Case Study: nZEC - New construction of 14 dwellings in Ravenscroft, Tye Green, Wimbish, UK

Short Description

Hastoe’s requirement was for a social housing development that met all standard needs, and minimised bills to alleviate fuel poverty. The brief to their design team in June 2009 was to achieve this by being Passivhaus certified and meeting the Code for Sustainable Homes level 4 standard. The development was a ‘rural exception scheme’, which meant all residents had to have a local affiliation; this made it easier to engage them and the local community in the project aspirations leading up to handover in June 2011. A post-occupation and in use building performance evaluation study was carried out with Technology Strategy Board funding over the following two years to see whether or not there was a ‘performance gap’, and to study all the aspects, from brief to use and maintenance, that contribute to the successful development. Performance covering owner and resident needs, and including issues such as thermal comfort and air quality alongside energy use of both the building and its systems.

Key Elements

Cooling

A passive cooling strategy has been employed to minimise risk of overheating in the summer. The highly insulated Passivhaus design, with large overhangs and brise-soleil reduce summer solar gain and heat transference through the fabric. The blockwork construction provides thermal mass that ameliorates daily fluctuation and slows gains in a heat wave. Residents are expected to use the external blinds to further reduce solar gains, and to open windows overnight to provide a through-draught to purge any excess heat when it is cooler outside. It is noted that the degree of overheating encountered is strongly dependent on resident actions; for example, extensive use of an inefficient plasma TV or tumble dryer will cause local heat gains.

Certification

Certified Passivhaus; EPC rating B (85 to 87). Client brief required delivery of a Certified Passivhaus; design and construction complied with this. Great attention to quality and detail throughout was required. In-use performance monitoring was conducted and covered assessment of energy use.

Project Management

The client chose to adopt a standard Design & Build Contract, but with the design developed to Stage F. Tendering included a pre-qualification stage to ensure that potential contractors could comply with the client’s requirements for a low energy sustainable development. The Architect, M&E Consultant and Passivhaus Consultant were retained by the client to monitor works post contract. In-use BPE was conducted to confirm the performance of the buildings and their technologies.

Capacity Building

As an early adopter of Passivhaus in the UK it was recognised that the required skill and experience would be difficult to find. It was an objective of the development to develop the necessary local capacity, from design professionals through to sub-contractors. Training off, and on, site was provided. This subsequently extended to site visits and education sessions for the East of England construction community, and for Hastoe support and maintenance staff.

Work with Residents

The need to raise resident awareness to ensure that they achieved the maximum benefit from the dwelling was acknowledged at the outset. Sessions were run for prospective residents advising them what a Passivhaus was and what they should expect. Detail, such as how to control the ventilation (and heating) system and how to use the external blinds to manage solar gain, was conveyed at handover and reinforced thereafter. The BPE study process also provided a number of opportunities to confirm understanding and correct where appropriate.

The awareness raising included energy advice. Handbooks were provided at handover to provide additional guidance. The BPE study team supplied feedback to residents on individual household performance, and in comparison to the other dwellings, both at 1:1 interviews and at a series of resident meetings. Hastoe support and the BPE team were available to provide additional advice as needed. It should be noted that the overwhelming reaction from the residents was delight at their low heating bills.

The homes were designed to comply with Code for Sustainable Homes level 4. Thus garden sheds were provided to store bicycles to encourage low carbon commuting, and rainwater butts were included for harvesting water for garden irrigation. Residents are also provided with a Home User Guide on move in, which provided tips and advice on a more sustainable lifestyle.

Partnership Working

Initially the Passivhaus Consultants provided expertise on minimising energy demand; as the project was developed this knowledge was passed to the other professionals involved. Where possible Passivhaus-certified products and materials were employed to minimise the risk of performance issues and to facilitate the certification process. Products were procured from suppliers with expertise in the products. The BPE study included research by a PhD student from UEA, and the lead evaluator concurrently undertook BPE work as an associate of the Adapt Low Carbon Group at UEA.

Thermal Insulation

The properties were built using 190mm thin joint blockwork on a reinforced concrete slab over 400mm of insulation. The walls are clad externally with 285mm of EPS insulation, with a 16mm render. Roofs are traditional pitched roof trusses with 500mm mineral wool insulation laid flat at ceiling joist level.  The design u-values of floor, walls and roof were 0.091, 0.088 and 0.078 W/(m2K) respectively. As a Passivhaus, thermal bridge free detailing was employed and attention was paid to avoiding thermal bypasses. The thermal insulation qualities and effectiveness were tested by the BPE study carrying out in-situ u‑value tests, and by use of thermal imaging.

Air Tightness

As a Passivhaus, minimising air leakage to meet the target of 0.6 ach is of paramount importance. The contractor provided an air-tightness champion to oversee the sealing and taping tasks on site. The design aimed to minimise penetrations, and a schedule was produced to ensure that they all had grommets and seals. All structural joints, particularly with the windows were taped. The plaster provided the final layer of protection. The air-tightness was tested by on-site blower-door equipment as soon as the shell of a dwelling was complete, was repeated once the M&E had been installed, and then again on completion. Tests were repeated after occupation when some windows needed to be replaced, and as part of the BPE study.

Windows and Shading

The windows are from the Internorm Edition range, being triple-glazed wood/aluminium with insulating thermal foam. The coated glass is laminated or toughened as required, there are thermix spacers, and they are argon filled. The u-value of the window is 0.86 W/(m2K). Solar gain was managed by large eaves overhangs, brise-soleil at ground floor and external blinds at first floor level. The brise-soleil is designed to passively shade the large south facing windows during summer but allow warm sunshine through in the cooler seasons when the sun is lower. Upstairs south-facing windows are fitted with an exterior venetian blind with a manual crank handle.

Ventilation

Well designed, supplied, installed, commissioned, used and maintained ventilation systems are essential to ensure good indoor air quality for residents, and to maximise the performance of the dwelling. Each dwelling has a Passivhaus-certified Paul Focus 200 MVHR unit, mounted within the thermal envelope on a north-facing external wall. Sound attenuated ducting distributes fresh air into the living and sleeping spaces, with extract ducts from kitchens and bathrooms. The insulated ventilation ductwork was contained within space joists at first floor level. The ducts rise through risers to first floor ceiling level to avoid the need for a high level service zone. The heavily insulated intake and exhaust ducts were taken out through the external wall. An electric frost-protection pre-heater was installed, with a wet system coil heater applied to the supply air duct for heating when required. Touchscreen controls were provided. The BPE study placed an emphasis on the efficiency of the system as a whole, including fan power and heat recovery, on the effectiveness in providing good air quality, and on the ease with which the residents could control the system. The impact on performance of the frequency of filter change was assessed.

Heating and Hot Water

Hastoe required that each house should have individual systems under the control of the resident, this prohibited consideration of communal systems. Solar thermal hot water is provided to meet summer demand, however this was not sufficient year round and so a small gas boiler was incorporated into each property. The solar thermal panels and gas boilers both provide heat to a 210l thermal store, which is used to provide domestic hot water and occasional top-up heating to the incoming air supply, the latter being controlled by a wireless portable room thermostat. The BPE study has monitored gas consumption by the boiler, and water usage for each property. Design and commissioning of controls to maximise solar supply, while ensuring adequate supply of hot water and minimising legionella risk is essential.

Solar Thermal

Provision of a solar thermal system aided meeting CSH4 requirements. A 2.3 m2 Baxi Solarflo In-roof Flat Plate Collector was chosen for the solar thermal system, the same for each property. Approximately 60% of the DHW demand was expected to be met by the solar thermal in the flats, and 48% in the houses.

Electricity Saving Products

M&E kit that was a fixture in the dwelling was chosen to be energy efficient; this included the MVHR fans, pumps for boiler and solar thermal systems, and compact fluorescent lighting throughout. Selection of appliances was the residents’ responsibility. Advice was given on the merits of choosing for energy-efficiency, however most had existing, often inefficient, appliances already; and those that did buy new tended to prioritise capital cost over lifecycle costs. The consequence being that electricity consumption is generally higher than the design expectation; this also contributes extra heat to the dwelling, undesirable in summer months.

Water Saving

To comply with Code for Sustainable Homes level 4 the aim was to reduce the consumption of potable water in the home to no more than 105 litres/person/day, through use of water efficient fittings and appliances. The design sought to minimise both hot and cold water usage to sanitary ware by way of fitting restrictors or installing shallower/lower capacity fittings; for example, WC dual flush at 4/2 litre, and shower feed limited to 6 litres/min. Rainwater is harvested in butts for watering the garden.

Funding

The development received HCA grant funding of £830,000.  The study was financially supported by the Building Performance Evaluation programme of the Technology Strategy Board (now Innovate UK), in addition to funds from Hastoe and initial support from the University of East Anglia (UEA).

Main Results

The two year Building Performance Evaluation Study has verified that the Wimbish Passivhaus development meets Hastoe’s objective of delivering very low heating bills that reduce the impacts of fuel poverty on their tenants, and have the potential to reduce rent arrears. The development performs as designed. The occupants have homes that are economic to run, healthy to live in and feel comfortable and spacious. This has shown that the attention to quality and detail in a Passivhaus delivers the expected benefits, justifying the modest additional cost. The delighted residents report that they have never felt cold, and some that their heating and hot water bills are only £120 a year. The project has shown that it is important for all parties to work together in partnership and to adopt a quality ethos. The engagement with the residents, which began at an early stage, brought dividends in that is important they understand how to get the best out of their home. The BPE has provided valuable insights; not only to ascertain which elements work well, and which less so, in achieving the overall design objectives, but also, where there are shortfalls, in diagnosing reasons and helping to confirm the success of remedies. BPE is important to provide feedback to developers for their future schemes.

Lessons learned

The client must set clear expectations for all parties. This is in terms of both the end deliverable and its performance, and the approach taken throughout the development. The design should be developed as a whole, especially ensuring that the approach to the M&E systems is integrated. The design should aim to minimise householder learning, such that the knowledge transfer is made simple. The monitoring approach should also be integrated into the design such that meters can be installed and commissioned with the M&E components. Value engineering during the works must be resisted and minimised, with the implications of any unavoidable change being fully assessed. The ventilation system is a vital component that must be given great care from design through to operation and maintenance. Minimising electricity use is important, both by selection of efficient fixtures such as pumps, and their controls, and by advice given to the residents in their choice and use of appliances.

Additional Information

Since developing Wimbish Hastoe has gone on to develop further Passivhaus schemes. Completed schemes are (with number of units in brackets): Ditchingham (14), Horseheath (2). Additional Passivhaus schemes are on site: Crawley Down (26), Burwash (10), Carhampton (20), Rattlesden (4), Hatfield Heath (11) and Outwell (14). Planned schemes are: Sharnbrook (13), Ashford (15), Piddletrenthide (6) and Wimbish II (11). Hastoe is applying the lessons learned from the Wimbish scand heme, for example, the need for residents to use external blinds to control internal temperatures has been removed in subsequent schemes, with the use of extended roof overhangs. Hastoe is also in the process of looking at alternative accreditation and cost models which provide similar results. For further information see www.wimbishpassivhaus.com/datasheets.html where there are plans of the development, along with copies of the presentation given to the UK Passivhaus 2014 Conference, and an Executive Summary of the final report prepared for the Technology Strategy Board.



Ms Lauren Gee
NHF - The English National Housing Federation
E-mail: lauren.gee@housing.org.uk
Web: www.housing.org.uk