# Double Skin Facades ## Table of Contents - [[#Overview]] - [[#Definition and Operating Principle]] - [[#Classification by Geometry]] - [[#Box Window Type]] - [[#Shaft-Box Type]] - [[#Corridor Type]] - [[#Multistory Type]] - [[#Airflow Modes]] - [[#Thermal Buffer Effect]] - [[#Natural Ventilation Through Double Skin Facades]] - [[#Acoustic Performance]] - [[#Fire Safety Considerations]] - [[#Daylighting and Visual Comfort]] - [[#Maintenance and Access]] - [[#Cost-Benefit Analysis]] - [[#Notable Examples]] - [[#Design Decision Framework]] - [[#Key References and Standards]] --- ## Overview A double skin facade (DSF) consists of two layers of glazing separated by a ventilated cavity, typically ranging from 200 mm to over 2000 mm in width. The cavity may contain solar shading devices, maintenance walkways, and ventilation pathways. Double skin facades offer a sophisticated environmental response -- combining solar control, thermal buffering, natural ventilation, and acoustic attenuation in a single system. However, they also introduce complexity in design, cost, fire safety, and maintenance that must be carefully evaluated. This article should be read alongside [[Building Envelope Fundamentals]] for general envelope principles, [[Natural Ventilation Principles]] for airflow mechanics, and [[Curtain Wall Systems]] for the glazing technology involved. --- ## Definition and Operating Principle A double skin facade operates by managing airflow and solar energy within the cavity between the two skins: 1. **Solar radiation** passes through the outer skin and strikes the shading device or inner skin 2. **Absorbed heat** warms the cavity air 3. **Buoyancy and/or wind pressure** drives heated cavity air upward and out (or the cavity is mechanically ventilated) 4. **The inner skin** experiences a moderated thermal environment, reducing heat gain in summer and heat loss in winter 5. **Shading devices within the cavity** are protected from wind, rain, and dirt, improving their durability and effectiveness ### Key Terminology | Term | Definition | |------|-----------| | Outer skin | The external glazing layer, typically single or double glazed | | Inner skin | The internal glazing layer, typically double or triple glazed with low-e coating | | Cavity | The air space between the two skins | | Shading device | Blinds or louvers located within the cavity | | Ventilation openings | Operable elements at top and/or bottom of the cavity for airflow | --- ## Classification by Geometry Double skin facades are classified by the way the cavity is partitioned and ventilated. The four principal types, as defined by Oesterle et al. (2001), are described below. --- ## Box Window Type ### Description The cavity is subdivided both horizontally (at each floor level) and vertically (at each structural bay or window module), creating individual box units for each window. ### Characteristics - Cavity width: typically 200-500 mm - Each box operates independently - No airflow connection between floors or bays - Cavity ventilated through openings in the outer skin at top and bottom of each box ### Advantages - **Fire safety:** No vertical cavity connection between floors; easiest type to achieve fire compartmentation - **Acoustic separation:** Each box is independent; no sound flanking between rooms - **Individual control:** Occupants can control their own cavity ventilation and blinds ### Limitations - Limited cavity width restricts maintenance access - Higher framing cost (more subdivisions) - Less effective buoyancy ventilation (short stack height) --- ## Shaft-Box Type ### Description Individual box-window units are connected to vertical shafts at regular intervals (typically every 1-3 bays). The shafts extend the full building height and terminate at a high-level outlet, creating a stack effect that enhances cavity ventilation. ### Characteristics - Box cavity: 200-400 mm - Shaft width: 400-800 mm - Shaft acts as a thermal chimney, drawing air from the box units - Air enters box at low level, passes through shading zone, exits into shaft ### Advantages - Enhanced buoyancy-driven ventilation compared to box window type - Maintains fire separation between boxes on the same floor - Good acoustic separation between rooms ### Limitations - Vertical shafts must be fire-stopped or designed as fire-resistant enclosures if they connect multiple floors - Shaft takes up facade area, reducing vision glass - More complex structural and weatherproofing detailing at shaft junctions --- ## Corridor Type ### Description The cavity extends the full width of the facade at each floor level, creating a continuous horizontal cavity (essentially a glazed corridor). Horizontal partitions separate each floor, but there is no vertical subdivision within a floor. ### Characteristics - Cavity width: typically 500-1500 mm (sufficient for maintenance access or walkway) - Horizontal partitions at each floor slab align with fire compartmentation - Cavity ventilated through openings in the outer skin at each floor level ### Advantages - Continuous maintenance access along the facade - Effective as a thermal buffer zone, reducing heating demand on exposed facades - Can serve as a circulation route in some configurations - Good fire compartmentation at floor levels ### Limitations - Sound can travel horizontally through the cavity between adjacent rooms on the same floor - Horizontal airflow may carry odours or pollutants between spaces - Wider cavity increases overall facade depth and reduces usable floor area --- ## Multistory Type ### Description The cavity is a single continuous volume extending across multiple floors and bays, with ventilation openings only at the base and top of the building (or at multi-storey intervals). ### Characteristics - Cavity width: typically 600-2000 mm - Full-height cavity creates maximum stack effect - Often combined with large-scale architectural expression (atrium-like character) ### Advantages - Strongest buoyancy-driven ventilation (tallest stack height) - Architectural drama and transparency - Efficient outer skin (minimal framing subdivisions) ### Limitations - **Fire safety:** Most problematic type; uninterrupted vertical cavity creates a chimney for fire and smoke spread between floors. Requires extensive fire engineering - **Acoustic:** Sound transmits freely between floors - **Overheating risk:** Upper levels of the cavity can become extremely hot in summer (cavity temperatures exceeding 60 degC have been recorded) - **Odour and pollutant transfer** between floors through the shared cavity --- ## Airflow Modes The cavity airflow can be configured in several modes depending on season, weather, and building requirements: ### Naturally Ventilated Cavity (Exhaust Air) - **Summer:** Outer skin openings allow air to enter at the base and exit at the top. Solar-heated air is expelled, reducing heat gain to the interior. - **Winter:** Outer openings closed. Cavity acts as a sealed thermal buffer, reducing heat loss. ### Mechanically Ventilated Cavity - Fan-driven airflow through the cavity at controlled rates - Allows precise control of cavity temperature - Used in tall buildings where natural stack effect may be excessive or unpredictable ### Supply Air Mode - Outdoor air enters the cavity, is pre-heated by solar gain, and is supplied to the interior as ventilation air - Reduces heating energy in winter - Requires careful filtration and contamination control ### Extract Air Mode - Indoor exhaust air passes through the cavity before being expelled - Recovers heat from the exhaust air stream to the cavity - Reduces condensation risk on the outer skin in cold climates ### Airflow Direction Summary | Mode | Airflow Path | Primary Season | |------|-------------|---------------| | Exhaust (natural) | Outside --> cavity --> outside (top) | Summer | | Buffer (sealed) | No through-flow; static air layer | Winter | | Supply air | Outside --> cavity --> inside | Winter (pre-heating) | | Extract air | Inside --> cavity --> outside | Winter (heat recovery) | | Mixed | Varies by zone and conditions | Transitional | --- ## Thermal Buffer Effect The cavity acts as a thermal buffer between interior and exterior conditions: ### Winter - Sealed cavity traps a layer of still air with greenhouse effect from solar gain - Cavity temperature is typically 5-15 degC higher than ambient in winter - Reduces the temperature difference across the inner skin, lowering transmission heat loss - Effective U-value of the total assembly is significantly lower than the inner skin alone ### Summer - Ventilated cavity removes solar-heated air before it can transfer to the interior - Combined with cavity-mounted shading devices, total solar heat gain can be reduced by 60-80% - Cavity ventilation rate must be sufficient to prevent excessive cavity temperatures ### Energy Savings Studies report typical heating energy savings of **10-30%** and cooling energy reductions of **20-40%** compared to equivalent single-skin facades, depending on climate, orientation, and operation. --- ## Natural Ventilation Through Double Skin Facades One of the key drivers for DSF adoption in tall buildings is enabling natural ventilation in conditions where single-skin facades cannot support it: - **Wind protection:** The outer skin shields operable inner-skin windows from high wind pressures at elevation - **Acoustic shielding:** The outer skin attenuates external noise by 10-20 dB, making window opening feasible near busy roads or flight paths - **Rain protection:** The outer skin prevents rain ingress when inner windows are open - **Controlled pressure:** The cavity moderates wind pressure fluctuations, allowing stable natural airflow See [[Natural Ventilation Principles]] for flow calculation methods applicable to DSF configurations. --- ## Acoustic Performance ### External Noise Attenuation | Facade Type | Approximate Sound Reduction Index (Rw) | |------------|---------------------------------------| | Single skin (double glazed) | 30-35 dB | | DSF, box window (outer single + inner double) | 40-48 dB | | DSF, corridor type | 38-45 dB | | DSF, multistory type | 35-42 dB | The box window type provides the best acoustic performance due to the sealed, compartmented cavity. ### Internal Acoustic Transfer - Box type: no flanking path between rooms (best) - Corridor type: horizontal flanking through continuous cavity (requires acoustic treatment) - Multistory type: vertical and horizontal flanking (worst; requires acoustic baffles or absorbent lining) ### Cavity Acoustic Treatment - Absorbent lining on cavity floor or ceiling at each level - Acoustic baffles between adjacent rooms - Cavity-mounted blinds provide some incidental absorption --- ## Fire Safety Considerations Fire safety is the most critical technical challenge in double skin facade design. ### Risk - The cavity can act as a chimney, allowing fire and smoke to spread vertically between floors - Glass breakage in a fire allows flames to enter the cavity - Shading devices (often aluminium or fabric) may contribute to fire spread ### Mitigation Strategies | Type | Fire Risk Level | Mitigation | |------|----------------|-----------| | Box window | Low | Compartmented by design; no special measures beyond standard floor/cavity sealing | | Shaft-box | Moderate | Fire-rated shaft enclosure or fire dampers at shaft connections | | Corridor | Moderate | Floor-level fire stops sealing the cavity at each slab; spandrel panel at floor zone | | Multistory | High | Requires comprehensive fire engineering: sprinklers in cavity, fire-rated spandrel zones, breakable glass panels for firefighter access, smoke ventilation strategy | ### Regulatory Considerations - Many jurisdictions require the cavity to be treated as an external wall or ventilated facade under fire regulations - BS 9414, EN 13501, and local building codes provide guidance on fire performance of facades - Fire-engineered solutions often require peer review and approval by fire authorities - Post-Grenfell (UK) and post-Lacrosse (Australia) regulatory tightening has increased scrutiny of all ventilated facade cavities --- ## Daylighting and Visual Comfort - The outer skin slightly reduces visible light transmittance (typically 5-15% reduction depending on glass specification) - Cavity-mounted blinds provide excellent glare control while maintaining view when open - The cavity depth provides a visual sense of layering and depth to the facade - Careful specification of glass tint and coating is needed to avoid excessive colour distortion --- ## Maintenance and Access ### Requirements - Both sides of the outer skin and the outer face of the inner skin require cleaning - Shading devices in the cavity require inspection and replacement - Ventilation openings require periodic clearing and adjustment - Condensation management (drainage, condensation gutters) ### Access Strategies | Cavity Width | Access Method | |-------------|-------------| | < 400 mm | External BMU (building maintenance unit) for outer skin; no cavity access | | 400-700 mm | Limited cavity access for blind maintenance; outer skin accessed externally | | 700-1500 mm | Walk-in cavity for maintenance of all components | | > 1500 mm | Full maintenance walkway; can double as circulation or amenity space | ### Design for Maintainability - Specify cavity width early in design -- changing it later affects structural, facade, and floor area calculations - Provide openable or removable outer skin panels for component replacement - Design drainage at cavity base for condensation and cleaning water - Coordinate BMU rail locations with both inner and outer skin attachment points --- ## Cost-Benefit Analysis ### Capital Cost Double skin facades typically cost **40-80% more** than equivalent high-performance single-skin curtain walls. The premium includes: - Additional glazing layer and framing - Shading devices within the cavity - Cavity ventilation openings and controls - Additional structural support for the outer skin - More complex sealing and weatherproofing ### Operational Savings - Heating energy savings: 10-30% - Cooling energy savings: 20-40% - Lighting energy savings from improved glare control: 5-15% - Extended blind life (protected from weather) - Potential reduction or elimination of perimeter heating ### Payback Typical simple payback periods range from **15-30 years** based on energy savings alone. The economic case is strengthened when accounting for: - Improved occupant comfort and productivity - Natural ventilation capability (avoiding or reducing mechanical ventilation cost) - Enhanced acoustic performance (avoiding secondary acoustic measures) - Architectural value and marketability ### When DSF Is Justified - Tall buildings on noisy, exposed urban sites where natural ventilation is otherwise impossible - Extreme climates where the thermal buffer significantly reduces energy demand - Prestige buildings where facade quality and occupant amenity command premium rents - Deep renovation of historic buildings where the outer skin can be an independent layer --- ## Notable Examples | Building | Location | Architect | DSF Type | |----------|----------|-----------|----------| | RWE Tower | Essen, Germany | Ingenhoven Architekten | Corridor/shaft-box | | GSW Headquarters | Berlin, Germany | Sauerbruch Hutton | Corridor, west facade | | One Angel Court | London, UK | Fletcher Priest | Box window | | Commerzbank Tower | Frankfurt, Germany | Foster + Partners | Multistory (atrium) | | Debis Tower, Potsdamer Platz | Berlin, Germany | Renzo Piano | Corridor | | Manitoba Hydro Place | Winnipeg, Canada | KPMB Architects | Multistory (solar chimney) | --- ## Design Decision Framework ### Step 1: Establish Need - Is natural ventilation required on a tall or noisy building? - Is the thermal buffer benefit significant for the climate? - Does the brief support the additional cost and complexity? ### Step 2: Select Type - Assess fire safety requirements -- this often determines the type - Assess acoustic requirements between spaces - Determine maintenance access strategy and minimum cavity width ### Step 3: Configure Airflow - Model cavity temperatures for summer and winter using dynamic simulation - Determine whether natural or mechanical cavity ventilation is required - Size ventilation openings using methods from [[Natural Ventilation Principles]] ### Step 4: Coordinate Disciplines - Structural: outer skin support, cavity loading - Fire engineering: compartmentation, sprinklers, smoke control - Facade engineering: thermal movement, weatherproofing, drainage - Building services: integration with HVAC, BMS control of cavity openings - Maintenance: BMU, cavity access, cleaning regime --- ## Key References and Standards - Oesterle, E. et al. (2001). *Double-Skin Facades: Integrated Planning* - Compagno, A. (2002). *Intelligent Glass Facades* - Poirazis, H. (2004). *Double Skin Facades for Office Buildings* (Lund University report) - CIBSE TM59 -- Overheating in homes (relevant to cavity temperature modelling) - BS 9414:2019 -- Fire performance of external cladding systems - EN 13830 -- Curtain walling product standard - CWCT Technical Notes -- Centre for Window and Cladding Technology --- #environment #facade