Soundproofing Your ADU: How to Keep Peace with Neighbors

Why Soundproofing Matters for Your ADU

Building an Accessory Dwelling Unit means adding another household to your property. Whether you're renting to tenants, housing family members, or creating a home office, sound separation between your ADU and main home—and between your ADU and neighbors—is crucial for privacy and peace.

Poor sound insulation is one of the top complaints in ADU living situations. Footsteps, conversations, TV noise, and even plumbing sounds can travel through walls, floors, and ceilings, causing tension between occupants and neighbors.

In this comprehensive guide, we'll cover everything you need to know about soundproofing your ADU, from understanding sound ratings to specific construction techniques and cost-effective solutions.

Understanding Sound Ratings (STC)

Sound transmission is measured using the Sound Transmission Class (STC) rating. Higher numbers mean better sound blocking.

STC Rating	What You'll Hear	Typical Construction
25-30	Normal speech clearly understood	Single drywall, no insulation
35-40	Loud speech audible but not understood	Standard interior wall
45-50	Loud speech barely audible	Good residential separation
50-55	Loud sounds faintly heard	Excellent separation
55+	Most sounds inaudible	Studio/professional quality

California Code Requirements

California Building Code requires a minimum STC 45 for walls and floors between separate dwelling units (including between an ADU and main home for attached units). However, we recommend targeting STC 50-55 for tenant comfort and better rental appeal.

Types of Sound to Block

Understanding sound types helps you choose the right solutions:

Airborne Sound

Travels through the air: voices, TV, music, barking dogs. Blocked primarily by mass and air gaps in walls.

Impact Sound

Caused by direct contact: footsteps, dropped objects, closing doors. Requires isolation materials like rubber underlayment.

Flanking Sound

Sound that travels around barriers through outlets, ductwork, gaps, and structural connections. Often the weakest link in soundproofing.

Soundproof Wall Assemblies

The wall assembly you choose has the biggest impact on sound transmission. Here are options from basic to premium:

Assembly 1: Standard Wall (STC 35-40)

• 2x4 studs, 16" on center
• R-13 fiberglass insulation
• 5/8" drywall both sides

Cost: $8-12/sq ft | Not recommended for ADU separation

Assembly 2: Improved Wall (STC 45-50)

• 2x4 studs, 16" on center
• R-13 mineral wool insulation (denser than fiberglass)
• Resilient channel on one side
• 5/8" drywall both sides

Cost: $12-18/sq ft | Meets code minimum

Assembly 3: Enhanced Wall (STC 50-55)

• Staggered 2x4 studs on 2x6 plate
• R-19 mineral wool insulation
• 5/8" Type X drywall both sides
• Green Glue compound between layers

Cost: $18-25/sq ft | Recommended for rentals

Assembly 4: Premium Wall (STC 55-60+)

• Double stud wall (two separate 2x4 walls with air gap)
• R-19 mineral wool in each cavity
• Double 5/8" drywall with Green Glue
• Acoustic caulk at all perimeters

Cost: $25-35/sq ft | Best privacy

Assembly	STC Rating	Cost/SF	100 SF Wall
Standard	35-40	$8-12	$800-1,200
Improved	45-50	$12-18	$1,200-1,800
Enhanced	50-55	$18-25	$1,800-2,500
Premium	55-60+	$25-35	$2,500-3,500

Floor/Ceiling Soundproofing

For two-story ADUs or attached ADUs above a garage, floor/ceiling assemblies are critical:

Impact Sound (IIC Rating)

California Building Code requires IIC 45 minimum for floors between dwelling units. Target IIC 50-55 for better tenant experience.

Floor/Ceiling Assembly Options

Component	Options	Impact
Subfloor	3/4" plywood or OSB	Structural base
Underlayment	Cork, rubber, or mass loaded vinyl	+5-15 IIC points
Flooring	Carpet > LVP > Hardwood > Tile	Carpet adds 20+ IIC
Joist Cavity	Mineral wool insulation	+5-10 STC/IIC
Ceiling	Resilient channel + double drywall	+10-15 STC

Windows and Doors

Windows and doors are often the weakest points for sound transmission.

Windows

Window Type	STC Rating	Cost Premium
Single pane	26-28	Baseline
Standard dual pane	28-32	Standard
Laminated glass dual pane	34-38	+$50-100/window
Triple pane	35-40	+$100-200/window
Acoustic-rated windows	45-50+	+$200-400/window

For ADUs near busy streets, laminated glass windows are an excellent investment. They also provide enhanced security and UV protection.

Doors

• Hollow core interior doors: STC 15-20 (poor)
• Solid core doors: STC 25-30 (acceptable)
• Solid core with seals: STC 30-35 (good)
• Acoustic doors: STC 40-50 (excellent)

Entry doors should be solid core or solid wood with quality weatherstripping. Door sweeps are essential—even small gaps dramatically reduce sound blocking.

HVAC and Plumbing Noise

Mechanical systems can transmit significant noise:

HVAC Considerations

• Choose a quiet mini-split system (look for units rated under 25 decibels)
• Install the outdoor unit away from bedrooms
• Use flexible duct connections to prevent vibration transfer
• Size the system properly—oversized units cycle frequently and create more noise

Plumbing Considerations

• Wrap drain pipes with pipe insulation or mass loaded vinyl
• Use quiet-fill toilets
• Install water hammer arrestors
• Avoid running pipes through shared walls where possible

Sealing Flanking Paths

Even the best wall assembly fails if sound travels around it. Common flanking paths:

• Electrical outlets: Use putty pads behind outlet boxes and avoid placing outlets back-to-back on shared walls
• Recessed lights: Use IC-rated acoustic enclosures
• HVAC ducts: Line shared ductwork or use separate systems
• Perimeter gaps: Seal all wall/floor and wall/ceiling joints with acoustic caulk
• Pipe penetrations: Seal around all pipes with fire-rated acoustic sealant

The ASTM International provides standards for testing sound transmission, and following proper sealing protocols is essential to achieving rated performance.

Detached vs. Attached ADU Soundproofing

Detached ADUs

Detached ADUs have a significant advantage—no shared walls, floors, or ceilings with the main home. Soundproofing focus areas:

• Exterior walls (traffic, neighbor noise)
• Windows and doors
• HVAC equipment placement

Attached ADUs and Garage Conversions

Garage conversions and attached ADUs require more extensive soundproofing:

• All shared walls and ceilings need STC 45+ assemblies
• Fire separation requirements often align with acoustic needs
• Seal all penetrations for plumbing, electrical, HVAC
• Consider impact sound if above occupied space

Budget-Friendly Soundproofing Tips

Not every project has unlimited budget. Here are cost-effective strategies:

Upgrade	Cost	Impact
Mineral wool vs fiberglass	+$0.50/sq ft	+3-5 STC
5/8" vs 1/2" drywall	+$0.15/sq ft	+2-3 STC
Acoustic caulk	$50-100 total	+3-5 STC
Solid core doors	+$100/door	+10-15 STC
Door sweeps/seals	$20-40/door	+5-10 STC
Carpet/rugs	$2-5/sq ft	+15-25 IIC

Frequently Asked Questions

What's the minimum soundproofing required for ADUs?

California Building Code requires STC 45 and IIC 45 for walls and floors between separate dwelling units. For detached ADUs, there's no minimum for exterior walls, but good practice suggests STC 30-35.

How much does soundproofing add to ADU cost?

Basic code-compliant soundproofing is included in standard construction costs. Upgrading to premium assemblies adds $3,000-8,000 for a typical 600 sq ft ADU.

Can I add soundproofing after construction?

Yes, but it's significantly more expensive. Adding a second layer of drywall with Green Glue can improve an existing wall by 5-10 STC points. It's always more cost-effective to soundproof during initial construction.

Is Green Glue worth it?

Yes, when used properly between two layers of drywall. One tube covers about 32 sq ft and costs about $25. It adds 5-10 STC points—one of the best cost/performance ratios available.

Do I need an acoustic consultant?

For most ADUs, a knowledgeable contractor can specify appropriate assemblies. For challenging sites (near airports, highways, or commercial areas), an acoustic consultant can perform site analysis and recommend specific solutions.

Conclusion

Investing in proper soundproofing during ADU construction is far easier and more cost-effective than addressing problems later. Good sound isolation increases tenant satisfaction, reduces complaints, and can justify higher rental rates.

Key takeaways:

• California requires STC 45/IIC 45 minimum for shared walls and floors
• Target STC 50-55 for excellent privacy in rental units
• Mass, air gaps, and isolation are the three soundproofing principles
• Seal all flanking paths (outlets, penetrations, perimeters)
• Doors and windows often require attention beyond walls

Ready to plan your soundproofed ADU? Our team can help you specify the right assemblies and connect you with experienced contractors.