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Soundproof Drywall: What QuietRock and a Second Layer Really Do, Resilient Channel, STC Ratings, and the Cheap Things That Work Better
Drywall, Taping & Painting
Soundproof Drywall: What the Expensive Board Does, What a Second Layer Does, and Why the Door and the Outlet Box Undo Both
Sound goes through a wall three ways: through the board itself, through the studs that connect one side to the other, and through every hole. Soundproof drywall helps with the first. It does nothing about the second and the third, which is why people spend $3,000 on QuietRock for a bedroom and still hear the television. The assemblies that work deal with all three: mass, decoupling and sealing. Here is what each product and method actually does, with the STC numbers and Ontario prices.
STC in one paragraph
Sound Transmission Class is a single number for how much a wall cuts airborne sound in the speech range. A standard interior wall, 1/2 in. drywall both sides of 2×4 studs, no insulation, is about STC 33: you hear normal speech through it. STC 40, you hear loud speech as a murmur. STC 50, loud speech is inaudible and you only get the bass from the home theatre. The Ontario Building Code asks for STC 50 between dwelling units, such as a basement apartment and the house above, and nothing at all between rooms in one house. Every 10 points is roughly half as loud to the ear. Impact noise through floors is a separate rating (IIC) and a separate problem.
What each thing does
| Upgrade to a standard 2×4 wall | Roughly | Ontario cost, per sq ft of wall face | Notes |
|---|---|---|---|
| Nothing: 1/2 in. both sides, empty | STC 33 | – | The baseline. Speech comes through. |
| Add batt insulation in the cavity | +3 to 5 | +$0.60 | Cheapest improvement there is. Do it in every bedroom and bathroom wall. |
| 5/8 instead of 1/2, both sides | +2 to 3 | +$0.40 | Mass. Modest on its own. |
| Second layer of 5/8 on one side | +4 to 6 | +$2.00 | More mass. With Green Glue between the layers, +8 to 10 instead. |
| Damped board (QuietRock 510, SilentFX, SoundBreak) | +5 to 8 (one side) | +$2.50 to $4.00 | A sheet with a viscoelastic layer inside; does what two layers and Green Glue do in one board. Expensive per foot, saves space. |
| Resilient channel on one side | +8 to 12 | +$0.80 | Decouples the board from the studs. Ruined by one screw that hits a stud, or by a baseboard screwed tight to the floor. Fussy, effective. |
| Sound clips and hat channel | +12 to 15 | +$1.80 | Better decoupling, more forgiving than channel. |
| Double stud wall or staggered studs | +12 to 18 | Framing cost, 2 to 5 in. of floor | The best decoupling. Plan it at framing or not at all. |
| Seal everything: acoustic caulk at plates, putty pads on boxes, no back-to-back outlets | Protects all of the above | +$0.20 | A 1% hole loses about half the wall’s performance. This is the one everyone skips. |
The assemblies I actually build
Bedroom to bedroom, or bathroom to bedroom: batt insulation, 5/8 board both sides, acoustic caulk at the plates, putty pads on the boxes, no outlets back to back. About STC 42 to 45 for an extra $1.00 a square foot. Add resilient channel on one side if the client wants it quieter and the trades can be trusted not to screw through it.
Home theatre or a teenager’s drum kit: double 5/8 with Green Glue on clips and channel, insulation, sealed, and a solid-core door with seals and an automatic bottom. STC in the mid-50s. The door is where this usually fails: a hollow door with a 3/4-in. gap under it is STC 20 and makes the wall irrelevant.
Basement apartment ceiling: the code wants STC 50 and a fire rating. Insulation in the joists, resilient channel or clips, 5/8 Type X, sealed penetrations, and impact noise is still the complaint, because footsteps are vibration and need a soft floor or a floating floor above. Fire-rated assemblies and legalizing a basement apartment; a garden suite in the yard needs the same thinking between the suite and its garage or workshop, covered on GardenSuiteOntario.com.
Where homeowners go wrong
- Buying damped board and skipping the insulation and sealing. $3,000 of QuietRock on an empty, unsealed wall with back-to-back outlets is an STC 38 wall. Insulation and caulk first; they are nearly free.
- Resilient channel installed by someone who has not done it. Upside down, screwed into studs, or the baseboard nailed through it to the plate. Every one of those shorts it out. Clips are more forgiving.
- Forgetting the door. Solid core, perimeter seals, a drop seal at the bottom. Otherwise the wall is a very expensive frame for a hollow door.
- Expecting a wall to stop bass and footsteps. Low frequencies and impact go through the structure. Mass and decoupling help; nothing in a sheet of drywall fixes a subwoofer on the floor above.
- Soundproofing after the drywall is on. A second layer over the existing board with Green Glue is the only retrofit that works without opening the wall, and you lose the sealing and the insulation.
Questions people ask
Does soundproof drywall actually work?
Damped boards like QuietRock add about 5 to 8 STC points on one side of a wall, which is real, roughly what two layers of 5/8 with Green Glue do. It does not decouple the wall and it does not seal the holes, so on its own it is a disappointing $3 a square foot. In a sealed, insulated wall it is a good product where there is no room for a second layer.
What is the cheapest way to soundproof a wall?
Batt insulation in the cavity, 5/8 board, acoustic caulk at the top and bottom plates, putty pads on every electrical box, and no boxes back to back. About $1 a square foot over a standard wall, and it takes a bedroom wall from STC 33 to the low 40s.
How much does it cost to soundproof a room?
A 12 by 12 bedroom with four walls upgraded to insulation, 5/8 and sealing is $400 to $600 extra at framing. The same room with clips, channel and double board is $2,500 to $3,500. The home theatre with a proper door is $5,000 and up.
Is 5/8 drywall more soundproof than 1/2?
By 2 to 3 STC points for the extra mass. Worth it, but insulation in the cavity is worth more and costs about the same.
Plan it at framing
Every assembly here except the extra layer is decided before the board goes on. The before-drywall checklist is the moment to walk the house with this page. The drywall calculator prices the 5/8 option across the house.
Sources and further reading
- Gypsum Association construction standards: GA-216 application and finishing (the hanger’s bible: fastener spacing, ceiling framing limits, joint treatment) and GA-600 fire resistance design manual, with the free downloads.
- CGC Inc.: the maker of most of the board and compound in Ontario; the product sheets give thicknesses, span limits for ceilings and the right compound for each coat.
- Ontario’s Building Code: the official page with the current regulation. Fire separations and garage walls are in Part 9; interior finish flame spread is 9.10.17.
