Passivhaus project case study - Mayville Community CentreDecember 2011

Roderic Bunn describes the background to a Passivhaus refurbishment project in Islington, London.

The science of climate change is presenting the global built environment with some stark choices. Carry on as we are, and buildings will contribute hugely to the rise in greenhouse gas emissions. Focus on zero carbon new build, and we'll only improve 1-2 per cent of the buildings every year.

The bare fact is that carbon reduction targets of 60 per cent or more cannot be achieved without a deep retrofit programme of existing buildings. Nearly 90 per cent of those buildings will still be operational in 2050, so tinkering around the edges with new build won't be enough.

A more radical approach to refurbishment is needed, where energy use is driven down so that remaining demands can be easily covered by renewable or low carbon fuel sources. The first step is to reduce heating loads by high levels of thermal insulation and fabric airtightness. The second step is to increase the efficiencies of the heating, cooling and lighting systems, and to ventilate naturally or by mechanical ventilation with heat recovery where appropriate. The third step is to use low carbon sources of fuel.

This, in essence, is what drives the Passivhaus approach. Widely practiced in Europe, Passivhaus is now making inroads in the UK. The key question is whether what works in mainland Europe is appropriate for the wetter, milder UK, with often densely occupied spaces and longer hours of operation.

The Mayville Community Centre project in Islington, North London, is a case in point. The project involves the complete refurbishment of an existing 19th Century brick building into the first non-domestic, fully certified Passivhaus refurbishment in the UK.

Procurement

Built in the 1890s as a generating station for London's tram network, the massively constructed building was rescued from dereliction in 1973 by the Mildmay Community Partnership and turned into a community centre for the local Mayville Estate.

In 2006 bere:architects was asked to refurbish and extend the rundown building. Initial thoughts had been to heat the existing building with a biomass boiler, but Justin Bere quickly calculated that they would need "juggernauts of wood" to cope with a leaky (asbestos) roof, leaky windows, and hardly any insulation.

Rather than simply extend the building, Bere reasoned that more space could be generated through efficient internal replanning and by adoption of Passivhus principles. The community trust agreed.

Procured under the JCT SBC/Q 05, refurbishment works to the building include internal space replanning to create an extra 35 per cent usable space for both local community use and renting to small fledgling businesses.

The building's accessibility was improved and a full upgrade of the fabric and its environmental systems was planned to meet the Passivhaus standard. The construction budget was £1.6 million.

Site aspects

The existing building, orientated due north south, is a three-storey concrete-framed building (basement, ground and first) with a nominal 600 mm-thick solid brick skin, pitched roof and a later single-storey extension. Although the building is situated within community gardens, they were not easily accessible. The rebuild aimed to improve this.

An enlarged single-storey entrance block with reception and dining area was added, along with access to the south garden. Enlarged openings to the ground floor south elevation also allows access to a south garden (increasing winter solar gain). A south elevation lightwell, down to basement level, allows in natural light, ventilation, and solar gains, and provides access to the basement.

The additions have increased the usable area by about 35 per cent. The treated floor area is 665 m2. Work started on site May 2010 and was completed in July 2011.

Building fabric

Bere Architects took an incremental approach to improving the building. Primarily this involved improving the efficiency of the fabric and a new roof.

As part of the refurbishment works, the external walls were damp-proofed down to the footings. Originally the slab had a damp-proof membrane, with the bitumin carried up the outside of the building. Unfortunately a 20 m drainage trench had to be cut though that membrane. Despite being made good, the replacement membrane remains a slight concern to the design team.

The Passivhaus Institute recommends insulating underneath new buildings, but here that wasn't practical. An alternative is to create what's termed a thermal bubble underneath the building, whereby insulation below the ground is taken out horizontally for a certain distance. At Mayville it was decided instead to carry the external insulation right down to the foundations and add 75 mm of high performance phenolic insulation on the building's basement concrete raft.

The walls above ground level were treated with 300 mm of expanded polystyrene block fixed to the external face of the brickwork and finished with a protective hardwearing Permarock render. Below ground the basement walls are externally insulated with 200 mm of extruded polystyrene insulation.

A replacement zinc pitched roof with 400 mm of insulation was installed over the top of the existing steel trusses. A layer of 300 mm Rockwool insulation was placed between the joists, with 100 mm of denser Rockwool over the top of the steel structure to avoid cold bridges. The top of the insulation is covered with a Tyvek breathable membrane and finished off with a standing seam zinc roof.

All windows were replaced with high quality, German triple-glazed windows with detailing that avoids thermal bridging. Careful attention was paid to both the detailing and the quality of insulation.

The window frames are positioned with inflatable bags, and fixed with screws that locate but do not put any pressure on the frame itself. The window frames were sealed into the openings with continuous tape. To ensure that contractors could follow instructions, the architect generated multiple work stage drawings for complicated junctions.

Two airtightness tests were carried out, the final test showing a value of 0.43 - remarkable for a 19th Century building.

Building services

The design team wanted to reduce energy consumption to the minimum, but as always there is a balance to be struck between highly carbon-efficient technologies, their capital cost, and the ease by which they can be subsequently managed and maintained in a building without on-site premises management.

Energy consumption (including unregulated loads) was modelled using the Passivhaus Planning Package (PHPP), a spreadsheet-based design tool for those designing to Passivhaus standard. The PHPP predicted a 7 kW heating load for the building over a continuous 24 h period.

There was no money in the tight budget for mechanical ventilation with heat recovery (mvhr) - a fundamental element of the Passivhaus approach - but the architects successfully argued for heat recovery ventilation to be part-funded from the £3 million Islington Climate Change Fund. This was originally intended only to fund heat pumps and photovoltaics.

The variable volume Paul Maxi mvhr unit is sized to deliver 5.6 litres/s per person, at a specific fan power of 1.86 W/l.s for each of the supply and extract fans. Heat recovery is via a corrugated plastic heat exchanger, said to deliver close to 90 per cent heat reclaim. The fan motor is in the airstream, which provides more heat.

In summer the building will be naturally ventilated through openable windows. The entire building will be mechanically ventilated in winter, with volumes based on carbon dioxide readings. The perimeter offices only have a supply, their extract being out to the hall.

In normal day-to-day use the hall will not need its own ventilation. At times of high occupancy, a carbon dioxide sensor in the return air ductwork will open up a ductwork damper to supply air directly to the hall. The small cafe area will also have boost ventilation.

A simple timer will bring on the ventilation to provide a base load to the offices, basement and extract from toilets, while other areas will come on with demand. The wet heating system will be used for early morning warm-up to avoid wasteful use of fan energy.

While grateful for the mvhr funding, the quid pro quo was an 8.4 kW Viessmann ground-source heat pump when the services designer, Alan Clarke, would have preferred a simple gas boiler. "We had to excavate three metres of soil for the ground-source brine pipework," he said. "It's a bit crazy to expend energy to do that."

The heat pump can also provide top-up for the hot water storage.The GSHP storage is sized at 200 litres to prevent the GSHP from modulating unduly.

Standard radiators, sized for a 45C flow, have been used for space heating. Decoupling space heating from air supply allows the mechanical ventilation to be turned off, reducing electricity consumption.

Islington provided 50 per cent funding for 116 m2 of grid-connected photovoltaics rated at 18 kWp. The building has limited need for hot water, so the solar thermal system is a modest single 3 kW panel connected to a 300 litre tank.

A rainwater harvesting tank will collect water from two small green roofs over the extension, and the roof of the main hall. The water will be used for garden irrigation and WC flushing respectively.

Electric lighting relies on a mixture of conventional T5 and compact low energy fluorescents. Daylight dimming controls and presence detection has been used where applicable. Large motorised south-facing roof-lights have been used to get daylight into the main hall and for summer night-purging.

The design team has paid specific attention to lighting controls, opting for a simple manual on, and auto and manual off approach. This is based on the principle of PIR-based absence detection rather than presence detection.

Energy calculations

The existing building was consuming 581 kWh/m2 per annum. Using the Passivhaus Planning Package, it was calculated that fabric improvements will reduce the energy demand to 127 kWh/m2 per annum, of which the heating demand will be 11 kWh/m2 per annum. The energy costs are anticipated to fall from £10 710 per annum to around £600 per annum.

The building will be certified by the Passivhaus Institute, and exceeds all Part L requirements. Using SBEM, the target emissions rate was calculated at 18.8 kgCO2/m2 per annum, with the as-designed building emissions rate set at 13.7 kgCO2/m2 per annum, an 87.5 per cent improvement on the old building.

The building's main energy end-uses break down as 15 kWh/m2 per annum for the heat pump, 0000 kWh/m2 per annum for the MVHR, and 0000 kWh/m2 per annum for all lighting. The solar panel is estimated to deliver 1341 kWh per annum, and the photovoltaics 14 400 kWh per annum.

Performance evaluation

With an eye to monitoring the performance of the building under the Technology Strategy Board's Building Performance Evaluation programme (BPE), the designers have installed flow heaters onto pipework, and are considering installing a wireless datalogger so the building's energy performance can be monitored remotely.

Sub-metering is extensive but not over-complicated. There are submeters serving the ground floor and basement lighting, and the first floor lighting. The photovoltaics, air handling unit, heat pump compressor, top-up immersion heater for the heat pump, rainwater and sewage pumps also have sub-meters.

The design team is aware that the mechanical ventilation system might be slightly too complicated for a building with many different zones, such as the offices, meeting rooms, catering kitchen and basement music recording studios. They thought hard about different planned activities through the week in different rooms to get a feel for what airflow rates would be needed.

The operating set point was based on higher rates for offices occupied all day, but fairly low densities and fairly intermittent occupancy for the community spaces. Average occupancy may be between 15-25 people, but the building's refurbishment and increased usable space will inevitably see greater hours of use and more people using it.

For a building with no on-site premises management, the user controls will need to be intuitive to use and well labelled. Highly visible information panels will be provided to help educate the building's users. There will also be a familiarisation programme for all building users.

Funding from the Technology Strategy Board will be used to monitor and evaluate the certified Passivhaus refurbishment. The air tightness value of 0.43 is exemplary. It may lead to useful guidance on specialist contractor training, construction drawings, and the design and installation of thermally and air-tight junctions.

Independent comparison and analysis of forecasted energy use (broken down by end use) will be compared with actual and predicted hours of operation using the CIBSE TM22 energy assessment method. Analysis of the Soft Landings handover, with training and long-term support by the professional team, will also be assessed.

BSRIA is part of the Mayville BPE project research team and will be reporting periodically on the building's performance.

BSRIA provides independent building performance evaluations and Soft Landings consultancy. For more details email bsria@bsria.co.uk or phone us on 01344 465600.

The Technology Strategy Board is a business-led executive non-departmental public body, established by the Government. Its role is to promote and support research into, and development and exploitation of, technology and innovation for the benefit of UK business, in order to increase economic growth and improve the quality of life. It is sponsored by the Department for Business, Innovation and Skills (BIS). T: 01793 442700 www.innovateuk.org.