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Building performance evaluation of passive solar schoolOctober 2012

Roderic Bunn investigates the design and construction of a passive solar school, and plans to analyse its in-use performance.

Here's a good question: when does a low energy building become a passive solar building? Is it when the thermal heating load is modelled at 15 kWh/m² per annum? Is it when an exemplary level of airtightness been achieved? Is it when reliance on fossil fuel has been largely replaced by renewables? Or is it a combination of all these things?

The project reported here is part of the Technology Strategy Board’s Building Performance Evaluation programme and acknowledgement is made of the financial support provided by that programme. Specific results and their interpretation remain the responsibility of the project team.

Well, the current fashion is that buildings can only claim to be truly passive when the PassivHaus Institut says they are, and a coveted certificate to that effect is hanging in the architect's reception.

The current enthusiasm for PassivHaus certification is certainly driving low energy design in the way that BREEAM did for many years, but where does it leave buildings that, for one reason or another, don't conform to the strict definition of PassivHaus? Are they lesser buildings by comparison? Have their design teams short-changed their clients by only achieving a 16 kWh/m² heating load?

That's the problem with any kind of threshold-based assessment based on theoretical numbers - it tends to categorise buildings into winners and losers before evidence is available. But low energy buildings should not be in some kind of race, where no-one remembers who came second.

The PassivHaus winners' circle of buildings will be surrounded by a large number of 'personal bests'. In any case, all passively designed buildings deserve to win their plaudits based on proven performance-in-use rather than through a post-completion calculation. We need to see how the buildings operate over the medium to long term before we can be sure that a building deserves an ultra-low or zero carbon classification.

Staunton-on-Wye Endowed Primary School. Local stone was used to cover the north elevation to fit the timber-frame building into its architectural context

Staunton-on-Wye Endowed Primary School in Herefordshire might not be certified PassivHaus, but it is certainly an eco-minimal building, including high standards of insulation throughout, a sedum roof, beneficial solar gain, and good levels of daylighting. The designers avoided materials and treatments high in volatile organic compounds, and inert construction materials were used as much as possible.


The voluntary-aided school has an interesting history, being set up by a local philanthropist to serve villages in the immediate area. The original school was set up in the 1700s by the original Jarvis Foundation, and a school building constructed in 1860.

By the turn of this Century the building was in a poor condition and desperately needed replacing. The school had already retreated into one corner of the building while other parts had been converted to residential use.

A nearby field came up as a possible site. The governing body of the school secured £800,000 from central government while the remainder was raised through determined local fundraising by the school and pre-school, and 25 per cent from the Jarvis Trust.

The client in this context was the school's board of governors. The head teacher, Pippa Lloyd, was a driving force, as was a local businessman, Ray Stone, who was involved in the area's Local Enterprise Partnership and was influential in the construction process.

The project was competitively tendered through the OJEU process. Architype acted as the architect and contract administrator, with E3 as services engineers. The building was constructed by Thomas Vale.

The construction phase wasn't plain sailing. The original electrical contractor went into administration during the contract and the replacement contractor had the unenviable task of picking up the project mid-way through, which inevitably led to programming issues. The mechanical contractor also went bust after project completion, which affected commissioning and the delivery of the operation and maintenance manuals.

Despite the upheavals, the project was completed in November 2010.

The original 1860s school building, since sold off to fund the construction of the new school

Construction and layout

The school combines a charity supported primary school for 78 pupils and 14 staff, and a separate pre-school facility catering for 26 children and 14 staff.

Accommodation in the main block includes a multi-use hall, classrooms, group rooms, a domestic-sized kitchen, administrative offices and toilets. The three primary school classrooms face south to optimise on daylighting, while the offices and main hall face predominantly north. The pre-school is a separate building, but shares the main school's biomass heating system.

The client's vision was for a highly sustainable, energy-efficient building. The brief was quite loose at the outset, but ultimately the building's orientation, position and design responded strongly to the site. Other design features, such as the control of ventilation, were developed during a consultation process.

The school's fabric is a form of breathing wall construction, with timber frame sandwiching 250-300 mm layers of Warmcell cellulose insulation. A bituminous fibre board serves as the external covering, with oriented strand board and taped joints as the internal airtightness layer. This led to a U-value of 0.12, while the roof achieved 0.1. The performance of the fabric elements will be assessed to see how close they are to the PassivHaus specification.

The building also vastly surpasses the regulatory air-tightness requirement by achieving an average of 1.7 m3 (h.m2) at 50 Pa. This is not as tight as the PassivHaus equivalent of approximately 0.6 m3 (h.m2), but the geometry wasn't ideal, and there were many junctions that would have needed significant investment in design to achieve the PassivHaus target.

With future proofing in mind, the designers have provided bulkheads in ceilings and notional ductwork space for the retrofitting of a mechanical ventilation and heat recovery system.

The main hall, facing north

Building services

The building services are very simple: apart from small extract fans serving the toilet areas, the building is naturally ventilated - single-sided for the offices and cross-ventilation in the classrooms. In the classrooms the ventilation is controlled by manual Teleflex winders for the fanlights on the south façade and for the extract through high-level clerestory windows.

The designers considered motorising the windows and placing them under BMS control. Not only was this deemed too costly, the head teacher preferred the teachers to have full manual control of their windows. Hence only the high level vents in the hall are motorised, with control via wall switches.

"Right from the start we wanted as much manual control has we could get," explained head teacher Pippa Lloyd. "We're trying to teach children to be responsible adults, and a system that operates itself is not the message we want to put across."

The lighting controls comprise presence detection (auto-on and off) for circulation areas, and manual switching with daylight-linked dimming to a preset level, and presence detection (auto-off) for the high-frequency fluorescent fittings in teaching areas.

The circulation lighting doesn't have daylight dimming when perhaps it might have benefitted from it. At completion the circulation lights were controlled by PIR absence detection, but as people move through the building constantly, the lights were found to be on even when adequate daylight was available via the skylights. A conventional manual switch for the lobby lighting was a post-contract addition.

The building's exemplary carbon performance is largely due to a 45 kW biomass boiler. As there is no gas to the site, an LPG-fuelled conventional boiler was an early design consideration. However, the client's enthusiasm for biomass and eagerness to chase whatever grants were available led to a grant of £50,000 being secured for a single wood-chip boiler.

The system is controlled by a Trend building management system, with monitoring, energy metering and temperature control. The system is largely run by Pippa Lloyd.

This serves lphw underfloor heating in both the main school and the preschool. Small radiators have been installed in some small rooms with a higher heat loss. Room temperatures are controlled by the Trend bms, with a two degree band of adjustment via room thermostats. The school hall is heated by a three-speed lphw fan convector.

Similar grants have been chased for photovoltaics and solar water heating, and the school has recently installed a 3.9 kW photovoltaics system purchased with money from a Rolls Royce Science Prize. "The feed-in tariff will be used to support science teaching in the school," says Pippa Lloyd.

It's fair to say that the school and the designers have a different view on the benefit of on-site renewables. While the school is keen on all forms of on-site renewable energy, the designers are more eco-minimalist, and argue for keeping things simple and reducing the maintenance burden. For example, the school's demand for hot water is very low and so the payback for solar heating would be quite long.

One of the three primary school classrooms

As Architype's Tom Mason said: "Sustainability doesn't have to be tangible, it can be invisible. And if people think they get energy for free, it can lead to more wastage."

For example," he added, "schools are putting up LCD screens to inform visitors about activities in their schools, whereas an old-fashioned pin board is perfectly adequate."

Energy calculations

The school's Building Emission Rate was calculated to deliver a 74 per cent reduction over the 2002 Part L requirement, and a 65 per cent reduction on the 2006 Target Emissions Rate. Although the school was not designed to the PassivHaus standard, the design team still ran the design through the PassivHaus Planning Package (PHPP) simulation tool to get a feel for how building compared. The rough calculation indicated a space heat demand of around 29 kWh/m² per annum (with mechanical ventilation and heat recovery - mvhr) compared with the PassivHaus standard of 15 kWh/m² per annum. Without mvhr the demand is estimated at 65 kWh/m² per annum.

Initial performance

The head teacher says the school has met expectations. "It works well, and we like it," said Pippa Lloyd.

Reservations were expressed about some of the systems, notably the heating. Problems with controlling the biomass system led to some overheating problems in the first winter of operation. In 2011, although the staff found the school a bit cold first thing in the morning, by 11 am it warmed up.

The primary school retains heat so well that the head teacher ran the underfloor heating sparingly during the 2011/12 winter. However, the preschool wanted its heating on, which meant that the 45 kW biomass boiler often only served a small area. In hindsight, say the engineers, a small boiler - even LPG-fuelled - may have been a more efficient way of heating the pre-school, leaving the biomass dedicated to the primary school.

The fanlights on the south-facing classrooms. These are on manual winders rather than motorised

The manually controlled ventilation system seems to be working well, if only because the schoolchildren happily act as mobile thermostats and draught sensors and will call for their teachers to open or close windows as required. The Teleflex controls are largely obvious as
to what they do, but in common with most installations do not have any labelling showing what they do, nor whether windows are open or closed.

The time clocks for the local 3 kW electric water heaters are located within cupboards, many of which are filled with classroom materials. Access to the time clocks is poor and have not been adjusted by the school since occupation.

Performance evaluation

Staunton-on-Wye Endowed Primary School will be subjected to a two-year building performance evaluation by the original design team, and led by the Architype and E3 Consulting Engineers, with BSRIA providing assistance with the research methods.

In addition, the research team will assess the quality of the internal environment, including the monitoring of temperature, relative humidity and carbon dioxide levels. Thermal imaging will also be carried out on specific areas of the building to assess insulation performance and air leakage paths.

The study will also measure all aspects of the efficiency of biomass boiler/plant configuration specifically the standing losses of the system, relative to the actual building demand, and determine the overall efficiency of the system. A review of hot water use could reveal whether the local timers could be adjusted to optimise delivery at times of demand.

All the incoming utility meters report back to the Trend bms, as do the submeters for the electrical distribution boards. Lighting and power is metered separately. A quick reconciliation exercise has found a 13 per cent disparity between the submeter totals and the main meter reading.

This error is just outside what might be considered allowable, and this will need to be investigated to find out whether one or more submeters are reading inaccurately, or whether an electrical load is not being metered.

The design team for Staunton have designed other schools to a low energy standard, plus the PassivHaus certified primary schools Bushbury Hill, Swillington, and Oak Meadow. These will provide useful benchmarks for the Staunton school, potentially providing an answer to the key question posed at the start: what is the real practical difference between eco-minimalist design and PassivHaus, and does the distinction matter more in theory than it does in practice?

Roderic Bunn is a building performance evaluator on the Technology Strategy Board's research programme. BSRIA will be reporting periodically on the performance of Staunton-on-Wye primary school.

BSRIA provides independent Building Performance Evaluation services for all types of buildings.

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


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