Lumber 4

Lumber Building 4
Kristiansand, Norway | Completed 2023
Lumber Building 4 is a visionary commercial and office building designed by Oslotre, that redefines sustainable construction in mass timber. Designed and built in just 12 months, this six-story structure demonstrates the potential of engineered wood to deliver performance, efficiency, and architectural warmth at scale.
The building's recessed ground floor houses retail spaces, while five upper floors are dedicated to flexible office environments. The structure is composed of Glulam columns and beams, and floors are formed using a hybrid Timber-Concrete Composite (TCC) system, combining 2/3 timber with 1/3 concrete. This approach harnesses the tensile properties of timber with the compressive strength of concrete, creating slender, efficient floor plates capable of long spans and improved acoustic and fire performance.
Prefabrication plays a central role in the project's environmental strategy. Engineered timber elements were factory-produced to precise BIM-based specifications, minimizing cut-offs and material waste while maximizing transport efficiency — achieving an 80% reduction in transportation needs compared to conventional concrete elements. On-site waste was minimized through the use of pre-insulated timber wall systems, and the entire build process was streamlined to limit emissions and disruptions.
Exposed timber interiors bring biophilic and thermal benefits to occupants. Wood’s natural ability to regulate humidity enhances indoor air quality and contributes to occupant well-being. Large floor-to-ceiling windows ensure optimal daylight access, while a central timber staircase in the building's atrium serves as both a vertical connector and architectural focal point.
The facade features prefabricated, curved pine elements treated with green pigment. As time passes, the shadows cast by linear eaves will weather the wood unevenly, giving rise to a subtle, ever-changing surface. Fire-treated pine cladding and white-pigmented spruce panels wrap the exterior, while wood fiber insulation boosts thermal performance throughout the envelope.
Lumber 4 was fully leased upon completion, affirming the commercial viability of sustainable timber construction. With its strong environmental metrics, material intelligence, and human-centered spaces, the building exemplifies what the next generation of urban architecture can and should be.
________________________________________ Technical Environmental Performance
- Climate Impact: Achieves a 53% reduction in greenhouse gas emissions compared to a conventional reference building (BREEAM standard).
- Energy Efficiency: Annual energy consumption is 92.4 kWh/sqm (GFA), with Energy Rating A for office areas. Ventilation is supported by a system with water-based heating coils.
- Envelope Performance:
- U-value for external walls: 0.21 W/m²K
- U-value for floors: 0.11 W/m²K
- U-value for windows/doors: 0.8 W/m²K
- Air leakage rate: 0.5
- Renewable Energy: Power is supplied by district heating and rooftop solar panels.
- Carbon Sequestration: Engineered timber (CLT and Glulam) sequesters carbon throughout its lifecycle, offsetting emissions associated with construction.
- Material Strategy:
- Structural frame: Glulam and CLT
- Floors: Timber-Concrete Composite (TCC)
- Insulation: Wood fiber
- Cladding: Fire-treated spruce and pine
- Interior finishes: Exposed timber and wood wool acoustic panels
- Waste Minimization: Factory-based prefabrication reduced site waste and transportation emissions; on-site materials were used with maximum efficiency.
- Digital Planning: Full BIM integration ensured construction precision, improved logistics, and reduced material excess.