When this log row house project was planned for the Honkasuo district in Helsinki, the aim was to be carbon negative. This meant the carbon handprint needed be larger than the carbon footprint. The natural way of building the house has aroused interest among professionals in the construction sector, consumers and in the media.

Text: Minna Aarnio and Puuinfo
Photographs: Anu Turunen

Read the article in Finnish: Hiilinegatiiviset Aarreaitat kunnioittavat perinteitä

The architecture of Honkasuo’s Aarreaitta homes is a simple but impressive example of “wooden brutalism”, as the contractor, architect Minna Aarnio calls it.

− With these homes, we have aimed at demonstrating that carbon-negative construction is indeed possible and can reduce the damaging carbon spike caused by new construction. The strongest influence on the site’s architecture and the entire Aarreaitta concept were the hand-carved solid logs. They store carbon in the most effective way possible and exemplify Finnish construction traditions at their best. The site’s curved layout and the choice of gravity ventilation also played their part, says Aarnio.

The row houses have seven structurally separate log frames built from 225 mm thick logs. Two of the homes have a pine wood frame and five are spruce. The air space between the frames is 50 mm wide, making the walls between the apartments 500 mm thick. The windows and exterior doors are made of wood, with the windows quadruple-paned to reach the desired U-factor of 0.8. As required by the municipal zoning plan, each unit’s facade is a different colour. The surfaces have been treated with ochre paints in various shades.

Each home has its own auxiliary building with a 150 mm thick solid log frame. These are supplemented by a shared recycling room and a shared sauna with a sheltered terrace for cooling off. All the auxiliary buildings are the traditional Falu red with matt black tin roofs with raised seams.

There is a sheltered courtyard between the row houses and auxiliary buildings, with a narrow path to the sauna crossing it in an east-west direction. Lying between the path and the row houses are gardens designed to further ecological diversity and to sequester carbon.

Solar power and renewable smart district heating

The site’s clay soil had already been stabilised with lime in earlier construction, so the foundation only needed steel piles for support. Pile anchors were cast on these steel piles, then base pillars on the anchors, and finally log frame racks were erected on the base pillars. The subfloor is ventilated, which means there is free space under the buildings as required by regulations.

The hand-carved, interlocking solid log structure is an all-in-one solution. Namely, the logs do it all, simultaneously providing the load-bearing frames, their stiffening, the facades and the interiors. The apartment frames are 225 mm thick, while the auxiliary buildings have 150 mm thick frames.

All the buildings have a P3 fire class, with a fire compartmentalisation of EI30 between the apartments. The latter was achieved by leaving sufficient distance between the double log frames and the auxiliary buildings, which means no separate partitioning structures were needed except in the attic. The attic has firebreaks to separate each apartment.

Every home will have 16 solar panels on its roof, which will generate more electricity than the household needs. And, the units come with air source heat pumps that can provide cooling and additional heating in the summer with the household’s solar power. Renewable smart district heating is used as the general heating system. The plot’s nine parking spaces will have power connections for charging electrical cars.

Towards carbon negativity

The design of the Aarreaitta concept began in late 2018 after seeing the latest report of the Intergovernmental Panel on Climate Change.

− The IPCC report made it clear that construction was not sustainable in its current form, which was interpreted to mean that Finnish wood needed to be redirected to longer-lasting uses. The project aimed for carbon negativity from the start, meaning a larger carbon handprint than footprint. This goal was more than achieved, since the project’s carbon handprint is 3.4 times the carbon footprint, says Aarnio.

The carbon footprint of the site is a third smaller than the concrete option or the wood and brick (726,000 kg CO_2e per 150 years). The site’s carbon handprint is almost six times that of the concrete version and nearly four times that of the wood and brick (-966,000 kg CO_2e per 150 years).

The Aarreaitta project sought to use immediately available materials and techniques and to keep solutions simple, with a narrow range of materials. The backbone of the concept combines Finnish wood with local wood-working techniques, low-tech building services such as gravity ventilation, and renewable energy production by the households themselves.

− In winter 2018–2019, the log carving contractor kicked off the wood procurement process by selecting the trees needed for two Aarreaitta units and two auxiliary buildings from a forest in Ähtäri. The architects at Arkkitehtitoimisto LPV modelled the plans with mathematical precision and created great conceptual drawings, says Aarnio.

Natural building arouses interest

The project has aroused a great deal of interest from the start. The excavation work began May 1, 2021, and already next autumn the project was widely reported by Finnish local and national newspapers and in the YLE Aamu-TV morning show.

According to Aarnio, the construction site has been visited by people from the Ministry of the Environment, the town planning department of Helsinki, engineering and architecture agencies, and by international students of wood architecture and construction. Also, the general public is thrilled.

− Many people who are looking for an apartment are interested in a more natural way of building, especially in cities. We have not yet advertised the apartments at all, because, according to the law, we cannot sell them until they are ready. In spite of this, as many as 80 families have already given their contact details to our real estate agent, says Aarnio.

The homes are expected to be completed in the summer of 2022. According to Minna Aarnio, the most significant lessons learnt in the project were the great potential of Finnish logs and local wood-working techniques – and the fact that sustainable construction is indeed already possible.

HONKASUON AARREAITAT

• Intended use | Residential building (row house with 7 homes)
• Developer/Client | Rakennusasiaintoimisto Aarre Oy
• Architectural design | Rakennusasiaintoimisto Aarre Oy / Minna Aarnio, Arkkitehtitoimisto LPV / Tarmo Peltonen, Rosa Paukio
• Structural design | Rakennusasiaintoimisto Aarre Oy / Jukka Reinikainen
• Main contractor | Rakennusasiaintoimisto Aarre Oy, project management contractor
• Wood component supplier | Saaren Hirsisaunat ja huvilat Oy, Sepa Oy, Lasivuorimaa Oy, Kaskipuu Oy, Tuulipuu Oy, Tähtiporras Oy
• Year completed | 2022