Under Washington State's landmark Middle Housing Law (HB 1110), single-family zones across Lacey, Olympia, and Tacoma allow duplexes, triplexes, fourplexes, and accessory dwelling units (ADUs). Building codes (IRC Section R302 and IBC Chapter 7) mandate a 1-hour fire-resistance-rated separation wall between dwelling units constructed with 5/8" Type X gypsum panels continuous from the foundation slab to the underside of the roof deck. Additionally, IRC Section AK102 mandates an airborne sound transmission rating of STC 50 or higher, requiring resilient sound channels (RC-1), acoustic insulation, and acoustic perimeter sealant. In 2026, South Sound fire-rated party wall assemblies cost $4.20 to $7.80 per square foot installed.
Washington's Middle Housing Revolution: The HB 1110 Density Shift
Passed by the Washington Legislature, House Bill 1110 (HB 1110) represents the most sweeping zoning reform in state history.
In cities like Tacoma, Lakewood, Olympia, and Lacey, traditional single-family zoning has transitioned to allow up to two, four, or six dwelling units per residential lot.
As developers, custom home builders, and property owners convert single-family parcels into attached duplexes, townhomes, cottage courts, and attached ADUs, interior drywall contractors face two non-negotiable statutory requirements:
- Life Safety Fire Separation: Preventing the spread of fire between attached residential units for a minimum of 60 minutes (1-Hour Fire Rating).
- Acoustic Privacy: Ensuring occupants cannot hear normal speech, television audio, or footfalls from neighboring units (STC 50+ Sound Isolation).
| Performance Requirement | Testing & Code Standard | Mandatory Drywall Assembly Spec |
|---|---|---|
| 1-Hour Fire Resistance | ASTM E119 / UL 263 / IRC R302 | 5/8" Type X gypsum panels continuous from slab to underside of roof deck; intumescent fire-caulking |
| STC 50+ Sound Isolation | ASTM E90 / ASTM E413 / IRC AK102 | Resilient sound isolation channels (RC-1), high-density mineral wool sound batts, non-hardening acoustic perimeter sealant |
1-Hour Fire-Rated Party Wall Assemblies Explained
Under International Residential Code (IRC) Section R302.2, common walls separating attached townhomes and duplexes must provide an unbroken 1-hour fire resistance rating tested in accordance with ASTM E119 or UL 263:
| Fire-Rated Assembly Component | Engineering Specification | Why It Is Mandatory |
|---|---|---|
| Gypsum Panel Spec | 5/8" ASTM C1396 Type X or Type C | Specially formulated gypsum core reinforced with glass fibers to maintain structural integrity under fire. |
| Continuous Vertical Run | Slab-to-underside of roof sheathing | Cannot stop at the ceiling plane; must extend continuously into the attic to prevent attic fire jump. |
| Fastener Spacing | 1-5/8" Type W/S screws spaced every 8" on edges, 12" in field | Prevents board from sagging or pulling away from studs during thermal expansion in a fire. |
| Joint Treatment | GA-214 Level 2 minimum fire-tape | All joints and screw heads must be taped and covered with joint compound to block smoke and heat transfer. |
| Penetration Protection | Intumescent fire-stop sealant | Electrical boxes on opposite sides of the wall must be separated by at least 24 inches or fire-wrapped. |
Acoustic Sound Isolation: Achieving STC 50+
Nothing generates more tenant complaints or lease cancellations in multi-unit housing than hearing the neighbors next door.
The Sound Transmission Class (STC) measures how well a wall assembly attenuates airborne sound. Standard single-stud interior drywall walls offer an STC of 33 to 35—meaning normal conversation is easily understood through the wall.
Washington building codes (IRC Appendix K / IBC Section 1206) mandate a minimum field rating of STC 45 (STC 50 laboratory tested) between attached dwelling units:
| Sound Isolation Level | STC Rating | What Occupants Hear | Code Compliance |
|---|---|---|---|
| Standard Interior Wall (Single 1/2" Sheet) | STC 33-35 | Loud speech easily understood; TV audio clearly audible. | Non-compliant for demising walls. |
| Insulated Wall (Fiberglass Batt in Cavity) | STC 38-40 | Loud speech audible but muffled; bass notes transfer. | Non-compliant for unit separation. |
| Resilient Channel Wall (5/8" Type X on RC-1) | STC 50-54 | Loud speech heard only as a faint murmur; high acoustic privacy. | Compliant with WA HB 1110 & IRC. |
| Double-Stud Decoupled Wall Assembly | STC 58-62 | Shouting barely audible; near-total sound isolation. | Luxury standard for custom duplexes. |
The 3 Proven Soundproofing Strategies Solis Installs
1. Resilient Channel Decoupling (RC-1 Systems)
Sound travels through drywall as mechanical vibrations. When drywall is screwed directly to wall studs, the studs act as acoustic bridges, transferring vibration instantly to the other side. Solis installs horizontal resilient sound isolation channels (RC-1) spaced 24 inches on center. The drywall hangs on flexible steel springs, decoupling the gypsum board from the framing and absorbing sound vibrations.
2. High-Density Mineral Wool Sound Batts
Standard lightweight fiberglass insulation offers poor low-frequency sound absorption. Solis installs high-density mineral wool sound batts (such as rockwool insulation) inside every stud cavity, trapping airborne sound waves before they bridge the wall.
3. Non-Hardening Acoustic Perimeter Caulking
Sound acts like water—if air can leak through a microscopic crack under the bottom plate or around an outlet box, sound will pour through, dropping an STC 55 wall down to STC 35 (acoustic flanking). Solis seals 100% of wall perimeters and electrical cutouts with heavy non-hardening acoustic sealants.
2026 Middle Housing Drywall Pricing in South Sound
Building high-performance fire-rated and soundproof party wall assemblies requires specialized materials, precise fastener schedules, and certified fire-stopping labor.
Based on 2026 Western Washington trade labor rates, here is what builders and developers can expect:
| Assembly Type | 2026 South Sound Cost per Sq. Ft. | What is Included in the Scope |
|---|---|---|
| Standard 1-Hour Fire-Rated Wall (Single Layer 5/8" Type X) | $3.60 - $5.80 per sq. ft. | 5/8" Type X panels both sides, code fastener spacing, fire-taping, acoustic sealant |
| STC 50+ Sound Isolation Assembly (RC-1 + 5/8" Type X) | $4.80 - $7.20 per sq. ft. | Resilient channels, mineral wool sound batts, 5/8" Type X board, acoustic perimeter seals |
| Double-Stud High-Privacy Demising Wall (STC 58+) | $6.20 - $8.90 per sq. ft. | Dual decoupled stud lines, double-layer 5/8" Type X, dual mineral batts, Level 4 finish |
| Architectural Level 5 Skim Coat Upgrade | +$1.45 - $3.00 / sq. ft. additive | Full-surface skim coat for open-concept great rooms under critical lighting |
Frequently Asked Questions (FAQ)
Does Washington State require an inspection for fire-rated drywall?
Yes. Local building departments (such as the City of Lacey, City of Olympia, and City of Tacoma) require a mandatory drywall fastening / fire-tape inspection before any joint compound or texture covers the drywall screws. Inspectors verify 5/8" Type X panel stamps, screw spacing (every 8" on edges), and continuous floor-to-roof coverage.
Can we add soundproofing to an existing wall without tearing it down?
Yes. We frequently install an additional layer of 5/8" Type X drywall over existing walls using specialized viscoelastic sound-damping compound applied between the layers. This dampens vibrations and increases wall mass, raising the STC rating by 10 to 14 points without invasive demolition.
Can Solis Interiors handle all multi-unit units under one contract?
Yes. Solis Interiors provides complete turnkey drywall packages for multi-family duplexes, triplexes, and townhomes across Thurston and Pierce counties—from framing insulation and resilient channels to fire-taping, texture matching, and corner-to-corner painting.
Authoritative Codes & Multi-Family Standards
- American Society for Testing and Materials (ASTM): ASTM E119 & ASTM E90 Fire and Sound Transmission Testing
- Washington State Legislature: HB 1110 - Concerning Increasing Middle Housing in Areas Traditionally Dedicated to Single-Family
- International Residential Code (IRC): IRC Section R302: Two-Family Dwelling Fire Separation & Appendix K Sound
- Gypsum Association (GA): GA-600: Fire Resistance and Sound Control Design Manual
- Underwriters Laboratories (UL): UL Fire Resistance Directory (Wall Designs U305, U411, U465)
Schedule a Middle Housing Drywall Walkthrough with Solis Interiors
Building a duplex, ADU, or townhome project in Lacey, Olympia, or Tacoma? Contact master finisher Jesus Solis for certified 1-hour fire-rated installation and STC 50+ sound isolation assemblies.
- Direct Phone: (360) 350-8616
- Online Quote Request: Request Fire & Acoustic Drywall Quote
- Service Areas: Lacey, Olympia, Tumwater, Tacoma, Lakewood, DuPont, Gig Harbor, Yelm, Centralia.