STC 50 lab / 45 field class of requirement. Read the edition on the job. Townhouse party walls and commercial tenant demising walls are not one spec.
What actually changes the number vs the hours
Board. Extra layers add mass (STC) and, if they are the right core in the listing, add fire time. Leftover 1/2-inch regular is neither. Abuse board is a surface, not an STC product. See choosing drywall type.
Stud. Interior drywall stud is the ASTM C645 / C754 partition. Depth and spacing show up in both fire listings and sound catalogs. Staggered-stud and double-stud walls are acoustical construction — they are not a 3-5/8 fire partition with the plates kicked off layout. Shaftwall and H-stud area separation are other rated systems; do not borrow their members to “help STC” on a corridor.
Decoupling. Resilient channel, isolation clips, and double-frame air gaps break the vibration path. They also change which fire listing you are in. Screw the board through the channel into the stud and you have neither the STC you bought nor the listing on the stamp. Channel orientation, fastener length, and “do not land a gypsum joint on the channel” are manufacturer notes, not a field habit.
Cavity absorption. Sound-attenuation blankets and mineral wool show up in both conversations. The fire listing either requires a named insulation, allows it, or is silent. The STC design usually wants the cavity filled with the product the catalog tested. Insulation is not interchangeable batt from the trailer. Johns Manville and peers publish the class; the listing publishes whether it belongs.
The seal. STC dies at the gap: floor track, head, outlets, uncaulked cutouts. Acoustical sealant is not firestop. Listed head-of-wall firestop is not acoustical caulk. Putty pads are an STC detail; through-penetration firestop is a fire detail. Buy both when the wall asks for both. Doors, frames, and hardware are flanking paths — an undercut trashes a demising STC no matter how many layers you hung.
Two different misses on the same partition
Fire miss, STC habit. Adding resilient channel or a clip system that is not in the listed design. Changing stud gauge or spacing off the listing. Substituting Type C where the design said Type X (or the reverse). Filling a listing that was tested empty. Omitting a layer on the shaft or corridor side because “the other side is two-hour.” Head-of-wall treated as deflection-track foam.
STC miss, fire habit. Building the listed fire wall and skipping the seal. Fastening through the decoupling layer. Leaving batt short of the deck or stuffing it so hard it bridges the channel. Stacking outlet boxes back-to-back. Treating a borrowed lite or a louver as if the partition still had an STC. Using the fire-caulk tube on every joint because it was on the cart.
Composite action (board both sides) is how many fire partitions reach height. Some STC designs want a resilient face that is not tied through. Height tables on the mill’s steel-stud sheet and the acoustical catalog can disagree. Design the wall type; do not average the tables.
Bid notes for the drywall sub
Price the wall type that is on the drawings, not a 5/8 Type X habit:
- Listed fire design number, layer count, and fastener pattern
- Acoustical construction (RC, clips, double frame, specified batt) as its own labor
- Perimeter sealant and listed head-of-wall — two products if the spec has two
- Box pads, acoustic putty, and extra studs at back-to-back boxes
- Door-frame grout, gaskets, and wrap only if they are in your number — see DFH
- Control joints that the fire wall still needs; an STC wall is not exempt from GA-216 movement rules
USG and the other gypsum houses publish fire and sound catalogs. ClarkDietrich publishes members that appear in both. Pull the current PDF for the mill on the submittal. Do not bid an STC from a blog table.
Commercial vs residential
Commercial wall and ceiling. Tenant demising, corridors, conference rooms, and rated shafts often stack fire + STC + abuse on one partition type. The W&C shop owns the metal stud, the board, and usually the batt. Firestop may be a separate sub. Acoustical consultant punch is real — budget the seal.
Residential production. Dwelling-unit sound language shows up on multifamily wood and townhouse party walls. A crew that only stocks 1/2-inch regular will not satisfy a listed 1-hour + STC demising wall. H-stud area-separation is a fire-wall class, not an STC product unless the documents say so. Single-family interiors almost never carry an STC stamp.
Related directory
Members: Steel stud / framing, ClarkDietrich. Board and sound/fire catalogs: USG, Manufacturers. Cavity fill: Insulation, Johns Manville. Openings that flank the wall: Doors / frames / hardware.
FAQ
If the wall is 1-hour Type X, is it automatically STC 50?
No. Fire time and STC are different tests. Some listed fire walls also have published STC values. Many do not. Read the wall type.
Can I add resilient channel to a UL fire partition to pick up STC?
Only if that channel is in the listed design you are building. A field-added RC layer is a different wall.
Is mineral wool the same as sound batt?
They overlap as a product class and they are not a free swap. Use the insulation the fire listing and the acoustical design both allow.
Does acoustical sealant replace head-of-wall firestop?
No. Perimeter STC caulk and a tested firestop joint are different materials and different details. Spec both when the wall needs both.
Do shaftwall and area-separation walls count as STC walls?
They are fire-rated systems with their own members. Some catalogs publish sound data on some of those designs. Do not assume. See shaftwall installation.