Field note
Sound-attenuation batts in light-gauge partitions: not a design by themselves
Stuffing leftover batt into a 3-5/8 metal stud is not an acoustic design. Sound-attenuation batts (SAFB and mineral-wool cousins) absorb energy in the cavity. They do not write an STC number, they do not seal flanking, and they do not replace a second layer or a resilient hang the listed assembly called for.
This page is for the drywall / wall-and-ceiling sub who often owns the insulation line on commercial partitions. It paraphrases public GA / ASTM / mill language on sound control in gypsum assemblies. It is not a laboratory report and not a substitute for the tested wall on the drawings. The rating lives in UL Product iQ, GA-600, and mill sound tables — not a trailer full of scrap.
What the batt is
Sound-attenuation fire blankets (SAFB) and mineral-wool / stone-wool cavity fill are unfaced or faced mineral-fiber blankets sized to friction-fit the stud bay. Fiberglass sound batts from Owens Corning and peers, and mineral wool from Johns Manville and peers, are the everyday commercial products. Directory: Insulation. Product families: Products.
Blanket products of this class are specified against ASTM C665 (mineral-fiber blanket). Board/batt mineral fiber used in fire and acoustic cavities also shows up under ASTM C612 language in mill data. Thermal R-value is not the bid criterion on an interior acoustic wall. Thickness that fills the cavity without compressing the hang is.
They are not spray foam (closed-cell foam that bonds both faces is a rigid path). They are not cellulose dumped in a metal stud, and not leftover thermal batts ripped short around boxes.
STC is the assembly
Sound Transmission Class (STC) is a laboratory rating of a full specimen tested to ASTM E90 and classified to ASTM E413. GA-600 and manufacturer sound manuals publish those tested walls: stud depth and mils, spacing, board type and layer count, orientation, insulation as named, decoupling if any, and the seal the lab used.
Change a layer, skip the batt the design named, swap fiberglass for a foam that was not in the test, or hang a different board than the table, and you are not looking at that STC anymore. The Gypsum Association’s public sound-control notes put insulation in its place: it absorbs energy in the cavity, and it works best after the faces are decoupled. If the two faces are rigidly tied through the stud, sound takes the stud.
Field performance is not the lab number. Flanking through an ACT plenum, an unsealed head, back-to-back boxes, or a door undercut will spend the rating before anyone blames the batt.
What batts do
- Break up the empty-cavity resonance between two gypsum faces.
- Meet the named insulation in a fire-rated or acoustic listed design so the listing still applies. GA-600 explanatory notes govern when you may add insulation to a fire design that was tested empty, and when adding or deleting insulation in a floor-ceiling hurts the rating. Read those notes. Do not add batt to a rated ceiling because “more is quieter.”
- Give the estimator a line item the GC already expects in a commercial partition package.
Friction-fit, fill the bay, no gaps at the web or at boxes, split around conduit. Compressing a batt into a resilient-channel or clip cavity so the channel cannot move is how you kill the decoupling the batt was supposed to help. See the hang of those members in the resilient-channel notes and steel stud framing.
What batts do not do
- They do not seal the wall. Perimeter acoustical sealant (the ASTM C919 class of joint sealant the gypsum manuals point at) at floor, head, and dissimilar abutments is the airtight line. Taped gypsum intersections help. A batt shoved into a crack is not caulk.
- They do not stop flanking. A partition that dies on the ACT grid leaks over the top. Either the wall goes to the deck as the acoustic drawing shows, or the plenum is a different design. Recessed boxes back-to-back or in the same bay are holes. Medicine cabinets, hose cabinets, and transfer grilles are holes.
- They do not replace mass or decoupling. A second layer, a constrained-layer panel, staggered or double stud, resilient channel, or isolation clips are different moves. The batt is the cavity fill in those designs, not a stand-in for them.
- They do not make Type X into an acoustic panel. Core type is fire. See choosing drywall type.
- They do not fix a bad hang. Fastener pattern and missing tape are hanging and finish problems. Level 1 fire taping is not an acoustical seal unless the documents closed the joint that way.
Sequence on a metal-stud partition
- Frame to the height table and layout. Directory: Steel stud / framing. ClarkDietrich and peers publish the members; insulation does not change mils.
- Board one face when the design is a conventional partition (or hang liner on shaftwall — different system).
- Fill the cavity with the named batt, full height, friction-fit, after MEP has a home. Do not wait until the second face is up and then “fish” scraps.
- Seal the perimeter as the acoustic / fire details show, then close the second face. Finish to the GA-214 level on the schedule — Finishing. Acoustics and paint smoothness are different paid scopes.
Commercial vs residential
Commercial W&C. Insulation in rated and acoustic partitions is often in the drywall number. Bid the batt, the hang time to fill cleanly, and the sealant if it is yours. Confirm who owns firestop at the head. A GC who says “throw some batt in” is not naming a GA-600 design.
Residential production. Interior sound batts between bedrooms or a theater wall are frequently the insulation sub or the GC, not the hang crew. When they land on the drywall bid, they are still a named product in a specified wall — not scrap from the exterior walls. Wood 2×4 and 3-5/8 metal are different bays; one SKU does not fill both.
Bid notes
- Own line: insulation type, thickness, facing, which walls. Not “insulate as needed.”
- Match the listed design. Empty-cavity vs insulated is a listing question.
- Do not convert a thermal batt leftover into SAFB on a hospital demising wall.
- Price fill labor on cut-up walls and boxes — not a clean-office production rate.
- Perimeter sealant and box putty, if in your scope, are not in the batt unit price.
Related directory
Insulation, Owens Corning, Johns Manville, Products. Framing: Steel stud / framing, Steel stud framing basics. Board: Choosing drywall type. Hang and finish: How to hang drywall, Drywall finishing levels, Finishing.
FAQ
If I fill every stud bay, is the wall an STC 50?
No. STC is the tested assembly. Batts are one component. Flanking and leaks spend the number in the field.
Can I use leftover R-13 thermal batts as SAFB?
Only if that exact product is what the design named. Thermal leftover is not a silent acoustic substitution.
Does mineral wool always beat fiberglass for STC?
The listed assembly names the product class. Do not swap fiber type to chase a rumor. Fire designs are even less forgiving.
Do I insulate a wall that stops at the ACT?
The batt in that short wall does not stop the plenum flanking path. Follow the acoustic drawing — often the wall goes to deck.
Is spray foam a better sound batt?
Not as a class substitute in a gypsum STC wall. Rigid foam that ties the faces is the wrong mechanism.
Who owns the batt on a commercial job?
Often the drywall / W&C sub. Write it in the bid. Residential production often does not own it.