Soundproofing insulation: quieter, not silent

Two different noise problems get called "soundproofing," and insulation does very different amounts for each.

The first comes from outside: jets over South San Francisco and San Bruno, planes in and out of San Jose, the steady roar of 101, 880 or 280, a BART line, a busy street. The second comes from inside: the TV through the bedroom wall, a home office next to the kids' room, the in-law unit downstairs, the drummer in the garage. Insulation helps with both. It is the main fix for neither on its own, and anyone who tells you otherwise is selling something.

Where is the noise coming in?

• Overhead, loudest outdoors too: aircraft. The roof and attic are part of the path; see the airport section below.
• From one side, worst near windows: a street or freeway. Windows, vents and gaps are often the weak point, more than the wall itself.
• From next door or the next room: a shared wall or floor, which is a different job from the outside shell.

What insulation actually does

Insulation in a wall or ceiling cavity absorbs sound bouncing around inside that cavity. What it can't do is stop sound that takes a shortcut: through the studs, through a gap under a door, around an outlet box, through a single-pane window.

The most useful research on this comes from Canada's National Research Council, which tested more than 250 gypsum board wall assemblies. The finding that matters for a normal house: when the drywall on both faces of a wall is fastened to the same row of studs, much of the sound goes through the studs, and sound-absorbing material in the cavity does relatively little for the wall's rating. Once one face is isolated, with resilient channels or staggered or double studs, the insulation, the cavity depth and the weight of the drywall all start to count. NRC also found the type of absorbing material has a relatively minor effect compared with those choices.

So filling an ordinary interior wall makes it quieter, and sealing its gaps makes it quieter still. Rebuilding it as an isolated assembly makes it much quieter, but that is drywall and framing work, not an insulation job. Better to know that before the quote than after the install.

Aircraft noise near SFO, OAK and SJC

The federal guidelines for sound-insulating homes near airports, written for the U.S. Navy and published by the FAA, are the most practical document on this. A few points from them:

• More noise generally comes in through windows and doors than through walls and ceiling or roof assemblies.
• Rooms with a roof or attic above them are louder than rooms with another floor above. Cathedral and vaulted ceilings let in more noise than ceilings with an attic over them.
• For accessible attics with less than six inches of insulation, they recommend adding batt or blown-in insulation.
• Attic hatches in living rooms and bedrooms are sound leaks; the fix is a heavier panel and a gasket. Whole-house fans need a cover panel.
• Their target measures focus on mid and high frequencies. Deep low-frequency rumble, the kind you get near the end of a runway under departures, needs additional measures.
• And the plain line: sound insulation is not sound elimination. You will still know a plane is overhead.

Bay Area airports have run programs for exactly this. SFO's noise insulation program dates to 1983 and treated more than 15,000 dwellings inside its 65-decibel noise contour, and since 2017 SFO has offered a replacement initiative for treated homes still inside that contour. San Jose's Acoustical Treatment Program soundproofed 2,675 homes and four schools between 1995 and 2009, but the airport says the program ended in 2010 and no funds remain. San Jose's airport also says Sunnyvale, Cupertino and Mountain View are mostly affected during south-flow operations, and that Communications Hill is affected because it sits high, under the arrival and departure corridor.

If you live near SFO, ask the airport whether your house was in its program before paying anyone, us included.

Freeway and street noise

Traffic noise is steadier than aircraft noise; the FAA guidelines point out that highway noise tends to fade into the background while each overflight calls attention to itself. For new construction, the California Building Code (section 1206.4 in the 2025 edition) limits noise from outside sources to 45 decibels in any habitable room, which is why new buildings along freeways often get upgraded windows and mechanical ventilation so the windows can stay shut. An existing house gets no such check unless you do it.

In an older house facing a freeway or busy street, the order of attack is usually the windows and doors first, then the gaps around them, then the walls and ceiling. Dense fill in the exterior walls on the noisy side is a reasonable supporting step, and it does double duty as thermal insulation; see existing-wall insulation. If the bedrooms are under the attic, the attic matters too.

Between rooms and units

This is where insulation earns its keep. Typical jobs:

• A bedroom wall shared with the living room or the TV.
• A home office that needs to stay quiet on calls.
• The ceiling between a downstairs in-law unit or ADU and the main house.
• A garage turned into a music room or gym.

When the wall or ceiling is open, mineral wool or fiberglass batts fill the cavities, and the gaps get sealed: outlet boxes, top and bottom plates, pipe and duct openings. When it is closed, dense-packed fill goes in through small holes. Doors matter as much as walls here; a hollow door with a gap underneath undoes a well-filled wall. Footsteps from above are a different kind of noise (impact rather than airborne), and cavity insulation helps less with them than with voices and TV.

For more on fitted batts, see batt insulation. If the room is over a garage, the garage ceiling page covers the fire-rated drywall you can't disturb casually.

Materials

Mineral wool doesn't burn and is heavier than fiberglass, which is why it is the usual upgrade for sound work. RemodelingExpense puts the national premium for mineral wool over fiberglass at $0.75 to $2.50 per square foot. Fiberglass batts and dense-packed cellulose or fiberglass also work, and per the NRC research the choice of material matters less than how the wall is built. The product goes on the written quote and carries its manufacturer warranty.

How we scope it

Tell us what you hear, where, and when. Planes at 6 a.m. over the bedroom, the neighbor's TV through a shared wall and traffic on the street side are three different jobs. You can book a free in-home inspection or send a short video, ideally with the noise in it, for an estimate.

The written quote says which walls, ceilings or floors get filled, with what, and what is outside its scope, such as windows or new drywall assemblies. Nothing is due before you sign. For a first range, the estimator can add soundproofing to an attic estimate.