Tuesday22 August 2017

B35 apartments by AGPS Architecture

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Hansjürg Leibundgut’s Zurich home is also a ground-breaking research project into energy use

B35 zero-emission apartments
AGPS Architecture
Zurich, Switzerland
July 2011

On a quiet street above Zurich’s university district, a pair of multi-family apartment buildings sit side by side on a site that had previously housed a 6,500cu m municipal water reservoir, hidden in the profile of the slope.

Despite its mute expression, the slimmer of the two, “B35”, is a radical piece of architecture. Its innovation, however, has been masterfully integrated into the fabric of the building rather than outwardly worn. The building,
completed in the summer, is the home and built manifesto of Hansjürg Leibundgut, professor for Gebäudetechnik (technical installations) at the ETH Zurich.

The focus of his research and practice is the overall energetic performance of buildings. B35 is one of an initial series of projects that articulate a new approach to energy use in buildings — one made possible by a range of newly effective technologies and a re-examination of fundamental concepts. Its two main goals are to create buildings that can be run with effectively zero emissions (carbon dioxide, radiation, noise) and “low exergy” (externally sourced energy). Added to these is the further requirement of the lowest possible increase in upfront investment. For the purpose of this analysis, CO2-free is defined as less than 1kg CO2 per sq m per year, in a country where the average is 46kg.

Typical apartment floorplan

In Switzerland, as elsewhere, progressive building standards have been generally directed at minimising net energy use and losses, almost at any cost. This means new buildings are ideally passive, while older buildings are being sealed and insulated where possible. This approach, Leibundgut argues, does not sufficiently differentiate between the nature of different energy sources, or provide a realistic framework in which our existing building stock might make the necessary transition to being able to be run CO2-free. The main opportunity, he argues, is that even in Europe the solar energy we receive represents an almost limitless source of renewable energy — but this is dependent on our being able to harness it. One significant step towards realising this potential is to be able to capture and store solar thermal energy efficiently, to replace the role combustible fuels now play in heating our buildings.

Ground source systems become significantly more efficient the larger they become

A number of strategies have been developed for this over the past few decades. The technology upon which they rest, the ground-source heat pump, is well known, but less popular in Britain than in other countries with a similar climate, partly because of the comparatively poor performance of regular domestic installations, and our convenient reliance on natural gas. The intensive collaboration with industry that underpinned the devices employed in project B35 (many of which are prototypes) demonstrates the exciting potential of this technology. In future, by thermally coupling buildings with the earth, we should be able to heat buildings solely with its ambient warmth and the excess solar energy delivered during the warmest months of the year.

Loggias in the south corner can be fully opened or closed to suit the weather.

Source: Photo: Rainhard Zimmermann

Loggias in the south corner can be fully opened or closed to suit the weather.

If this sounds like too much faith in technology, then it should be noted that the projects largely prove the general concept and its economic feasibility. Two are already completed — the other is a renovation of a large university building — and two more are under way. Such ground-source systems become significantly more efficient the larger they become, even able to serve the needs of an entire campus or neighbourhood. The heat-pumps installed in the house, which achieve an average COP performance ratio of 8, are due to be replaced with even more efficient models within two years.

The top floor apartment is vessel-like, looking out over the city.

Source: Photo: Rainhard Zimmermann

The top floor apartment is vessel-like, looking out over the city.

The building

South-east elevation

The building has five floors above ground, with a total usable floor area of 830sq m. The ground floor is dedicated to office space, where a section of the reservoir has been retained to create a cavernous hall onto which the new volume has been grafted.

The apartments on the floors above are typically refined, characterised by generous living spaces and a deftly arranged sequence of rooms set in the centre of the plan and divided by sliding partitions.

The Leibundguts themselves live in the loft apartment. Its character is defined by the complex form of the roof — a pair of canted volumes that fit neatly within the planners’ height restriction and reflect the main role of the roof as the centre of energy collection.

Sustainable technologies and strategies used in the B35 apartments

Bim modelling

Bim was used from the outset to serve as the basis for the analysis and simulation required to optimise the design and the technical performance of the building.

Thermal coupling

Seventy-three separate loops of underfloor heating and other piping arrays create thermally active building elements, centrally controlled to be able to deliver and collect heat where and when required.

Advanced ground source heat pump system

Three heat pumps draw on two deep-bore vertical loops to allow the system to store and deliver thermal energy throughout the year.

PV hybrid solar collectors

Photovoltaics are used because they capture a higher proportion of solar energy than any other means.

Panels covering 28sq m should generate 9,200kWh of electricity each year (one-third of the total requirement), and 24,000kWh of thermal energy. A second array is due to be installed in 2012 to power three electric vehicles for the house.

Deep thermal reservoirs

Technical installations being arranged before a floor is cast.

Technical installations being arranged before a floor is cast.

The main loop is 380m deep. The reason for going so deep is to create a reservoir of sufficient scale (38,000kWh) and of the highest possible ambient temperature. Between 200m and 400m the ambient temperature ranges between 15°C and 18°C, an ideal basis. The higher the temperature of the reservoir, the more efficiently the heat pumps can operate. The output temperature of the water from the heat pumps is capped at 42°C — which also improves the performance of the pumps.

COP = the ratio between the energy delivered and the external energy (Exergy) required. At the kitchen sink the water is heated as required to 62°C, to clean dishes properly.

Off-site renewables

Some 24,000kWh/a of electricity is produced in part of a solar farm near Seville, Spain. This is used to offset the energy from the grid, solely for use by appliances and lighting. The investment was included from the outset as part of the building costs. That the power is generated elsewhere is not a discrepancy, but an argument that investment in renewable energy should be made where it can operate most effectively, as losses due to transport are negligible. It also recognises that architecture can’t solve everything, but must rely on coordinated action elsewhere.

“Airbox” passive ventilation units with integrated heat exchangers allow fresh air to enter the building. They operate at a low pressure differential, minimise heat losses in winter, and can capture excess warmth in summer and channel it to the reservoir system.

Cooling in summer

Cool water from the shallow reservoir (150m deep) is circulated when needed, and the heat gained is transferred to the deep reservoir. Automatically controlled external blinds are used to minimise unwanted heat gains, along with advanced glazing technology in the larger-format windows with so-called M-glass (g-value of 0.19), and triple-glazed windows as standard elsewhere.

Thermal resistance

An innovative outer wall construction was devised, made of two layers of insulating concrete with an embedded layer of rigid foam insulation.

The walls were built up in a single-cast process, and the outer layer was given a dark grey hydrophobic wash.

The build-up provides sufficient thermal resistance to maintain comfortable interior conditions with acceptable energy losses given the amount available to the system.

The insulation values are thus less than the most demanding standards, with a R-value of only 0.16m2K/W.

Digital current

The entire electrical circuitry in the house is also used to transmit information, allowing the monitoring and control of devices. This means the energy use of appliances can be optimised centrally, and
unused devices can automatically be disconnected, rather than left on standby.

Best-of-class energy-efficient appliances

All electrical devices and appliances, as well as light fixtures have been selected on the basis of their efficiency rating.

Heat source


Project Team
AGPS Architecture, Client Hansjürg Leibundgut, Building services engineering Amstein & Walthert and Mettler & Partner, Support structure planning Büro Thomas Boyle



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