What Is Window Wall?
Window wall is a hybrid system that combines elements of curtainwall and window systems. It is installed floor-by-floor, with windows (complete units) installed in a mullion framework that spans each floor. The system is anchored to the structural frame at each level, much like curtainwall, but uses off-the-shelf or semi-custom window units rather than framing members alone.
Window wall has become the standard for mid-to-high-rise multifamily projects because it provides superior thermal performance compared to curtainwall, offers cost efficiency compared to full curtainwall systems, and allows for large operable windows that residents expect.
Understanding Window Wall System Types
Standard Window Wall: Standard window wall uses commercial-grade window units—typically aluminum-framed, casement or projected windows—installed in a frame that spans each floor. The frame is anchored to the structural slab, creating a weather-tight envelope floor-by-floor. This is the most common window wall type for residential projects and offers a good balance of cost, performance, and install-ability.
Thermally Broken Window Wall: Thermally broken systems incorporate thermal breaks in both the window frame and the window wall mullion framework. This dramatically improves thermal performance and reduces energy loss through the framing members. Most high-performance residential window wall today includes thermal breaks.
Structural Glazing Window Wall: Structural glazing (silicone-bonded) window wall eliminates visible frame members on the exterior, creating a seamless glass facade. This is an aesthetic choice used on premium projects but requires more sophisticated engineering and longer lead times. Structural glazing is more common on high-end residential and hospitality projects.
Unitized Window Wall: Unitized systems pre-assemble window units with their supporting mullions at the factory, then ship complete units to the site for installation. This approach reduces on-site assembly, improves quality control, and accelerates installation schedules. Unitized systems are increasingly popular on large multifamily projects.
Key Performance Criteria for Multifamily Window Wall
Thermal Performance (U-Value): This is paramount in multifamily residential. Residents expect low heating and cooling bills. Modern window wall systems typically achieve U-values of 0.25-0.35, with high-performance systems reaching 0.20 or lower. Thermal breaks in both windows and mullions are essential. Single-pane or non-thermally-broken systems are now outdated for residential applications.
Air Infiltration (Air Leakage): Measured as CFM at 75 Pa (cubic feet per minute at 75 pascals of pressure), air leakage affects both comfort and energy costs. Target specifications are typically 0.3 CFM or lower per square foot of window area. Better systems approach 0.1 CFM.
Water Tightness: Window wall must be completely water-tight. Testing per ASTM E331 (static water tightness) is standard. All window wall systems are tested and rated, but installation quality is critical. ACG's installation procedures ensure water tightness is maintained.
Wind Resistance: Design wind speed depends on building location. Most multifamily buildings in Florida require systems rated for 150+ mph wind speeds. Window wall is tested per ASTM E330 (structural performance under wind loads) and ASTM E1996 (impact resistance in high-wind areas).
Noise Isolation: Exterior noise (traffic, aircraft, mechanical equipment) can affect resident satisfaction. Window wall with laminated glass and thermal breaks provides superior noise reduction. STC ratings (Sound Transmission Class) of 35+ are typical, with high-performance systems achieving 40+.
Selecting the Right Manufacturer
Your window wall manufacturer choice affects cost, schedule, performance, and long-term reliability. Consider these factors:
Track Record in Multifamily: Select manufacturers with proven experience on residential projects of similar size and climate. Ask for references from recent multifamily projects and follow up with conversations about performance and support.
Thermal Performance Rating: Compare U-values and SHGC (Solar Heat Gain Coefficient). High-performance systems cost more but deliver lower operating costs. The economics depend on climate and utility rates, but in Florida's warm climate, minimizing SHGC is also important.
Lead Times and Production Capacity: Large multifamily projects with hundreds of windows create significant manufacturer demand. Verify that your selected manufacturer can meet your project timeline. Longer lead times (12-16 weeks) are common on large, custom projects. Budget accordingly.
Technical Support and Field Service: A responsive technical team during fabrication, installation, and startup is valuable. Manufacturers that provide field support, training, and troubleshooting reduce installation problems and accelerate learning for new crews.
Warranty and Service Network: Verify warranty terms—particularly coverage for thermal breaks, seals, and finish. Establish a post-installation service relationship for maintenance and repairs during the warranty period and beyond.
Coordination With Other Trades
Window wall installations affect and are affected by multiple trades. Successful coordination requires clear communication and defined responsibilities:
Structural Steel/Concrete: Anchor points must be set accurately during structural work. Verify slab elevations, anchor locations, and tolerance stacks early. ACG coordinates with structural crews to ensure anchor installation meets window wall requirements.
Exterior Insulation and Finish Systems (EIFS) and Cladding: Window wall integration with cladding systems requires careful detailing. Ensure flashing, sealant, and cladding interfaces are coordinated to prevent water intrusion. The window wall contractor and cladding contractor must work together on details.
Interior Framing and MEP: Window sills integrate with interior floor systems. Coordinate with mechanical trades on penetrations and routing of utilities around windows. Early coordination prevents field conflicts and rework.
Testing and Commissioning: After installation, window wall systems are tested for air infiltration, water tightness, and structural performance. Plan for testing schedules and coordinate with the general contractor and design team on test protocols and acceptance criteria.
Budget and Schedule Considerations
Window wall cost per square foot ranges from $75-$200+ depending on system type, performance level, and complexity. Thermally broken standard window wall is typically $100-$150/sf. Premium structural glazing or high-performance systems cost more. Get firm pricing early and factor in any design changes.
Installation schedule depends on building height and window quantity. A 10-story multifamily building with 400-500 windows typically requires 8-12 weeks installation. Budget for testing, rework, and coordination time. Large projects benefit from early manufacturer engagement to refine details and optimize the install sequence.
Common Issues and How to Prevent Them
Condensation: Interior surface condensation can occur on poorly insulated windows in humid climates. Specify high-performance thermal breaks and ensure interior ventilation is adequate. This is a resident satisfaction issue, not a product failure, but it can create disputes.
Water Intrusion at Sills: Improper sill installation or flashing is the most common cause of water problems. Ensure installation procedures include proper sill sealing and integrate with interior floor protection.
Installation Quality Variation: Large projects with multiple installation crews can have quality variation. Establish clear quality standards, conduct regular inspections, and provide training to all crews. ACG maintains consistent quality across all project phases.
Schedule Delays from Coordination Issues: Delays in structural anchor installation, cladding coordination, or testing scheduling are common. Establish a project timeline with clear milestone dates for all trades. Weekly coordination meetings prevent surprises.