The quick answer
Mass loaded vinyl (MLV) blocks airborne sound like voices and TVs. It's weak against footsteps, which is what most people below a carpeted floor actually hear. For footsteps, a thick carpet pad or a rubber underlayment does more. The MLV makers' guides I could find cover walls and ceilings, not under-carpet installs, so treat the steps below as general practice, not a maker's procedure.
MLV is a heavy, flexible vinyl sheet that adds mass without much thickness. It's easy to see why people want to slip it under a carpet: the floor is bare, the neighbor below complains, and MLV is sold as a soundproofing material. But the noise that carries through floors comes from two different sources, and MLV only addresses one of them.
For the bigger picture on noise between homes, see our soundproofing guide.
Airborne Noise vs Impact Noise
The Sound Transmission Class (STC) rating describes how well a partition blocks airborne sound, according to Riverbank Acoustical Laboratories. Impact sound, such as footsteps and dropped objects, is rated separately as Impact Insulation Class (IIC). A floor can pass one test and fail the other. Multifamily codes commonly expect at least IIC 50 and STC 50 between units, per Commercial Acoustics and Riverbank.
That split decides what MLV can do. Pure Eco, an MLV seller, says MLV is primarily for airborne noise and is not enough on its own for impact noise like footsteps. Its own suggestions for footsteps are floating floors, acoustic subfloor mats, resilient channels, or isolation clips with hat channels.
What MLV Is and How Much It Blocks
MLV is sold in 1 lb and 2 lb per square foot weights. Second Skin, a maker, lists its 1 lb roll at about 1/8 inch thick and STC 26, and its 2 lb roll at STC 32. Those are the material's own ratings. A floor with MLV in it will test differently, and I found no maker data for MLV under carpet.
It's heavy. Pure Eco says a 4 by 25 foot roll at 1 lb per square foot weighs about 100 pounds, so plan for help. High-quality MLV is described by the seller as low in VOCs and safe indoors. That's a seller claim with no test data cited, so check the product's data sheet if odor or chemicals concern you.
Installing It Under Carpet
None of the maker guides I read describe an under-carpet install. They cover walls and ceilings. Second Skin even says MLV in floors is less common, and for most floor projects it recommends a rubber underlayment. What follows is general practice, and the first two points come from how those guides treat any MLV barrier.
- Seal the edges and seams. The makers stress that sound leaks through any crack, gap or seam, so MLV has to be both dense and sealed. On walls they butt seams together and seal them with acoustical sealant, rather than overlapping them over studs.
- Leave a small perimeter gap and seal it. They leave about 1/8 inch at edges and fill it with acoustical sealant.
- Lay it on the clean subfloor, under the carpet pad. This is my judgement: the pad is the soft layer that cuts footfall noise, and the carpet and its fasteners sit above. Check with the carpet installer on tack strip and stretching, since stretching a carpet over a heavy sheet is a hands-on job.
- Cut with a utility knife and straightedge. Both makers recommend a utility knife and T-square.
Wall installs use plastic washers and fasteners into framing every 8 inches. Do not copy that on a floor without asking, because screws or staples into a subfloor can matter to a landlord or an installer.
Better Choices for Footsteps
Commercial Acoustics reports that a bare 6 inch concrete slab tests around IIC 25 to 35, while the same slab with carpet and pad tests around IIC 65 to 75. Carpet and a good pad already do most of the impact work. A thicker pad, or a rubber underlayment under the carpet, is the first upgrade, and it avoids layering heavy sheet goods under a stretched carpet.
On a hard floor with a wood subfloor, the fixes are bigger: a floating floor or isolation clips with hat channels in the ceiling below. Those are construction projects. If you rent, the ones you can do without permission are thick rugs on pads, felt on furniture feet, and a word with the neighbor.
Will It Fix Echo?
No. DDS Acoustical separates blocking, measured by STC, from absorption, measured by the Noise Reduction Coefficient (NRC) and used to address echo inside a room. A dense vinyl sheet under carpet blocks a little and absorbs nothing in the room. For echo, see how to reduce echo in a room.
Frequently Asked Questions
Does mass loaded vinyl work under carpet?
It adds mass against airborne noise, but it doesn't fix footstep noise on its own. Pure Eco says it isn't enough for impact noise, and no maker guide I found covers an under-carpet install.
Does MLV work on concrete floors?
No maker source I found tests it on concrete. A bare concrete slab with carpet and pad tests far better for impact noise than bare concrete, per Commercial Acoustics, which suggests the pad is doing the work.
Does MLV smell?
One seller says high-quality MLV is low in VOCs and safe indoors, with no test data cited. Check the data sheet and air the room.
What should I look for when buying MLV?
Pick the weight for the job. 1 lb and 2 lb per square foot are the common sizes, and the 2 lb version rates higher (STC 32 vs 26 at one maker). Ask for the maker's data sheet, and confirm whether it covers floor use.
Sources
- Pure Eco: How to Install Mass Loaded Vinyl (weights, roll weight, airborne vs impact, sealing)
- Second Skin Audio: How to install MLV (STC 26 and 32, seams, floors, underlayment)
- Riverbank Acoustical Laboratories: Understanding STC ratings (what STC measures, code minimum)
- Commercial Acoustics: IIC Rating 101 (IIC vs STC, carpet and pad, IBC minimum)
- DDS Acoustical: Differences between STC and NRC (blocking vs absorbing)
MLV sellers publish most of the figures here. No independent lab test for MLV under carpet was found.