Can You Put MLV Directly on Drywall?
Updated
Yes, you can put MLV directly on drywall, as long as you screw it through the drywall into the studs, seal every seam and cover it with a new layer of 5/8 in. drywall. Leave it bare and it adds just 1 lb/sq ft and leaks at every open seam. Cover it, and that side of the wall picks up about 3.2 lb/sq ft of sealed mass: 1 lb of vinyl plus 2.2 lb of USG Firecode X (opens in a new tab).
Acoustiblok's wall instructions (opens in a new tab) are written for open studs, not existing drywall, but the same details carry over:
- Fasten into the framing with plastic or "tin cap" washers under each fastener, and run the sheets horizontally.
- Overlap seams at least 1 in., with sealant inside the lap and no fasteners in it.
- Leave a 1/8 to 3/8 in. gap at the floor, ceiling and side walls, and fill it with sealant.
- Cover with 5/8 in. or thicker drywall, keeping the vinyl and fasteners flush so you don't get dimples, and seal its perimeter gap too. Put in box extenders first, because your outlets will end up recessed.
Watch out: Keep Green Glue off the vinyl. Its maker says the compound "will not work in direct contact with a layer of MLV" (Green Glue FAQ (opens in a new tab)), so it belongs between two drywall sheets instead.
Expect a few STC points, not silence. A whole second drywall layer took an insulated 2x4 wall only from STC 34 to 37 in USG lab tests (opens in a new tab). Does mass-loaded vinyl work explains when it's worth the money, and MLV soundproofing panels covers other spots where it fits. If you want bigger gains, read up on decoupling in the soundproof panels guide, and check the soundproof panels FAQ for more answers.