Future Affordable, Dunfermline, Fife

Introduction

A modest terrace of three houses in Dunfermline, Fife, demonstrates a solution to the massive problem of building large numbers of houses which are affordable, efficient and sustainable. Intended for all housing markets - social, rented and private – they meet the requirements of the 2013 Low Carbon & 2016 Zero Carbon targets set by the Scottish Government. The key to their affordability and sustainability is Scottish timber. Scottish C16 Sitka spruce, one of the most carbon efficient building materials available, is used for the wall, floor and roof structure and also for the services core structure.

The three two-bedroom houses are part of a larger development of 27 houses, the Fife Housing Innovation Showcase, built for the Kingdom Housing Association. The three houses are the first built examples of a new building system ‘Future Affordable’, a collaboration between David Blaikie Architects, Kraft Architecture, Springfield Properties and Edinburgh Napier University’s Centre for Offsite Construction + Innovative Structures (COCIS). The system has been developed for easy adoption by private housing developers and registered landlords.

Future Affordable combines two elements; a prefabricated timber envelope, the KII wall system developed by Kraft Architecture, and a prefabricated ‘massive timber’ service core developed by David Blaikie Architects and known as e.CORE. Testing and analysis of the KII and e.CORE systems were undertaken by COCIS as part of their research into technical solutions and products for timber systems of the future. The house plans used - one of Kingdom Housing Association’s standard house types - were modified to suit the new system.

David Blaikie was already familiar with the concept of including all major services in a core, vertically stacked for a two storey house. An earlier house he had designed for Scotland’s Housing Expo 2010 competition, also based on a prefabricated core, was the winning entry and was subsequently built. This experience was used for the new project with Kingdom Housing Association and their standard house plans were adapted by David Blaikie to suit the prefabricated system.

For each of the three two-bedroom houses, the e.CORE consists of two off-site manufactured, structural service modules, one for the ground floor and one for the first floor. The ground floor contains a shower and WC, all plumbing and wiring for the kitchen and a small service-control compartment which contains electrical distribution board, smart meter, central heating controls, the whole house mechanical ventilation and heat recovery system, the electrical systems and control gear and micro-renewables devices. One side of the e.CORE is adjacent to the kitchen so that kitchen services can be incorporated into the module. The bathroom on the first floor has a bath, WC and washbasin.

 

The development of e.CORE

One of the key problems of prefabricated service pods is the need to make the enclosure rigid enough to be lifted by a crane; as a result many are steel framed. To the team, the ideal timber product to achieve high strength and stiffness with low capital investment was a stacked plank system, where solid timber planks are nailed together to form panels. This has the advantage of using small-thickness material which is often a lower value product, such as fast-grown softwood. Lower grade timber can be used as the large number of planks work together to reduce the influence of defects in any one piece, such as large numbers of knots.

The system developed and tested at COCIS uses 47 x 75mm C16 planed timber planks nailed together to produce a solid panel system. The panels were used in e.CORE to create a very rigid ground and first floor structure, a six-sided cube 2.2 metres square but only 70mm deep. Its rigidity is such that it can be craned into a house without distortion. The panels are not only rigid, they provide acoustic and fire separation performance and low embodied energy qualities. A prefabricated component such as e.CORE can reduce and may often eliminate site work by service trades.

In a traditional timber kit house the external timber frame is the structural element. In a Future Affordable house the e.CORE contributes to the structural rigidity to the external frame, with the result that smaller and fewer timber studs are needed, making it easier to fit insulation and reducing areas of cold bridging. The massive timber structure of the core helps improve the indoor air quality as it acts as a water vapour buffer for the living spaces of the house. The e.CORE contains 1 tonne of timber, which evens out the heating and cooling cycle of the house through thermal mass (fabric energy storage), reducing heating bills and improving the living environment. With this amount of timber being used, the level of carbon capture within the building is improved as each core sequesters 600kg of carbon. Incorporation of e.CORE in a house will reduce the site construction period of a typical house by 4 weeks.

 

The KII timber frame

The KII System is a whole house system developed by Kraft Architecture for off-site manufacture to improve efficiency of labour and materials and to minimise embodied carbon. Scottish homegrown timber floors and truss frames are key components of the structural envelope. These prefabricated components, developed in Scotland, allow services to be integrated; in addition, the pitched roofs are constructed of frames which allow the loft space to be easily converted in the future into a home office or additional bedroom and there is an option of using cassette panels to form rooms in the roof.

The system has undergone structural testing in Scotland to maximise the use of home grown C16 timber while minimising both repeating and linear thermal bridging, achieving affordable 2013 / 2016 ‘Fabric First’ standards for mass housing. The thermal insulation break of the KII system contributes to the overall building performance by reducing the repetitive and linear thermal bridging co-efficient value by more than 50 per cent compared to Scottish accredited detail values.

The KII studs are designed to suit different performance requirements by accommodating a variety of insulation materials, but on this project a ‘breathable’ or ‘vapour open’ arrangement was used which allows the building envelope to adapt to different conditions, filtering excess moisture, controlling air infiltration and reducing energy loss.

 

The choice of timber

The timber was Scottish grown FSC certified C16 spruce, primarily supplied by BSW Timber. As it was processed locally, the cost of transport was lower and the Life Cycle Analysis impact was greatly reduced.

Scottish Sitka spruce is normally graded as C16, its relatively low strength and stiffness make it the lowest grade acceptable for structural applications. But by using modern material measurement equipment to determine the density and dynamic modulus of elasticity of the actual timber used, the COCIS team was able to optimize its performance and improve the structural capability of the system.

As well as its low C16 grade, the use of homegrown timber, specifically thinner section material, is also limited because of its lack of dimensional stability on drying. Timber for components which are fabricated offsite has to be dried down to approximately 12 to 14 per cent. Working with the supply chain partners BSW Timber and MAKAR construction, these challenges were overcome by the use of appropriate drying cycles.

 

Construction of the e.CORE modules

The panels of stacked planks were made, tested and subsequently sheathed to enhance stiffness. Twelve panels were required for each house; the ground floor and first floor modules were fabricated separately and transported to the building site. Once the foundations and ground floor external walls were completed, the ground floor module was lowered into position by crane. The first floor and walls were built and then the first floor module was also craned in.

 

Collaborative partners

COCIS, through European Regional Development funded programmes, the Institute for Forest Product Research (FPRI), the Wood Products Innovation Gateway (WPIG), the Institute for Sustainable Construction (ISC) and the Low Carbon Building Technology (LCBT) Gateway, assisted with the supply chain integration and conducted additional testing and analytical work to ensure system compatibility. The information collated from the tests has been used to populate a generic data base which is forming a whole-house engineering mechanism being developed in CSC (UK) Tedds software to facilitate future system evolution and house type optimisation. The work is being undertaken via the EPSRC grant ‘Structural Optimisation of Timber Offsite MMC’. The collaborative team included Ochill Timber and Living Solutions, a Fife-based social enterprise tackling long term unemployability, which undertook the fabrication of the KII studs and the e.CORE modules.

Completion Date:

April 2012

Year Published:

July 2014

Building Type:

Housing

Location:

Dunfermline, Fife

Client:

Kingdom Housing Association

Architects:

David Blaikie Architects (e.CORE) and Kraft Architecture (KII system)

Structural Engineer:

Hannah Reed Associates

Main Contractor:

Springfield Properties

Timber Supplier:

BSW Timber Ltd, Ochill Timber

Timber Elements:

Structural frame, roof truss, floor truss, windows, structural service module

Timber Species:

Scottish grown FSC (Forest Stewardship Council) certified C16 Sitka spruce

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