WCES - Taisei Corporation Kansai Branch Buliding

Energy performance

52.2 %

Energy savings

74.0 kWh/m²/year

Absolute performance

26.4 kgCO₂/m²/year

Carbon intensity

Operational Context

The Taisei Corporation Kansai Branch Building is an urban office building in Osaka, Japan, consisting of Building 1, completed in 1992, and Building 2, completed in 1998. It has two basement floors, nine above-ground floors and one penthouse floor, with a total floor area of 13,651.55 m². Renovation work began in April 2022 and was completed in March 2023. The building remained operational throughout construction, implementing a renovation approach that did not require the wholesale relocation of its occupants.

Significance of Green Renewal ZEB: Improving the existing building stock, which far exceeds annual new construction, is essential for Japan to achieve carbon neutrality by 2050. Taisei Corporation’s Green Renewal initiative seeks to improve the well-being of people, buildings and the planet through six concepts: energy saving, energy generation, decarbonization, wellness, smart solutions, and safety and security. Green Renewal ZEB refers specifically to initiatives emphasizing energy saving, energy generation and decarbonization. This project was designed as a replicable model for ZEB renovation of medium-sized urban office buildings.

Energy performance under Japan’s ZEB evaluation system: The project combines improved building-envelope insulation, solar shading, high-efficiency building services and photovoltaic generation. Its energy performance was evaluated using the Building Energy Index (BEI) employed in Japan’s building energy efficiency framework. BEI is the ratio of the assessed building’s annual primary energy consumption to that of a standard reference building calculated under the Japanese methodology (BEI = assessed building / reference building primary energy consumption); a lower value indicates better energy performance. It does not directly represent the measured energy saving between the pre- and post-renovation periods. The assessed pre-renovation design BEI was 0.93, and the post-renovation design BEI was 0.37, equivalent to approximately 63% lower primary energy consumption than the Japanese reference building. During post-renovation operation, the primary-energy performance indicator, accounting for photovoltaic generation, reached 0.25, corresponding to approximately 75% less primary energy than the Japanese reference building and performance equivalent to Nearly ZEB.

Key measures implemented

The main actions implemented to reduce the energy consumption of this building.

Efficiency Investments

Comprehensive architectural and building-services renovation: The renovation combined enhancements to the passive performance of the building envelope with improvements in the efficiency of active building systems. Measures included supplementary insulation on the existing reinforced-concrete and autoclaved lightweight concrete (ALC) external walls, argon-filled low-emissivity double glazing, and inner sashes where appropriate. Building 1 uses a central heat-source system, while Building 2 adopts high-efficiency packaged/VRV air-conditioning, providing alternative approaches that can be examined for ZEB renovation of existing buildings. LED lighting and building-integrated photovoltaics (BIPV) were also installed.
Multifunctional facade unit: A multifunctional facade unit was added to the existing south-facing exterior wall, integrating four functions: solar shading, BIPV electricity generation, daylight admission through daylighting blinds, and greenery. By addressing weaknesses in the existing envelope while considering daylight, views and planting for occupants, the add-on system was designed to be transferable to other renovation projects.
Exhaust-utilizing window system: Based on the principle of dynamic insulation, the exhaust-utilizing window system adds an inner sash to the existing window and passes surplus conditioned indoor exhaust air through the cavity before discharging it outdoors. This limits both heat gains from outside and heat losses from the room, while facilitating installation from inside an occupied building. In winter mock-up tests, its internal surface temperature remained around 20°C, compared with approximately 15°C for the existing window. An operational U-value of approximately 0.33 W/(m²·K) was confirmed. In a separate summer comparison with and without airflow, heat ingress excluding transmitted solar radiation was reduced by approximately 53% (14.3 W/m²). These figures represent window-system tests, not the building-wide energy-saving rate.
Light-diffusing radiant air-conditioning duct: A slim radiant air-conditioning duct approximately 100 mm deep was developed by combining lightweight corrugated air ducts with aluminium radiant panels, light-diffusing ribs and phase-change material. The system is designed for installation below existing ceilings. In mock-up comparisons with conventional air conditioning, the vertical temperature difference was reduced from 11.1°C to 6.4°C in winter and from 1.8°C to 0.8°C in summer, with PMV values closer to thermal neutrality. Desk-level illuminance was approximately 2–3% greater than with a conventional embossed panel.
Medium-temperature chilled water and high-efficiency equipment: In Building 1, spray-assisted air-source heat-pump modular chillers supply medium-temperature chilled water (11°C supply / 19°C return). Under the reported conditions, heat-source efficiency was approximately 1.4 times that of conventional 7°C chilled-water supply. Together with high-efficiency air conditioning in Building 2, photovoltaic generation, battery storage, hot-water system upgrades and LED lighting, these measures improved overall system performance.

Technical Optimization

AI and cloud-based BEMS optimisation: The cloud-based Building Energy Management System (BEMS) uses equipment and energy metering data, indoor and outdoor sensor readings, and weather forecasts to predict thermal loads. It optimises the number of operating chillers and supply-water temperatures to improve the overall system coefficient of performance (COP). Nighttime heat storage and early-morning discharge using a sealed buffer tank also help level the high heating start-up loads after winter holidays.
Personalised thermal comfort control: Occupant location information from human-presence sensors and beacon tags is combined with individual thermal preferences to operate variable-air-volume (VAV) terminals and air-swing fans through predicted mean vote (PMV) control. The system targets PMV values between −0.3 and +0.3 to support comfort, responding to actual occupancy and preferences rather than applying uniform indoor conditions.
Monitoring, maintenance and measured energy performance: Energy consumption is metered by floor and by equipment, consolidated in the cloud-based BEMS, displayed through digital signage and accessible for remote monitoring. A specialist team analyses the data and advises on operational improvements. During the reported monitoring period, actual monthly primary energy consumption was below the design estimate in every month. In April, an intermediate-season month, consumption was 52% below the corresponding monthly design estimate. This is not a comparison against the pre-renovation monitoring period submitted to WCES.

Use Sufficiency

Renovation during occupancy and continued use of existing building stock: The project renewed the building envelope, windows and equipment while the office remained in use. Reusing the structural frame reduced the environmental burdens associated with demolition, replacement construction and complete relocation. The inner-sash window method and slim radiant air-conditioning duct were also specifically developed with installation in existing buildings in mind.
Workplace improvements and occupant feedback: Biophilic interior design, planting, semi-outdoor areas and activity-based working (ABW) settings provide occupants with a choice of places to work. These were combined with PMV-based control reflecting personal comfort. Post-renovation occupant surveys showed improved satisfaction with thermal conditions, air quality and spatial environment, and reduced complaints about winter downdrafts near windows. A correlation was also observed between higher overall office ratings and self-reported reductions in time required to complete work.

Technical context

Floor area 13,651 m²

Number of floors 9

Activity type Office

Year built 1992-2026

Certifications

EDGE Zero Carbon ()

Taisei Corporation Kansai Branch Buliding