Fire-rated assemblies save lives. The International Building Code (IBC) mandates specific fire resistance ratings for walls, floors, and ceilings based on occupancy type, building height, and construction classification. These ratings dictate how long an assembly must contain fire and prevent structural collapse—typically 1 hour or 2 hours for most commercial and multifamily residential projects.

Soundproofing and fire protection often compete for the same space inside wall and ceiling cavities. Adding materials that have not been tested as part of a listed assembly creates an unknown variable. If a fire occurs and the assembly fails to perform as rated, the contractor faces legal liability, the building owner faces insurance denial, and occupants face danger.

The good news: many soundproofing materials—including quality MLV—carry fire ratings that allow them to be incorporated into listed assemblies without compromising fire performance. The key is knowing which products are rated, which tests they have passed, and how to document everything for the Authority Having Jurisdiction (AHJ).

Fire testing for building materials falls into two distinct categories: surface burning characteristics (how fast flame spreads across the material surface) and fire resistance (how long an assembly withstands fire exposure).

This is the primary test for individual materials including MLV. A 24-foot sample is mounted in a Steiner tunnel and exposed to a controlled flame. The test measures:
• Flame Spread Index (FSI): How rapidly flame travels across the surface (Class A = 0-25, Class B = 26-75, Class C = 76-200)
• Smoke Developed Index (SDI): How much smoke the material produces (must be ≤450 for most applications)

Most building codes require all materials installed within wall cavities, plenums, and concealed spaces to achieve Class A (FSI ≤25, SDI ≤450). This is the benchmark contractors must verify before installing any soundproofing product.

This test evaluates complete wall or floor/ceiling assemblies—not individual materials. A full-scale assembly is constructed and exposed to fire following a standard time-temperature curve. The assembly must maintain structural integrity, prevent flame passage, and limit temperature rise on the unexposed side for the rated duration (1 hour, 2 hours, etc.).

Underwriters Laboratories publishes tested assembly designs (e.g., UL U305, UL U411) that specify every component—stud type, insulation, drywall layers, and any additional materials like MLV. Using a UL-listed design gives contractors and inspectors confidence that the assembly will perform as rated. Deviating from the listed design—even by substituting a different insulation brand—technically voids the listing.

Quality mass loaded vinyl achieves Class A fire ratings under ASTM E84, with typical test results showing flame spread indices of 10-20 and smoke developed indices well below 450. This places MLV among the safest soundproofing materials available for concealed installation.

The PVC polymer matrix in MLV is inherently fire-resistant due to its high chlorine content—chlorine atoms suppress combustion by interrupting the chemical chain reaction of fire. Manufacturers further enhance fire performance by adding antimony trioxide or alumina trihydrate fire retardant compounds during production.

Not all MLV products carry identical fire ratings. Budget products that use excessive plasticizer or recycled polymer content may produce higher flame spread and smoke values. Always request the manufacturer's ASTM E84 test report—not just a claim of 'Class A'—and verify the test was conducted by an accredited laboratory (UL, Intertek, or equivalent).

Our MLV Regular and MLV Pro both carry verified ASTM E84 Class A ratings with full test documentation available upon request.

The most effective approach combines fire resistance and acoustic performance in a single tested assembly. Here are proven configurations that deliver both:

• 3-5/8" steel studs at 24" o.c.
• 3-1/2" mineral wool insulation (non-combustible, melting point 2,150°F)
• 1 lb/ft² MLV membrane stapled to one side of studs
• 5/8" Type X gypsum board on each side
• Acoustic sealant at all perimeters and penetrations

This assembly achieves STC 50-55 while maintaining a full 1-hour fire rating. The mineral wool insulation is critical—it is non-combustible and maintains its structure during fire exposure, unlike fiberglass which melts at approximately 1,000°F.

• 3-5/8" steel studs at 24" o.c.
• 3-1/2" mineral wool insulation
• 1 lb/ft² MLV membrane
• Double layer 5/8" Type X gypsum board on each side
• Staggered joint pattern on drywall layers
• Fire-rated acoustic sealant at all perimeters

The double drywall layers add both mass (improving STC) and fire resistance. Staggering the joints ensures no single crack path exists through the assembly. This configuration is standard for multifamily party walls, hotel room separations, and commercial tenant demising walls.

Floor/ceiling assemblies in multifamily buildings must typically achieve both STC 50 and IIC 50 for airborne and impact noise, plus a 1-hour or 2-hour fire rating. MLV integrates effectively into these assemblies:

• Above the subfloor: MLV underlayment beneath finished flooring adds mass without significantly raising floor height. Combined with a resilient underlayment layer, this addresses both airborne and impact noise
• Below the floor joists: MLV draped over the ceiling framing before drywall installation adds significant airborne noise reduction to the assembly
• Within the cavity: Mineral wool batts between joists provide both fire resistance and sound absorption

Critical detail: MLV must not be the only layer between fire-separated spaces. It must always be covered by a fire-rated finish material (Type X gypsum board) on the exposed side. MLV alone does not provide a fire resistance rating—it provides a surface burning classification only.

For floor applications, our Floor Underlayment Regular pairs with MLV to deliver both impact isolation and airborne noise reduction within fire-rated assemblies.

Contractors frequently encounter soundproofing products that do not meet fire code requirements. Knowing what fails saves time, money, and rework:

• Acoustic foam (open-cell polyurethane): Most acoustic foam is Class C or unrated. It ignites easily, produces toxic smoke, and is prohibited in concealed spaces in virtually all jurisdictions. Never install acoustic foam inside wall cavities
• Mass loaded vinyl without documentation: Unbranded or imported MLV sold without ASTM E84 test reports cannot be verified and will be rejected by diligent inspectors
• Spray foam insulation in fire-rated assemblies: Unless the specific spray foam product is included in a UL-listed assembly design, it typically voids the fire rating
• Recycled rubber mats: Many recycled rubber products lack fire testing entirely. Their composition varies by batch, making consistent fire performance impossible to guarantee
• Cork underlayment: Natural cork has poor fire performance (Class C typically) and is not suitable for concealed installation in fire-rated assemblies without an intumescent barrier

The common thread: any product installed in a concealed space must have ASTM E84 Class A documentation. If the manufacturer cannot provide it, do not install it behind drywall.

Retrofitting soundproofing into existing fire-rated assemblies requires careful planning to avoid voiding the rating:

• Do not penetrate fire-rated drywall unnecessarily: Every screw, nail, or anchor point through Type X gypsum is a potential failure point. Use adhesive attachment methods for MLV when possible, or ensure fasteners are covered by the finish layer
• Seal all penetrations: Electrical boxes, plumbing pipes, and HVAC ducts passing through fire-rated walls must use listed firestop systems (putty pads, intumescent caulk, or fire-rated collars). Acoustic sealant alone is not a firestop
• Maintain air cavity integrity: Do not compress insulation or overfill cavities. Fire-rated assemblies are tested with specific cavity configurations—changing the air space changes the performance
• Use mineral wool, not fiberglass: In fire-rated assemblies, mineral wool maintains its structure at temperatures up to 2,150°F. Fiberglass melts around 1,000°F, potentially creating voids that allow fire passage
• Document everything: Photograph the assembly at each stage of construction. Keep material data sheets, ASTM test reports, and UL listing references on-site for inspector review

Passing fire inspection on a soundproofed assembly comes down to documentation and visible compliance. Here is what Authorities Having Jurisdiction typically verify:

• Material test reports: Inspectors want to see current ASTM E84 test reports for every material inside the assembly. Have these printed and ready before calling for inspection
• UL listing compliance: If you are claiming a specific UL design number, every component must match the listing exactly—stud gauge, insulation type and thickness, drywall brand and thickness, and fastener spacing
• Firestop at penetrations: This is the #1 failure point. Every pipe, wire, duct, and box penetrating a fire-rated assembly must have a listed firestop system. Inspectors will reject entire walls for a single unsealed penetration
• Drywall installation quality: Proper screw spacing (typically 8" on edges, 12" in field for Type X), no damaged or cracked boards, and fire-rated joint compound at all seams
• Labeling: Many jurisdictions require fire-rated assemblies to be permanently labeled with the UL design number and fire resistance rating. Check local requirements before closing walls

Pro tip: build a relationship with your local AHJ early in the project. A pre-construction meeting to review your proposed assemblies can prevent expensive surprises during final inspection.