The case for Centres of Excellence in sustainable building design

Lessons from Elm Tree Mews
In 2005, The Joseph Rowntree Foundation commissioned, Elm Tree Mews, a
small development in York aiming for “an exemplar for 21st Century suburban
homes”. The properties cover the full spectrum of housing types from threeand
four-bedroom houses, to one- and two-bedroom apartments.
The construction and subsequent occupation was monitored by Leeds
Metropolitan University [Bell et al 2010b]. These studies revealed that,
although the project sought to achieve a standard equivalent to the
Government’s carbon emissions target for 2013, the performance achieved
was only marginally in advance of 2006 regulations. The measured carbon
dioxide emission was 6.4 tonnes per annum compared with the 2.7 tonnes
predicted in the design. In 2010 the foundation published “Low Carbon
Housing - Lessons from Elm Tree Mews” to allow the industry to learn from
their experiences.
The problems experienced were primarily due to a lack of consistency
between the thermal modelling calculations and the actual design and
construction of the buildings. The calculations underestimated the amount of
timber framing in the construction and failed to account for thermal bridging
or heat loss via the cavity party walls. As a result the fabric heat-losses were
54% higher than predicted. Furthermore the measured air leakage for the
completed properties averaged 7m 3 /h.m 2 , more than twice the leakage rate of
3m 3 /h.m 2 specified in the brief and used in the energy calculations.
Further problems occurred due to the lack of a chain of custody for energy
performance throughout the design and construction. This meant that, for
instance, the energy impact was not evaluated when an alternative window
supplier was selected with a lower performance than that originally specified.
Finally, the heating design relied on a communal ground source heat pump
system. It appears that the heat pump was not properly integrated with
the conventional heating and hot water systems in the dwellings and this
resulted in overall performance some 30% below the rating assumed in the
design. The Foundation concluded that if the development had instead
been heated with ordinary gas condensing boilers, carbon dioxide emissions
would have been about one tonne per annum lower.
The report concluded that many processes and cultures within the industry and
its supply chain need to change if zero carbon housing is to become a reality.
• Design processes should be improved so that they:
• take a much more rigorous approach to detailed design including
undertaking and checking thermal calculations,
• focus on as-constructed performance taking into account the
interactions of the different components in both fabric and systems,
• give more consideration to the needs of householders and provide
controls that are designed in accordance with ergonomic principles.
• Services design should focus on whole system performance.
• The planning and control of construction needs to be improved and
include in-production testing.
• Improvements are required in services commissioning, testing and
monitoring to ensure effective performance.
• More support and guidance is required across the house-building industry.
• Policy aspirations will not be achieved without rigorous evaluation of low
carbon schemes and ensuring that lessons are embedded in all parts of
the industry.
24 The Royal Academy of Engineering