Last edited 07 Dec 2020

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Performance in use of National Trust’s Heelis building and Woodland Trust’s headquarters

Article from the October 2014 edition of the CIBSE Journal written by Bill Bordass, Pete Burgon, Hester Brough and Matt Vaudin.


[edit] Introduction

Experience gained from the post-occupancy evaluation of the National Trust’s Heelis building have been fed into the design of The Woodland Trust’s headquarters. The project team compares the in-use performance of both.

To maintain the ‘golden thread’ from design intent to reality when creating the 2,727m2 head office for The Woodland Trust, Max Fordham – and architect Feilden Clegg Bradley Studios – used post-occupancy findings from the Heelis building, in Swindon. The outcomes have now been studied, thanks to funding from Innovate UK – formerly the Technology Strategy Board – and its Building Performance Evaluation programme.

In 2002-04, the environmental engineer and architect formed part of a research team investigating the potential for soft landings (1), and discovered the importance of maintaining the ‘golden thread’ – starting with inception and briefing, then managing expectations throughout the procurement process, building on initial aftercare, post-occupancy evaluation, and closing the feedback loop.

[edit] Gathering data

At that time, Max Fordham and Feilden Clegg Bradley Studios were working together on Heelis, the National Trust’s 7,605m2 (gross) head office in Swindon. The project followed a strong sustainability agenda, though this was somewhat softened by the requirements of the developer, which procured the building after the scheme design had been agreed with the client.

As part of the reality checking advocated by soft landings, a matrix was developed by Feilden Clegg Bradley Studios and Max Fordham, to allow design ambitions for sustainability to be reviewed at project meetings. Max Fordham was also appointed to fine-tune the operation of the mixed-mode building for two years after handover. The Heelis findings were published in Building Services Journal (2) in November 2007.

Feilden Clegg Bradley Studios also commissioned William Bordass Associates and Building Use Studies to undertake a post occupancy review, similar to the Probe studies published in Building Services Journal from 1995-2002 (3).

[edit] Learning from Heelis

A key nding from Heelis was to avoid unmanageable complications. Measures adopted at The Woodland Trust included:

[edit] Occupant satisfaction

BUS Methodology occupant questionnaire surveys were completed. For consistency, BUS 2011 office benchmarks are used for both buildings (2004 benchmarks were originally used for Heelis). The profiles are similar for both buildings.

In detail:

  • Heelis rated worse on winter temperature and summer air quality. This partly related to initial control problems. The situation was improved the following year
  • The lower perceived productivity at Heelis was probably because many staff had relocated from other parts of the country. The Woodland Trust moved just 200 metres
  • Noise at The Woodland Trust was the only indicator significantly worse than average. This reflects a general deterioration in noise perceptions, owing to more open planning, and higher occupation densities. Many Trust staff were also unaccustomed to open-plan offices. In addition, The Woodland Trust needed to use a space that was acoustically connected to the offices for large meetings and training sessions – activities not anticipated in the brief
  • The average score for health at the Trust was linked to less window opening than anticipated, and – perhaps – some initial problems with outgassing. Ventilation control was subsequently improved.

[edit] Energy performance

Using standard Building Regulations assumptions, the predicted annual gas consumption was 37.5kWh/m2 GIA at The Woodland Trust. The total in 2012-13 was 32.6kWh/m2, with the heating component, 25.7kWh/m2, almost identical to the design estimate – a very good result in this exceptionally cold year. This also compared well with the 90kWh/m2 for heating and hot water at Heelis, with an additional 24kWh/m2 for its catering kitchen.

Figure 2 shows annual electricity consumption – in kWh/m2 –for the same period, broken down by end use. Starting at the bottom, this shows:

[edit] Issues in operation

Generally, The Woodland Trust building performed well, but some problems arose in use – of which server-room cooling was the most critical. Value engineering at a relatively late stage replaced the independent airside and chilled water cooling systems with a packaged chiller, with both refrigerant and free cooling. Although the chiller was highly specified, it failed every few months, bringing down the Trust’s entire ICT, thin client, and telephone systems.

The manufacturer was remote and local support proved difficult to obtain, so a basic DX backup system had to be added. The chiller’s controls proved to be vulnerable to brief power interruptions, which are common in Grantham. Although the problems were eventually fixed, simple, more standard equipment would have been preferable.

Using the boiler plant for both heating and hot water also led to some energy wastage, because of interactions between the two systems. A reduction of nearly 10% in gas consumption was estimated had the systems been completely separated.

The natural ventilation system initially caused problems. In winter, occupants opened windows less than expected because of draughts (the main openings are at desk level). When CO2 levels rose above 1,200ppm, the automated system opened higher-level windows, but the minimum setting of 10% was also draughty. The BMS was eventually altered to give facilities control over minimum window opening.

In summer, night cooling was also disappointing initially, partly because of a complicated control logic and convection currents reaching the external air temperature sensor when the sun’s rays fell on the building’s timber rain-screen cladding, elevating the detected temperature by as much as 8°C. The situation was improved by relocating the sensor, simplifying the logic, and allowing the facilities manager to decide whether heating or night cooling was required. The function of the concrete radiators was explored in 2012-13, using heat-flux sensing and time-lapse infrared thermography. The average rate of heat absorption during office hours was 5W/m2, rising to 10W/m2 on hot days. An important finding was a need to close windows an hour before the office opened, so temperatures could equilibrate, otherwise the air and furniture could feel too cool when people arrived.

[edit] Conclusions

The building achieved many of its design objectives: good quality at a normal cost (£1,800/m2), and good levels of occupant satisfaction – though with shortcomings in relation to noise, in particular. Initial problems with air quality and summertime temperature have been tackled, with potential for further fine-tuning.

Lower energy use than Heelis was achieved for all building-related end uses, especially heating and lighting. However, despite ‘thin clients’ and other efforts, the electricity used by ICT systems was higher. Future projects would benefit from the services of an ICT energy-efficiency consultant.

The ‘keep it simple and do it well’ approach could be taken further still, particularly by improving the usability of control systems, and avoiding over-complication in the name of energy efficiency – especially in the design of the server room cooling system. In terms of procurement, future projects should:

This article was created by --CIBSE.

  • Bill Bordass FCIBSE, of William Bordass Associates and the Usable Buildings Trust.
  • Pete Burgon CEng MCIBSE, of Max Fordham.
  • Hester Brough, of Feilden Clegg Bradley Studios.
  • Matt Vaudin, of Stonewood Design.

For the full article on the CIBSE website click here.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External references

  • (1) The Soft Landings Framework published by BSRIA and UBT in 2009, drew upon this research work, subsequent case studies, and the activities of the user group hosted by BSRIA. Download it from
  • (2) G Nevill, So, how are you doing? Building Services, Building Services Journal, 32-37 (November 2007).
  • (3) The original Probe articles can be downloaded at

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