Pohjois-Pasilan Proosa

This project emerged from an open architectural competition held in 2018. The competition, “Housing Reform Helsinki – Housing 2020,” focused on developing new housing solutions with wooden structures for a specific site in North Pasila’s new Postipuisto residential area in Helsinki.

The local plan for the area was already confirmed before the competition, and the winning design significantly differed from it. Since the local plan couldn’t be changed, considerable effort was required to adapt the complex’s design, which used wooden CLT spatial elements, to the strict constraints of both the plot and the existing plan.

The narrow block is designed with building masses that ascend around central stairwells. The first floor encircles the block, effectively enclosing the street space and creating a private courtyard within. Between these building masses are “light courtyards” – green roofs framed by climbing walls on the street-facing side. This clear architectural concept proved to be a robust and flexible foundation for developing detailed implementation designs.

Each building’s stairwells feature common areas on their street-facing facades. Designers envision these small spaces as a daily extension of residents’ living areas, hoping they’ll offer valuable flexibility as apartment sizes become more compact. Interestingly, the fixed furniture in these areas is crafted from CLT waste pieces salvaged from window and door cutouts.

From the designers’ perspective, this project was an inspiring balancing act among the chosen construction method, the competition-phase solutions, and the site’s environmental constraints. The working group’s consistent goal was to create an architecturally coherent and economically viable complex of bright, multi-directional apartments, ultimately aiming to encourage more wooden apartment building construction.

The project is part of the City of Helsinki’s ongoing apartment building development program.

Technical solutions

The building’s load-bearing frame primarily utilizes 80 mm thick cross-laminated timber (CLT) panels. In specific spatial elements, particularly where large openings are desired (such as in multi-element apartments), laminated timber has been incorporated for columns and beams. The external walls are thermally insulated CLT, and the facades are site-clad with wood. While a horizontally boarded design could have allowed for factory pre-fabrication, this approach wasn’t taken. Concrete was used for parts of the first floor, including some of the single-story light courtyards, though the overarching goal was to maximize the use of wooden structures throughout the building.

The first-floor apartments feature wooden subfloors with ventilated crawl spaces, necessitating a floor height of at least 700 mm above ground level. To ensure accessibility, the courtyard is elevated, with entry into the building facilitated by separate walkways that span a shallow, lower intermediate space.

Both the upper and intermediate floors are made of wood. The ventilated upper floor is insulated with blown wool. To ensure weather protection during construction, sloped roofs with a membrane were used. This allowed the roof to be raised at the end of each day, consistently shielding the installed floor elements.

The block is situated in an acoustically challenging location, specifically north of the Ilmala depot area.

Strict airborne sound insulation requirements were a key consideration in both the structural design and the selection of window and door types.

Construction project

Exceptionally, the competition organizer did not have to assign a contractor for the building. After the competition, the first task of the architectural design team assembled by Lassi Mustonen was to find developers and a builder for the site. The project was decided to be promoted with ARA operators and a KVR contractor.

The contractor decided to build the project using prefabricated CLT space elements. These elements formed apartments of one, two, or three units, assembled on-site. A key objective was to deliver apartments as factory-finished as possible, including fixed furniture. Bathrooms, also pre-manufactured, were seamlessly joined with the wooden spatial elements at the factory.

The block’s three large, metal-structured green walls were fabricated in a metal workshop and assembled on-site only after the spatial elements were installed.

Prefabricated element construction required a forward-oriented design, as changes to the elements could not be made in practice during the construction phase. The first part of the project was received in late 2023 and the entire project was completed in December 2024.