How to Soundproof Between Your Main House and ADU

How to Soundproof Between Your Main House and ADU

One of the most overlooked aspects of ADU design and construction is soundproofing. Whether you are renting your ADU to a tenant or housing a family member, managing sound transmission between the main house and the ADU is critical for maintaining privacy and a peaceful living environment. Sound issues that seem minor during the planning stage can become major sources of frustration once someone moves in.

This is especially important for attached ADUs, garage conversions, and ADUs built close to the main house. Even detached ADUs can have sound issues if they are positioned near bedroom windows, outdoor living areas, or other noise-sensitive zones.

In this comprehensive guide, we will cover the science of sound transmission, practical soundproofing strategies for every budget, California building code requirements for sound, and specific solutions for different ADU types. By the end, you will have a clear plan for achieving the sound isolation your project needs.

Understanding Sound Transmission

Before diving into solutions, it helps to understand how sound travels and how it is measured. Sound transmission in buildings occurs through two primary paths:

Airborne Sound

Airborne sound includes voices, music, television, and other sounds that travel through the air. These sounds enter walls, floors, and ceilings and cause them to vibrate, transmitting the sound to the other side. Thin walls, gaps around doors, poorly sealed windows, and ductwork are common pathways for airborne sound.

Structure-Borne Sound (Impact Sound)

Structure-borne sound occurs when physical impact creates vibrations in the building structure. Footsteps, closing doors, dropping objects, and plumbing noise are common examples. These vibrations travel through solid materials (wood framing, concrete, pipes) and can be heard as low-frequency rumbling or thumping in adjacent spaces.

How Sound Is Measured

California's building code (Title 24, Part 2, based on the California Building Code) requires a minimum STC 50 rating for walls and floor/ceiling assemblies between dwelling units in multi-unit buildings. While detached ADUs are not technically required to meet this standard, using STC 50 as a target for attached ADUs and a benchmark for detached ADU walls facing the main house is a sound (pun intended) practice.

Wall Soundproofing Strategies

For attached ADUs and garage conversions, the shared wall between the ADU and the main house is the most critical area to address. Even for detached ADUs, the walls facing the main house benefit from enhanced soundproofing.

Strategy 1: Insulation Selection

Standard fiberglass batt insulation provides some sound reduction, but upgrading to higher-performance insulation can make a significant difference.

Recommendation: Mineral wool (Rockwool) is the best value for soundproofing. It is denser than fiberglass, provides excellent sound absorption, and is naturally fire-resistant. For the shared wall in an attached ADU, mineral wool is the top choice among acoustics experts.

Strategy 2: Double Drywall with Green Glue

One of the most effective and cost-efficient soundproofing techniques is adding a second layer of drywall with Green Glue (a viscoelastic damping compound) between the layers. This approach converts sound energy into heat, dramatically reducing transmission.

A standard 2x4 wall with single 5/8" drywall on each side and fiberglass insulation achieves roughly STC 35-39. Adding a second layer of 5/8" drywall with Green Glue on the ADU side can push the rating to STC 50-55, meeting or exceeding code requirements for multi-unit buildings.

Cost: Approximately $2-3 per square foot for the additional drywall and Green Glue. For a shared wall of 200 square feet, that is $400-600 in materials, a very modest investment for significant sound improvement.

Strategy 3: Staggered or Double-Stud Wall

For maximum sound isolation, a staggered-stud or double-stud wall eliminates the direct structural connection between the two sides of the wall. In a staggered-stud wall, alternating studs are attached to different top and bottom plates, so no single stud touches both sides of the wall. In a double-stud wall, two completely separate stud walls are built with a small air gap between them.

These approaches can achieve STC ratings of 55-65, approaching the sound isolation of a professional recording studio. The trade-off is that they use more floor space (the wall assembly is 8-12 inches thick instead of the standard 4.5 inches).

Strategy 4: Resilient Channel

Resilient channel is a thin metal channel that is screwed to the studs, and the drywall is then attached to the channel rather than directly to the studs. This creates a "floating" drywall layer that is decoupled from the structure, reducing structure-borne sound transmission.

Adding resilient channel to a standard wall assembly can improve the STC rating by 5-10 points. It costs approximately $0.50-1.00 per square foot in materials.

Important note: Resilient channel must be installed correctly to be effective. If even one screw penetrates through the channel and into the stud (creating a "short circuit"), the entire system's performance is compromised. Use experienced installers who understand acoustic isolation principles.

Window and Door Soundproofing

Windows and doors are typically the weakest links in any soundproofing strategy. Sound will always take the path of least resistance, so even the best wall assembly will be undermined by a single-pane window or a hollow-core door.

Window Upgrades

Recommendation: For ADU windows facing the main house, laminated double-pane windows offer the best balance of cost and performance. The laminated interlayer (a sheet of PVB or EVA plastic between the glass panes) adds mass and damping that significantly reduces sound transmission. As a bonus, laminated glass also provides better security and UV protection.

Door Upgrades

Exterior doors for the ADU should be solid-core doors with proper weatherstripping and a door sweep at the bottom. A hollow-core door has an STC rating of only 20-25, while a solid-core door with proper seals can achieve STC 30-35. For even better performance, consider a door with an acoustic seal (compression gasket) system.

Outdoor Sound Buffering

For detached ADUs, outdoor strategies can reduce noise transmission before sound even reaches the building envelope.

Landscaping as Sound Barriers

Dense vegetation can reduce noise by 5-10 decibels per 100 feet of planting. While this may not sound like much, a reduction of 10 dB is perceived as roughly cutting the noise level in half. Effective sound-buffering plants include:

• Evergreen hedges (Arborvitae, Leyland Cypress, Podocarpus)
• Dense shrubs (Photinia, Oleander, Pittosporum)
• Bamboo (clumping varieties to prevent spreading)
• Ornamental grasses (Fountain Grass, Miscanthus)

Solid Fencing

A solid fence (not a lattice or slatted fence) between the main house and ADU can reduce noise by 5-8 dB. The fence should be as tall as local codes allow (typically 6 feet for side yards, 3-4 feet for front yards) and have no gaps at the bottom or between boards. Tongue-and-groove or overlapping board fences are most effective.

Water Features

A small fountain or water feature between the main house and ADU does not actually block sound, but it provides "white noise" that masks unwanted sounds. The gentle sound of flowing water can effectively make conversations and low-level music from the ADU imperceptible from the main house.

Frequently Asked Questions

How much does it cost to soundproof an ADU?

Basic soundproofing (mineral wool insulation, double drywall with Green Glue on shared walls, solid-core door) adds approximately $2,000-5,000 to construction costs. Advanced soundproofing (staggered studs, laminated windows, acoustic doors) can add $5,000-15,000. For most detached ADUs, the basic package is sufficient.

Can I soundproof an existing ADU that is already built?

Yes, but it is more expensive and disruptive than incorporating soundproofing during construction. Options include adding a second layer of drywall with Green Glue to interior walls, replacing windows with laminated glass, adding door sweeps and weatherstripping, and installing secondary interior windows.

What STC rating should I target for my ADU?

For attached ADUs (including garage conversions), target STC 50 or higher for shared walls and floors. For detached ADUs, STC 40-45 for walls facing the main house is usually sufficient. STC 50 means that loud speech is inaudible and music is barely perceptible.

Does soundproofing affect resale value?

Yes, positively. A well-soundproofed ADU is more attractive to potential tenants and buyers. It demonstrates quality construction and attention to livability, which supports higher rent and property values. See our guide on whether ADUs raise property value.

Is soundproofing required by California building code?

For attached ADUs, the California Building Code (CBC) requires STC 50 for wall and floor/ceiling assemblies between separate dwelling units. For detached ADUs, there is no specific STC requirement between the ADU and the main house, but meeting STC 50 for the ADU's walls and windows is a best practice for livability.

What is the single most cost-effective soundproofing upgrade?

Adding a second layer of 5/8" drywall with Green Glue to shared walls. At $2-3 per square foot in materials, it provides the greatest STC improvement per dollar spent. This single upgrade can improve a wall's STC rating by 10-15 points.

Budget vs. Premium Soundproofing: A Side-by-Side Comparison

Choosing between budget and premium soundproofing solutions depends on your noise reduction goals, construction timeline, and overall ADU budget. The table below outlines the key differences between the two approaches for California homeowners.

Budget Soundproofing Options ($500 to $3,000)

• Mass-loaded vinyl (MLV): A thin, dense material that can be added to walls, ceilings, and floors to block sound transmission. Cost is approximately $1 to $2 per square foot for materials.
• Acoustic caulk and weatherstripping: Sealing gaps around doors, windows, and electrical outlets with acoustic sealant can reduce noise transfer significantly at minimal cost, usually under $100 for a full ADU.
• Heavy curtains and rugs: Adding dense curtains to windows facing the main house and thick area rugs with padding can absorb airborne sound and reduce impact noise. Budget approximately $200 to $500.
• Solid core doors: Replacing hollow interior doors with solid core doors reduces sound transmission between rooms and between the ADU and the exterior. Each door costs $150 to $400 installed.
• Basic insulation upgrade: Adding standard fiberglass batt insulation to shared walls or ceilings provides moderate sound reduction. Material cost is $0.50 to $1.50 per square foot.

Premium Soundproofing Options ($3,000 to $15,000+)

• Double-stud or staggered-stud walls: Building walls with two separate stud rows eliminates direct structural connections between surfaces, dramatically reducing sound transmission. This is one of the most effective soundproofing methods but adds $2,000 to $5,000 in framing costs.
• Resilient channel or sound isolation clips: These decoupling systems attach drywall to the framing through flexible connectors that break the vibration path. Cost is $1.50 to $3 per square foot installed.
• Acoustic mineral wool insulation: Roxul or similar mineral wool products offer superior sound absorption compared to fiberglass. Cost is $1 to $2 per square foot but provides significantly better noise reduction.
• Double or triple-pane windows: Upgrading to laminated glass or adding a second pane of glass with an air gap provides excellent noise reduction from exterior sources. Budget $400 to $1,200 per window depending on size and glass type.
• Floating floor systems: Installing a floating floor with an acoustic underlayment prevents impact noise from traveling through the floor structure. Cost is $3 to $6 per square foot for materials and installation.

For most California ADU projects, a combination of budget and premium strategies delivers the best results. Investing in proper insulation, acoustic sealant, and solid core doors during construction is far more cost-effective than retrofitting soundproofing after the ADU is complete. If your ADU will share a wall with the main house, such as a garage conversion, prioritize wall and ceiling soundproofing in your construction budget.

Frequently Asked Questions About Soundproofing Between a Main House and ADU

What is the most cost-effective soundproofing method for an ADU?

Sealing air gaps with acoustic caulk and adding mass-loaded vinyl to shared walls provides the best noise reduction per dollar spent. These two measures alone can reduce sound transmission by 10 to 15 decibels, which makes a noticeable difference in everyday comfort. For ADUs being built from scratch, specifying acoustic mineral wool insulation in the walls and ceiling during construction adds minimal cost but significant sound reduction.

How much does professional soundproofing cost for a California ADU?

Professional soundproofing for a full ADU typically costs between $3,000 and $12,000 depending on the scope. A basic package covering shared walls and ceilings with resilient channels and mineral wool runs $3,000 to $5,000. A comprehensive package including floating floors, premium windows, and full decoupling systems can reach $10,000 to $15,000. These costs are generally included in the overall ADU construction budget when planned from the start.

Does soundproofing increase my ADU's rental value?

Yes. Tenants are willing to pay a premium for quiet, well-insulated ADUs, especially in dense urban areas of Los Angeles. Soundproofing can justify $100 to $200 more per month in rent, which adds up to $1,200 to $2,400 per year in additional income. Over the life of the rental, this premium far exceeds the initial soundproofing investment.

Can I soundproof an existing ADU without major construction?

Yes, several retrofit options are available that do not require opening walls. Adding a second layer of drywall with green glue compound on shared walls, installing acoustic panels or heavy curtains, replacing hollow doors with solid core doors, and sealing all gaps with acoustic caulk are all effective retrofit strategies. These measures can reduce noise by 5 to 12 decibels without major construction disruption.

Is soundproofing required by California building code for ADUs?

California building code requires a minimum Sound Transmission Class (STC) rating of 50 for walls and floor-ceiling assemblies between dwelling units. This applies to ADUs that share a wall or floor-ceiling with the main house, such as attached ADUs or units built above a garage. Detached ADUs do not have a shared wall STC requirement, but the distance between structures and standard wall construction usually provides adequate noise separation. Check with your local building department for any additional requirements specific to your city.