Shattering the energy barrier

Germany's Passiv Haus Institut (PHI) has become a leading centre in developing a specification for the next generation of energy efficient buildings – the Passive House Standard. It has tested and established a variety of ways in which such buildings can be constructed to meet this underlying aim. In 400 residential buildings the standard has reduced total energy consumption to 12% of the UK norm. The approach has now been applied to a 2 200 m2 office and factory in Colbe, Marburg.

Wagner, manufacturers of solar energy equipment and rainwater collection systems, wanted a building which contained offices, meeting rooms, catering and factory floorspace which demonstrated the firms' environmental ideals.

In 2000, a typical budget for German offices is £800/m2, excluding fees, land, and 16% vat. Wagner's budget was no different.

The three-storey structure was completed in autumn 1998. It is a straightforward combination of concrete-frame external walls with calcium silicate block infill, in situ concrete intermediate floors and a timber I-beam roof. The most heavily-glazed wall is steel-frame.

The timber-clad external wall insulation panels are bolted to the outside of the frame. To reduce air leakage, a sealed polyethylene air-vapour barrier is fitted between the wall cladding panels and the frame.

The insulation levels are 400 mm mineral fibre in the roof, 300 mm in the walls and 240 mm cellular glass under the floor. The windows have an overall U-value of 0·75 W/m2K. Insulated doors greatly reduce heat loss.

One of the most lasting impression was the impeccably-fitted and durable finishes – polished granite stairs, tile skirtings and ceramic tile floors with perfectly-aligned joints. This gives it a distinct edge over UK standard office buildings. Linoleum is used for the 'soft' floors while all the walls have solvent-free paints and a specially reinforced plaster with acoustic insulation properties.

The office only reached the space heating energy budget of 15 kWh/m2y by vigorous intervention from the PHI which provided expertise in reaching very low energy use.

There were two serious errors: the wrong windows were delivered, requiring the hollow plastic profiles to be injected with foam on site, and the timber roof was designed in detail before the architects had fully appreciated the implications for air leakage. It needed extensive sealing for the building to meet the PHI standard of 0·6 ac/h at 50 Pa. On the Wagner building this is equivalent to 0·8 m3/m2h at 50 Pa.

Heating and ventilation
The mechanical ventilation/heat recovery (mvhr) system uses four cross-flow exchangers in series, giving 80% overall efficiency. Such exchangers were used in preference to a heat wheel. Utilising a system with no risk of leakage between the inlet and exhaust airstreams allowed all the toilets to be connected to the central mvhr system; there are no other fans except in the kitchen, which has a separate exhaust. The mvhr system beats the PHI electricity target which is 0·45 Wh/m3 (1·6 W/l/s).

The building has four parallel earth-preheating tubes. Each is a 32 m length of concrete pipe with an internal diameter of 500 mm. Half their total length is outside, while half lies below the ground floor slab. In this climate, tubes 1·5 m deep deliver air to the heat exchanger at between 7°C in winter and 18°C in summer.

The building needs a small input of heat in winter, which is simply added to the normal flow of ventilation air. The gas is burned not in a condensing boiler but in a small chp system. This is the size of a lawnmower engine, with outputs of 5 kW(e) and 12 kW(t).

In other words, the heating system installed capacity is 5 W/m2 floor area which is 75% below the 21 W/m2 of the Elizabeth Fry Building, which itself broke UK records.

Some space and water heating comes from an active solar system. The central stairwell contains a vertical heat store of 80 m3, or 1% of the building's volume. This is fed from 70 m2 of collectors.

In hot summer weather, the small clerestory windows are opened at nights to cool the building mass and are almost closed-up during hot days. This strategy has consistently kept the building interior air below 27°C, even when the outside temperature reached 33°C.

Lighting and electrical
The lighting is mostly ceiling-mounted compact fluorescents. At the time of the visit, in cloudy mid-March, the perimeter luminaires in the entrance hall were dimmed to compensate for available daylight. The lighting was off in upstairs rooms which had ample daylighting from the clerestory windows.

Local discomfort in the building's telephone exchange led Wagner to retrofit a 300 W(e) cooling plant for the space. Other equipment was chosen from PHI's regular lists of more energy-efficient equipment on the German market.

Measured energy consumption
The sum of gas and electricity consumption is 38 kWh/m2/y. In other words, 55% lower than the Elizabeth Fry Building. Gas consumption in the first year was 13 kWh/m2/y – 30% more than predicted, but still exceptionally low.

For at least part of the monitoring period the structure would have been drying out. It will be surprising if gas consumption does not fall with further fine-tuning. Consequently, the sum of gas and electricity may reach 35 kWh/m2/y.

The telecommunications system and its associated cooling system consumes roughly 3·3 kW(e), but PHI experts feel that such devices could be re-designed to use 100-200 W. If so, Wagner's electricity use might fall by 50% to 13 kWh/m2/y if the telephone exchange is replaced.

Even the present figure is striking enough to make this a landmark in the history of energy-efficient European buildings. Figure 1 sets out some salient UK figures for typical and good practice. A like-for-like comparison is not straightforward – Wagner and the Elizabeth Fry Building provide filtered air for at least the bulk of the year, offering some of the advantages over the Type 1 fully air-conditioned building. (Type 4 reflects the energy performance of a prestige air-conditioned building).

Covering the costs
Over and above its normal expenditure, Wagner received external public sector support of £30 000 towards marginal design and construction costs and £250 000 towards thorough monitoring of the building's energy use and the performance of individual items of equipment, for example the earth tubes.

The £30 000 amounts to an input of 1·7%, but the extra construction cost was nil. The money went on optimising the design, enabling the PHI and the team members to produce a building which cost no more and has outstanding energy performance.

Many energy-efficient UK buildings are still being designed to target levels of 100 kWh/m2y, the sum of gas plus electricity. Of the buildings examined by PROBE, only the Elizabeth Fry Building and Woodhouse Health Centre clearly perform at this level or better. So Wagner not only breaks this energy barrier, it shatters it.

The historic background of support for energy-efficient initiatives in Germany and the external support available to landmark or pioneering projects probably played a role in Wagner's success. It is important to receive corresponding support and commitment here if we are to keep up with the constant move forward of international best practice.