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Does a building lose its airtightness over time?April 2012

Roderic Bunn presents the results of an airtightness test of the Elizabeth Fry Building, 14 years on.

Does a building lose its airtightness over time? Logic would suggest that it would, as seals deteriorate and differential movement and shrinkage create new air gaps in the fabric. Buildings age, and arguably they should be forgiven if they can't hold their breath as well as they used to.

In any case, is such degradation really that significant? There will always be other reasons for a building's energy consumption to gradually increase. Partial changes of use, ageing building services, and more electrical equipment such as laptops and vending machines all play their part in a gradual reduction in efficiency. It reaches a point where decline in a building's systems can only be arrested by a significant retrofit.

What better way to test this hypothesis than to re-visit one of the exemplar low energy buildings of the 1990s - the Elizabeth Fry Building at the University of East Anglia. The Elizabeth Fry Building was lauded at the time for being one of the "best buildings ever," an epithet coined by Building Services Journal less as a statement of truth and more in relief that a post-occupation evaluation of a building with low energy aspirations seemed to have delivered on its promises. In many ways Elizabeth Fry was the construction industry's Higgs Boson - rare proof that it was possible to build both an energy efficient and highly comfortable and usable building. (The puzzle has been the industry's inability to repeat the exercise on a regular basis, but that's another story.)

The rear south-facing elevation. The compriband seals to the metal profiled cills were found to have perished, creating a new air leakage path. The lobby in the background gives access to the air handling plant room and lecture theatre doors.

To test whether the building's exemplary airtightness has stood the test of time, BSRIA was invited by the University of East Anglia (UEA) and builder Willmott Dixon to conduct an airtightness test nearly 14 years after that carried out for the PROBE series of building revisits.

Elizabeth Fry - the building

Elizabeth Fry is a four-storey rectangular building of gross internal area 3250 m2 and a treated floor area of 3130 m2. Its principal elevations face almost north onto a main campus road, and south onto a courtyard car park.

Back in 1994, way before airtightness testing was mandated in Part L of the Building Regulations, the building's architects John Miller + Partners and services engineers Fulcrum Consulting set out to optimise thermal insulation and minimise air leakage. The walls were insulated with 200 mm mineral fibre cavity fill, and fitted with tripleglazed, aluminium-clad timber windows with mid-pane blinds. The roof had 300 mm of insulation.

With a design heat loss of only 15 W/m2, two 24 kW domestic wall-hung condensing boilers provided all the heat required, with a third in reserve. Heating and cooling is entirely through the mechanical ventilation system and openable windows. Four air-handling units incorporate heat recovery. Two cross-flow units serve the lecture rooms, while more efficient flow-reversing regenerators were used for the offices and seminar rooms.

Following initial monitoring, six 200 W electric heaters were added in six rooms with exposed floors, corners or roofs.

Even by 2012 standards the building's air leakage index was impressive: the post-completion pressure test came in at 4.2 m3/(h.m2) at 50 Pa. BSRIA's re-test for the PROBE analysis in early 1998 reported a slight deterioration in performance of 6.53 m3/(h.m2). This reduction was attributed to air leakage at the stairwell roof at both ends of the building, an access hatch to the roof, and leakage around windows and door thresholds. Nevertheless, the result still compared extremely well with airtightness standards of the time.

BSRIA’s Tom Jones delivers pressure test results of the Elizabeth Fry Building to Dr Bill Bordass of the Usable Buildings Trust and Martyn Newton of the University of East Anglia

The 2011 airtightness test

The UEA is leading a European Project, Build with CaRe, to help build capacity to produce low-energy buildings. While the team was exploring the history of Elizabeth Fry and subsequent Termodeck buildings on the UEA campus, it was decided that BSRIA would carry out a third pressure test to find out whether the building had become less airtight over time. This was performed on 18 September 2011, nearly 14 years after the PROBE test.

The test results need to be considered in the context of the building's current use. Since 1998, the uses of some internal areas have changed. The staff and student common rooms on the first and second floors have been converted to offices and seminar rooms. In 2008, the kitchen and dining area on the top floor was converted into a densely-occupied, open plan, post-graduate administrative office. As a consequence the kitchen plant was removed and the extract sealed off.

In summer 2011 the ground floor seminar rooms and their blockwork walls were stripped out to provide an administration centre serving a large number of faculties.

Pressure test results

Much to everyone's surprise, the 2011 test revealed the Elizabeth Fry Building to be more airtight than it was in 1998, achieving 5.32 m3/(h.m2) at 50 Pa.

The degradation of door and window seals, plus the deterioration of mastic seals between window frames and the blockwork, might reasonably have led to a higher air leakage rate. But little air leakage was detected. The mastic compriband seals between the cills and the blockwork had fallen apart and this had led to detectable air leakage.

BSRIA believes that the removal of the rooftop kitchen extract plant will have cut out a significant source of leakage. There is also a suspicion that ground floor lecture room ventilation plant may also not have been sealed off in 1998 as effectively as it was in 2011. In 1998, the temporary sealing of the air handling plant relied on the dampers, whereas in 2011 it was sealed with polythene and tape.

The accuracy of the airtightness of the building may be affected by some uncertainty in the envelope area calculations, as these are dependant on the accuracy of the drawings supplied.

Some of Elizabeth Fry’s areas have been converted to quite densely occupied office space, a use that was not intended

Nevertheless, what the air leakage index results show is that 14 years of wear and tear (and changes of use) have not altered Elizabeth Fry's high levels of airtightness. The test results from 1994, 1998 and 2011 can never be totally comparable, as there are many variables:

  • the accuracy of the testing equipment (which has improved in the intervening period)
  • the degree to which intended openings, such as air handling plant, can be sealed
  • and of course changes in the building's use.

The other question is whether Elizabeth Fry's exemplary energy efficiency has continued. Sadly, the building's halo has slipped a little.

Annual energy use in 1997 was 61 kWh/m2 for electricity, 33 kWh/m2 for gas and 4.2 kWh/m2 for domestic hot water. In 2008, electricity consumption rose to 72 kWh/m2, but data after that are unreliable owing to metering faults. Gas consumption has risen to 35 kWh/m2, plus 11 kWh/m2 for domestic hot water that now runs permanently at 55°C (65°C on Sundays). Much of the consumption is thought to be due to standing losses, and some uninsulated pipework.

Lighting and office equipment energy use have gone up owing to the increased occupancy levels. Room occupancy has also increased and the staff and student common rooms on the first and second floors have been turned into offices and seminar rooms. In 1997, the building contained 70 office workstations. In 2010, there were 120.

BSRIA provides airtightness testing and consultancy as part of its package of building compliance services. For more information contact BSRIA Airtightness:

T: 01344 465615

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