This multifamily house with its innovative wood–straw construction would not have been possible without the professional involvement of the builder and architect over the past ten years. Builder Ulrich Steinmeyer is an active board member of Ökoplus, the trade association for suppliers of natural building materials. Architect Dirk Scharmer founded the Professional Association for Straw Bale Construction (FASBA) in Germany. Together with other experts, they built the flagship project: the five-story North German Center for Sustainable Building. The center was featured in the magazine “WOHNUNG+GESUNDHEIT” (156), published by the Institute for Building Biology + Sustainability IBN.

Innovative straw construction

Based on this experience, the builders chose a compact structure with a gabled roof to maximize energy and cost efficiency. The long side of the building faces south-southeast to optimize the use of solar energy by the photovoltaic system and the rooms. The sustainability concept focused on a well-insulated building enclosure, avoiding thermal bridges, and orienting the windows for optimal “passive” solar use. More than half of the apartment windows face south, while the small bathroom windows face north. Thus, in winter, the windows use the incoming sunlight to heat the building. The large roof overhang and balcony provide good shade in summer.

The building enclosure consists of a load-bearing wood frame with straw bales filling the spaces between the studs. Most of the wooden studs for the frame are rough-sawn, and the straw bales are BauStroh-certified, meaning transport costs were minimal. This construction method provides excellent thermal insulation for the exterior walls, which are plastered with lime on the outside and clay on the inside.

This results in very low energy consumption during the use phase. In addition, using natural building materials, which require minimal processing, significantly reduces primary energy consumption during construction compared to conventional buildings.

1 South view of the straw bale house with photovoltaic panels and a large roof overhang for shade
2 The embedded window frames are plastered by hand with lime.
3 On the inside, the wall assembly includes a reed mat, clay plaster, and wall heating pipes.
4 Delivery of the straw bales

Energy concept with local heating

The overall energy concept aims to prevent energy loss during construction and use, as well as generate the remaining energy needed for the permanent use of the building or supply it via renewable energy sources. The concept is based on a small local heating network connected to two other wood houses in the neighborhood. The three houses, which have a total of ten apartments, are connected for their entire energy supply, including electricity and heat. The combination of a pellet heating system, a solar thermal system with a large buffer storage tank, and a large photovoltaic system is sufficient to cover the remaining energy requirements.

The local heating network operates at 45 degrees Celsius (113 degrees Fahrenheit) with heat exchangers in each apartment. This reduces heat loss in the ground and supply lines, allowing the solar thermal system to be used for longer. In addition, a ventilation system with heat recovery serves as a supplement in winter. It is equipped with a ground register to preheat the air in winter and cool it in summer.

The photovoltaic system generates nearly all of the electricity required by the households. Around 50% of this electricity is consumed directly. Any excess electricity is fed into the public grid and can be drawn from there at other times.

Comparative life cycle assessments

To illustrate the ecological differences between various building methods and use concepts, architect Scharmer compared the Verden wood–straw house to a typical regional clinker brick house of the same size and shape. He made two comparisons: one with the same building services systems and one with conventional building services systems. Scharmer calculated the manufacturing and use phases over the first 50 years, taking into account construction and maintenance costs, as well as the residents’ heating and domestic hot water requirements. He also considered the building’s electricity consumption and production.

Scharmer’s calculations of the global warming potential showed that the straw house would emit 15 tons of carbon dioxide (CO₂) over 50 years due to manufacturing, construction, maintenance, and heat consumption. However, for around 40 years, the building will not cause any carbon dioxide emissions, including heat consumption and maintenance.

Compared to a conventional building that complies with the same energy standards and has the same building services systems, this results in a difference of around 220 tons of carbon dioxide after 50 years of use. This difference is mainly due to the different building materials used. Scharmer has calculated that “this amount would be enough to drive a mid-range car around the world 45 times.” In addition to its positive environmental performance, the straw house has a significantly lower energy consumption than conventional buildings. The energy saved by using natural materials instead of conventional ones during construction is enough to heat the building for around 100 years. In other words, the same amount of non-renewable primary energy used for a conventional house could build two straw houses of the same size.

Construction data for a multifamily house in Verden

* CO2 emissions of a mid-range car: 120 g of CO2 per km