Employing Lessons Learnt: Key Construction Details And Innovations
There are a number of special innovations explored on the Wilkinson Primary School project which has contributed to the success of the building.
With the hindsight and experience of delivering previous passivhaus schools, we found that relying heavily on the Building Management System (BMS) was actually less effective than allowing the occupants to regulate the internal conditions on their own. At Wilkinson, we made all of the opening windows within classrooms manually operated, with only the clerestory windows controlled by actuators.
We persuaded the Passivhaus Institute to accept a higher internal gain factor, due to the higher density of children in UK schools compared to German schools, enabling the whole design to be optimised. This led to reduced areas of glazing, a smaller heating system, better comfort and lower energy consumption
Quote: 'The finished school is an absolute delight for staff and pupils, who moved in to a school they were already familiar with, given the involvement they had all had in the design and construction phases. The building flows perfectly with everything to hand.' Marc Webb, Sites and Building Strategy Manager, Wolverhampton City Council (Client)
The Foundation Slab
he foundation detail is a repeat of the earlier Passivhaus schools, with a continuous insulation layer around the building, starting underneath the slab and vertically up the walls. The timber frame design allows for the structural element to sit on the slab thus transferring the loads, with the thermal 'duvet-layer' aligned with the insulation formed around the slab. The internal Air-tightness layer is also continuous, using 18mm OSB3 boards taped at the joints with air-tightness tape.
First Floor Detail
This detail has been revised since the first Passivhaus schools and is now more simple to construct. The previous schools were designed so that the first floor was supported by installing directly onto the ground floor walls, thus making the air-tightness line convoluted and exposed to potential damage during construction. On Wilkinson School, we worked closely with the timber frame sub-contractors at an early stage and designed this detailed so that the air-tightness line was a continuous line of OSB, resulting in the frame becoming a type of 'Balloon frame system'.
Windows and Curtain Walling
The windows and curtain walling are Guttmann triple glazed units, however we worked extremely closely with the cladding sub-contractor to find a economical and aesthetically pleasing window surround detail. This detail allowed the window-surround to be glazed in, forming a designed edge to each of the two cladding materials. The windows surrounds also incorporated their own drainage system, which prevented the potential for staining from the Corten cladding material, which is an issue over the first few years of occupation.
Sprinkler Pump House
Through monitoring of the first Passivhaus schools (Oak Meadow and Bushbury Hill), it was found that the industry wide standard non-insulated GRP sprinkler pump houses were consuming approximately half of the energy of the entire schools to keep them heated at the required regulation temperature.
This monitoring allowed us the opportunity to learn and revise the way the pump house was designed for Wilkinson School. Instead of using standard off-the-shelf GRP pump housing, we designed a very simple well insulated box to house the pump. This was based on Passivhaus principles in so much as thermal bridging was eliminated by the use of a continuous insulation layer and air-tightness was included within the design.
This has been highly successful and consequently, this is now being applied to our previous Passivhaus Schools.
Energy Demands and Post Occupancy Monitoring
Measured Energy Performance
Airtightness result: 0.34 ach @ 50 Pa
Predicted gas consumption on PHPP= 12 kWh / m2 p.a
Actual gas consumption Jan ‘14- Jan’15 = 10.96 kWh / m2 p.a
Actual electricity consumption Jan ‘14- Jan’15 = 50.4 kWh / m2 p.a
Total Primary Energy = 143 kWh / m2 p.a*
Annual energy demand for heating in monetary values: 12kWh/m2 x £0.0429 / kWh = £0.51/ m2**
Total Annual CO2 emissions / m² treated floor area = 12.6 kgCO2/m2
* we are identifying the cause of this being over 120 kWh/sqm.a through post occupancy monitoring, and expect to reduce this during the 2nd and 3rd year of operations.
**(based on the average price of gas from the Energy Saving Trust, last reviewed Feb’15)
EPC Rating = A (25 points)
Typical External Wall = 0.133
Flat Roof = 0.153
Ground Floor Slab = 0.152
Pitched Roof = 0.103
Architype have been monitoring the thermal comfort of Wilkinson Primary School during its first year of occupancy, as well as collecting the same data from the first generation Passivhaus schools, (Oak Meadow and Bushbury Hill) pre-passivhaus schools, (St. Luke's and The Willows), and a standard school building built to UK Building Regulations in the 1970's to compare.
The results have been outstanding, showing clear improvements in each succeeding school; with all showing huge benefits upon the standard Building Regulations school. Wilkinson Primary in-particular has best overall results, having employed the lessons learned in design, construction and Soft Landings from the predecessors.
Annotations on Graphs
Graph 1 Graph showing the total thermal energy consumed for space heating and hot water in Pre-Passivahus, 1st generation Passivhaus and 2nd generation (Wilkinson Primary) Passivhaus schools.
Graph 2 showing the winter temperature in a similarly located classroom in each school. The red line shown 21°C is the lower limit for learning, and as you can see from the graph, Wilkinson Primary begins each school day at this level, rising to a comfortable and consistent 23°, compared to the conventional Building Regulations school which never reaches the lower limit.
Graph 3 showing the summer temperature during the same week in all studied schools in June. Wilkinson is the most controlled classroom of all of the studies, rising no more than 1.5° during occupied hours. Natural cooling and ventilation help to prevent large fluctuations and keep the temperature consistent. Classrooms stay cool and fresh in the afternoons, helping concentration of pupils and preventing fatigue.
Graph 4 indicates the CO₂ levels in the test classroom during the same week in winter. The winter is the most difficult time to control CO₂ levels because people do not want to open windows in the cold weather, so the air becomes stagnant very quickly. The recommended healthy amount of CO₂ in classrooms is 1500 particles per million. As you can see from the graph, the conventional school far exceeds this, and Wilkinson Primary remains consistently below the entire week.
Graph 5 indicates the CO₂ levels in the test classrooms during the same week in summer. As you can see from the graph, all classrooms do much better in controlling the CO₂ content as they can open the windows. Again, Wilkinson has the best results with least fluctuation thanks to the night time cooling strategy which lets classrooms be ventilated securely throughout the night.