Code 5 House, Cardiff/Swansea/London

Achieving Code Level Five with Concrete and Masonry A mainstream approach for high performance, low-cost social housing.

Since the introduction of the Code for Sustainable Homes in 2006, the housing sector has made good progress in developing solutions to meet its tough performance standards, at all levels of the Code. Going forward, the next challenge is bringing the lessons learned into mainstream housing, in a cost-effective and practical manner. For social housing this challenge is particularly demanding, since energy efficiency enhancements to the fabric and services must not impact on the core design requirements within this sector, which include: • Affordability • Low maintenance • Simplicity (easy to operate) • Use of systems with a proven track record • Robust and durable solutions

Key findings • The specification of conventional concrete and masonry construction presented no particular thermal performance limitations at code level 5, notwithstanding the need for larger cavities and enhanced measures to minimise cold bridging. (This point is also relevant for code level 6, because the difference between code levels 5 and 6 largely relates to the provision of renewable energy.) • Many of the newer systems and technologies associated with code level 5/6 performance were discounted early on as they did not fit well with the particular needs for social housing i.e. for simple, durable, cost-effective solutions with a low maintenance requirement. • The specified concrete and masonry construction elements attractedgood scores in the BRE Green Guide, which is used to assess the environmental performance of materials within the Code for Sustainable Homes. • Most suppliers of concrete and masonry products operate environmental management schemes compliant with ISO 14001 (or equivalent). This attracts points in the ‘Responsible Sourcing’ category of the Code. Further points can be scored by specifying materials from suppliers compliant with the new standard for responsible sourcing of construction products (BES 6001).

The design exercise underlying this document highlighted that: • The house building industry using concrete and masonry solutions can make the step change required to deliver zero carbon housing by 2016. • Code level 5 homes built with concrete and masonry are affordable and scalable. • Code level 5 homes built with concrete and masonry can use existing construction methods and skills with a simple renewable energy strategy. • The local supply chain, short lead times and a flexible programme offered by concrete and masonry construction continue to be a key benefit.

The Concrete Centre, together with a registered state landlord and their chosen contractor and design team, has developed concept designs for concrete and masonry homes that achieve Code Level Five of the Code for Sustainable Homes. The designs provide a mainstream approach for high performance, low cost social housing.

Since the introduction of Code for Sustainable Homes in 2006, the housing sector has made good progress in developing solutions to meet the tough performance standards, up to Level 4 and higher. The challenge now is to translate those solutions for mainstream housing to achieve level 4 and higher.

For social housing this challenge is particularly demanding since increased energy efficiency must not have an impact on the core design requirements of affordability, low maintenance, simplicity, use of systems with a proven track record or the provision of robust and durable construction. The concept designs take full account of this and in addition they meet current public social housing design standards such as Lifetime Homes, Building for Life and the augmented space standards. Furthermore, the designs not only take account of current legislation but also future foreseeable changes to SAP which is likely to take more account of thermal mass.

A major consideration in meeting these requirements was the determination to use readily available and proven methods of concrete and masonry construction and design solutions that meant the concept could be built in any location.

The concept designs are for a 5 person, 3 bedroom, house of 2 stories with a floor area of 85m2. The plot was assumed to have a north/south orientation and low flood risk.

For the ground floor, two types of beam and block system were considered, one using polystyrene blocks and the other concrete. Both had a 90mm screed finish containing underfloor heating pipes. To achieve a design U-value of 0.12W/m2K, the concrete block option requires around 200mm of insulation below the screed, whilst the polystyrene option only needs an additional 50mm of insulation, resulting in a much lower overall depth. However, this was not considered to be a particular design issue for the project.

Other differences centred on cost and environmental rating, for which the polystyrene options scores an A+ in the BRE Green Guide. A beam and concrete scores a B, although the potential exists to upgrade this to an A rating by specifying screed with a high recycled content. With little to choose between the two options, concrete blocks were ultimately selected.

The design U-value of 0.15W/m2K for the external walls was achieved using a 200mmm cavity fully filled with polystyrene beads and medium density aggregate block. Had wall thickness and lower thermal conductivity been more of a driver, the greater insulating properties of aircrete blocks would have reduced the depth by some 10mm, or 15mm if using a thin-joint system, which would also offer possible construction programme advantages.

Concrete blocks, timber studs and steel studwork were all considered for the internal walls, but a combination of factors lead to the specification of block walls. These included a tougher finish, greater mass and good fire and floor resilience. It also enabled the preferred option of a wet plaster finish to be applied to both the external and internal walls.

The pitched roof provides optimum performance in terms of weathering, whole life costs, and the use of solar panels to achieve Code Level 5 targets. 450mm of mineral wool was determined to be the most economical way of achieving the required U-value of 0.12W/m2K. A traditional roof covering was a fundamental design requirement in order to meet the aspirations of the RSL for architectural designs that are acceptable to planning authorities on a national basis.

Timber and UPVC windows were evaluated for thermal performance, maintenance requirements, ease of cleaning and durability. Despite lower levels of embodied CO2, timber was ruled out due to higher maintenance requirements an issue of particular importance to the RSL. The decision to use UPVC windows was also influenced by their recent Green Guide environmental rating improvement from C to A to reflect the material's good whole life performance and ability to be recycled at the end of life.

Whilst thermal mass does not fall within the scope of the Code, making use of it was an important design requirement in order to reduce the risk of summertime overheating and so provide a degree of adaptation to our warming climate. Depending on orientation, it also has the potential to improve energy efficiency during the heating season. Accordingly, the design optimises the inherent thermal mass of the materials used helped by a wet plaster finish on the walls and a tiled floor where practicable.

The main RSL design requirements influencing the choice of heat source and renewable technologies centred on the following issues:

Ease of maintenance Cost Use of a known technology Code score Compatibility with other aspects of the design

These design requirements lead to the selection of a photovoltaic system and a class 5 gas condensing boiler, which reduce the case also including a solar hot water system. Instead, the entire renewable energy requirement is met by a 27m2 photovoltaic (PV) array. Whilst relatively expensive, the benefits of PV in terms of simplicity and reliability were felt to be more significant. Having a single renewable technology also reduces the overall complexity and maintenance requirements of the house.

The concept designs from The Concrete Centre clearly demonstrate that existing trade skills together with readily available construction materials can easily achieve Code Level 5.