Healthy results for Southmead

Southmead Hospital in Bristol has demonstrated its energy efficiency credentials, through a programme of air tightness testing conducted by BM TRADA.

The £425 million Southmead Hospital project is one of the largest privately funded construction projects in the UK. Built by Carillion, the building has taken some 44 months to construct. Covering a total footprint of 115,000m2, the state of the art hospital offers 800 acute beds, 24 state-of-the-art theatres, a helipad for the emergency services and an accident and emergency department capable of coping with large numbers of patients at a time

Although the scale of the building led to a number of challenges in undertaking mandatory airtightness testing, the hospital was shown to achieve air permeability ratings well under the minimum level of 10m3/h/m2, required by Part L1A of the Building Regulations for England and Wales.

Why airtightness testing?

With sustainability a high priority for the design team, ensuring excellent energy efficiency for the building was a key goal for the project and limiting air leakage was seen as essential to achieving this aim.

“Our vision is for Carillion to be the leading sustainable construction and support services company in the UK,” says Carillion Project Director Keith Hutton.

“We have defined six positive outcomes which form the basis of out Sustainability Strategy. These include enabling low-carbon economies, protecting the environment and supporting sustainable communities.

“The Southmead hospital is currently our largest construction contract in the UK, and targeted to be the most sustainable development of its type in the country.”

BM TRADA Head of Operations Tom Gregory explains why airtightness testing is important in achieving energy efficiency:

“Air permeability is a measure of the infiltration of conditioned air into the building or the loss of conditioned air from inside through gaps, cracks and holes in the building fabric.”

“The loss, cooling or heating of this conditioned air through uncontrolled ventilation affects the energy consumption of the building, since when air leaks from buildings additional energy is required to maintain temperatures.

“By limiting the leakage of conditioned air from buildings, it is possible to reduce energy consumption and costs.”

The challenges

Testing a building of this size and complexity is not a simple process. BM TRADA worked closely with the design team, building control and the site management team to decide how the hospital would best show compliance with the Building Regulations Approved Document L1A, Conservation of Fuel and Power.

Inspections and detailed reviews of the design were carried out for almost a year before any part of the hospital was tested. The sheer scale of the building meant that the building had to be tested in sections in order to prove compliance.

This presented a real challenge for the site team since testing in sections meant that it was essential to ensure internal walls were air tight, as they would be relied upon during testing. This is not normal practice.

Fortunately, the design of the hospital lent itself to being divided up into areas that minimised the use of internal walls as the air barrier. Testing of multiple wards at one time allowed BM TRADA to ensure that around 90% of the air barrier was formed from external elements of the construction.

Conducting the tests

BM TRADA completed the first set of air testing in January 2013. The objective of the tests was to measure the volume of conditioned air escaping through the building envelope via uncontrolled ventilation at an induced pressure difference of 50Pa.

The tests involved setting up multiple fans in multiple locations to pressurise the building. Numerous pressure difference readings and fan flow rates were recorded, taking into account environmental conditions.

“Put simply, the fans are used to blow air into the building to create a positive pressure difference of 50Pa when compared to the external pressure,” says Mr Gregory .

“Unfortunately, air does not behave in a uniform way as air density is affected by temperature and barometric pressure; both factors that can change rapidly in tall buildings as heat naturally rises. Furthermore, the pressure difference created must be uniform across the entire section of the building being tested.”

Measurements were taken in over 20 places for each test to ensure that that the air pressure was increased across the entire volume of the building, and not just in localised areas, since this could give rise to false readings.

Against the mandatory requirement of less than 10m3/h/m2, the area weighted average of all tests conducted was an impressive 8.41, meaning that the client was able to apply to Building Control for a Completion Certificate, without conducting any remedial work.

BM TRADA Senior Test Engineers Stephen Hayden and Scott Bartholomew, who between them carry a combined 14 years of experience, oversaw the project.

“This is both one of the most technical sets of testing we have carried out and, without doubt, one of the most enjoyable. Carillion’s site team are a credit to their company and made us feel welcome at all times,” recalls Mr Hayden.

Praising the Carillion project, Mr Bartholomew adds:

“From day one, Carillion listened to our expert advice which made the whole process smoother. Their ability to listen, understand and do is known throughout the industry.”

Commenting on the service provided by BM TRADA, Mr Hutton says:

“By working closely with the BM TRADA team we were able to overcome the challenges presented by this large and complex project.

“The expertise of the engineers was invaluable in helping to plan for the testing process, enabling us to conduct the tests with minimum disruption and achieve air permeability ratings well under the levels required.”

For further information on air and acoustic testing at BM TRADA, contact testing(Replace this parenthesis with the @ sign), 01494 569800 or visit