Carrowbreck Meadow: Passivhaus rules

Sheltering in a landscaped clearing in the middle of thick Norfolk woodland lies the largest Passivhaus scheme in Greater Norwich and one of the biggest in eastern England. Carrowbreck Meadow is a development of 14 Passivhaus homes designed by interdisciplinary architect Hamson Barron Smith for Broadland Growth, a novel joint venture between integrated property services provider NPS Group and Broadland district council, the local authority.

Carrowbreck exhibits many of the features one would expect from a Passivhaus scheme. For instance, it provides a thermal bridge and draught-free building envelope that exceeds airtightness regulations five times over. Fresh filtered air is also delivered to all the homes, using a heat recovery system capable of achieving more than 90% efficiencies.

But the reason Carrowbreck sets a benchmark in terms of sustainable design is because of the potential role it could play in influencing national policy and procurement when it comes to the delivery of Passivhaus homes. Shortly after the Carrowbreck scheme was tendered, a £300m “Fabric First” framework was set up by Norwich council and managed by Hamson Barron Smith (HBS) to bring together contractors committed to delivering Passivhaus quality. The framework is now being used to deliver a number of other projects including a larger HBS project called Bowthorpe Three Score, also in Norwich.

Bowthorpe Three Score proposes more than 1,000 homes over the next 10 years. Phase two started last month and aims to deliver 172 mixed-tenure dwellings, 112 of which are to be constructed to the Passivhaus standard. Therefore, one of the key objectives of Carrowbreck is to provide a test-bed for larger Passivhaus developments.

Accordingly, the development has been designed using a soft landings framework and will offer full post-occupancy evaluation, including detailed monitoring of data such as real-time energy consumption and comfort levels. From doing this, the scheme attempts to increase understanding of the technical performance and physiological impact of Passivhaus homes and could eventually provide critical information to inform future similar projects nationwide.

In order to analyse what this might mean for sustainable housebuilding elsewhere, below are four key areas in which Carrowbreck Meadows satisfied its Passivhaus brief.

Orientation

Carrowbreck sits in a sheltered clearing in the middle of a wood, so its site instantly provides the natural shading required to prevent overheating – a central Passivhaus pursuit. Nevertheless, the houses on the scheme have been carefully orientated to benefit from the rich woodland setting and provide as much natural shading as possible. The footprint was developed in close conjunction with the planning team and is the result of several detailed shading and daylighting analyses. Equally, on all the homes, effort has been made to ensure that the principal facade is within 30° of south – or ideally within 15° – in order to provide optimal daylighting and maximise useful solar gain through the heating period.

Site plan

Most of the homes are clustered into groups of three and located in a clearing furthest away from the densest, thickest trees in order to provide as much daylight as possible. South-facing windows feature brise-soleil to prevent glare and manually operated Venetian blinds have been supplied to all east- and west-facing windows.

By introducing this level of solar control, the design is not restricted by the “small window syndrome” that some see as a feature of Passivhaus. But as Sarah Lewis, HBS associate architect for sustainability, points out, “Passivhaus homes can be orientated in any direction, it’s simply a case of making any necessary compensation elsewhere in the design.”

Construction and materials

Carrowbreck meets exemplary standards of airtightness. The homes achieve 0.3 air changes per hour, twice the accepted Passivhaus standard and five times better that the rate required by building regulations. Moreover they comprise draught-free building envelopes with minimal thermal bridging. These achievements were realised as a result of an imaginative construction approach that prioritised the fabric-first approach and, crucially, sought to fully commit the contractor, R G Carter Construction, to the Passivhaus ethic.

Two core principles helped guide the construction of the houses, as Lewis explains: “We decided against prefabrication primarily for cost and programmatic reasons, with a site-based system providing more flexibility in terms of delivering the show house in advance of the remaining units.

“Additionally, this gave the contractor an opportunity to gain specific Passivhaus skills that could be passed on to apprentices through training and deployed on future schemes.”

In order to achieve impressive airtightness standards, the external walls of the houses are constructed from loadbearing hollow clay blocks with fully breathable lime plaster internal finishes and a vapour-permeable EPS (expanded polystyrene) insulation membrane, finished with lime render to the exterior.

This arrangement kept the airtightness line on the internal finish and required that penetrations to external walls be kept to a minimum. It was also based on a methodology that sought not to rely on an arbitrary membrane to provide insulation but to construct instead a high-performance envelope whose very composition provides a continuous insulation itself. The clay blocks were also something of a novelty. They are fully recyclable at end of use and require up to 95% less water to make than conventional bricks. They require almost no mortar and can be constructed rapidly – Carrowbreck regularly achieved speeds of 30m² to 40m² of wall construction per day.

Heating and ventilation

A mechanical ventilation with heat recovery (MVHR) system filters fresh air continuously throughout the day across all houses on the site, achieving heat recovery efficiencies of well over 90%. Because the system is based on continuous fresh air intake, there is no recirculation of air prior to it being filtered. Additionally, air extracted from wet rooms such as bathrooms is filtered before being channeled into habitable rooms.

The Passivhaus Institute traditionally maintains that the heat lost from its buildings is so minimal that heating requirements can be as little as 10% of the energy used in conventional buildings. The same principle has been applied at Carrowbreck. While gas boilers provide heating for hot water, the heating is based on a fairly traditional Passivhaus system which anticipates low heat loading and uses the MVHR as the primary source of heat. Many of the homes at Carrowbreck have only one radiator heating the entire first floor. This helps to explain why the fan power of the MVHR unit uses an equivalent energy requirement of only 20p per day.

Carrowbreck Meadow may only be a relatively small development of 14 homes but it sets a benchmark that could well be applied to much bigger schemes on a national scale. First it proves that, in design terms, Passivhauss can be convincingly applied to all manner of visual contexts, even a rich woodland setting. And secondly, it shows that achieving Passivhaus is more than just the sum of technical parts but a collaborative venture that is as concerned with skills training and occupiers psychological wellbeing as it is with energy performance.