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The history of non-domestic airtightness testingFebruary 2015

Written by Tom Jones, Technical Manager Airtightness, BSRIA Compliance.

Tom Jones has many years experience in design reviews and testing for air leakage

In the last century when consultants were calculating the heat load of a new building they would use design data provided by the CIBSE guide with respect to the average air infiltration or natural ventilation of a building. These values were hypothetical and were widely used with the HVAC industry. A survey of the ventilation rates of modern housing had just been completed, and it was considered that similar information would be useful for commercial buildings.

In the 1980’s BSRIA undertook a project to develop a method of measuring the natural ventilation of large buildings. Initially this was confined to single cell buildings (factory/warehouse type buildings) and then extended to large multicellular building such as offices. The results of this research indicated that the actual air change rate of a building may be greater than those suggested by CIBSE.

However the database was very small, and insufficient to make any definitive conclusions. The technique itself did not lend itself to large scale surveys of infiltration rates within buildings. Typically for a single cell building it would require about 2 man days to install the equipment, typically 8 – 10 hours to undertake the test, then a further day to dismantle the sampling tubes. During the test procedure large quantities of tracer gas would be used, which had to be transported around the country, and at the end of the 4 days one measurement would have been recorded, and this value was a function of the building, internal – external temperature difference, and wind conditions (both speed and direction). While it provided some data, it was difficult to use to compare different buildings. It was therefore obvious that a different method was required.

During the measurement of ventilation rates on domestic dwellings, the air leakage using a pressurisation technique had been used to compare different dwelling types. It was therefore a natural progression that this technique to be developed for measuring the air leakage of larger building.

Unfortunately during the development of the tracer gas technique, it was quickly appreciated that significant air flow rates would be required, and any equipment would need to readily transportable, capable of attaching to a double doorway, and self-contained (it was considered unreasonable to expect to attach to a building’s power supply, given the size of the fan motor required). Then in a moment of inspiration, Nigel Potter the lead engineer, remembered from his college days that Land Rovers were sometimes used to power agricultural machinery using a power take-off. Further investigation however indicated that the power used by this machinery was much less than the power we required for the proposed mobile fan. After some discussions with Land Rover they agreed to upgrade the power train, with the then largest engine for that vehicle, a 3500cc petrol.

However this was not without its tribulations, the Power take-off shaft was re-routed to emerge at the rear of the vehicle, which was where the petrol tank was located. Two tanks were therefore installed under the front seats of the vehicle, which was where the battery was located, so this was moved to beneath a bench seat at the rear of the vehicle.

Multiple fan rovers can be needed for testing the very largest buildings

Those of us that had the pleasure of filling both tanks got used to pulling up at a petrol station, removing the seat pads before prostrating one-self across the inside of the vehicle with a petrol nozzle, while trying to ensure there were no drips of fuel within the interior of the cab. Although ironically given the ventilation of the cab these fumes did not last very long, as during the cold winter days it was not unusual for the engineers to travel with their coats on, and the heater at its maximum output.

Eventually both fan and vehicle were ready for use, and the BSRIA laboratories were used as a trial. During one of these trials a representative of Land Rover attended, and appeared quite content until they looked beneath the vehicle, only to observe a red hot exhaust.

After some modifications the vehicle was ready, and a survey of 12 industrial buildings was carried out. This survey confirmed the earlier finding using the tracer gas results that the magnitudes of the natural ventilation rates within this type of building were generally greater than those suggested for sizing heating etc. Upon completion of this survey a method of comparing the air leakage of different sizes of buildings was required.

Various methods of normalising the air flow rate required to pressure a building to 50 Pa were investigated, and after some deliberation the surface area of the walls and roof was used and the introduction of what is now known as air leakage index conceived. One of the reasons this parameter was chosen was that most commercial buildings have concrete floors, which was surmised would have a very low air leakage, therefore the air supplied would be egressing from the rest of the structure. In devising this parameter, there was never any distinction made between a ground bearing concrete slab, and a ‘suspended’ concrete slab.

The natural ventilation rate was predicted using a computer program developed by BSRIA in the 1970’s called CRKFLO.

A second survey was commissioned to investigate the air leakage of office buildings, and again this proved that quoted natural ventilation rates were generally less than those suggested. One person described it as a lottery as to whether the building constructed would have reasonable airtightness, or an airtightness 3 or 4 times the current limiting factor of the current Building Regulations. A second survey was commissioned by BRE, for office buildings, increasing the database. This further reinforced the existing data that the magnitude of the air leakage of office buildings (and hence the natural ventilation rate) was generally greater than design values quoted by CIBSE design information. Upon completion of this last survey, this technique ceased being a purely instrument of research, and was used commercially.

Initially this was to assist in identifying problems within a building, normally where the heating was failing to maintain the required air temperature. Around the same time a major retailer was interested in the removal of revolving and bi-parting doors at their stores, and introducing methods by which customers could enter their stores more efficiently, and with less irritation. To accomplish this, buildings with a low air leakage were required.

HVCA consultants also started to appreciate the benefits of buildings which they knew were not going to have excessive natural ventilation, and started specifying air leakage rates for buildings. At this stage reduction in energy was not the main priority, but rather ensuring buildings would perform within their required parameters. With respect to retail, other companies within this sector started to see some of the benefits of stores with lower air leakage rates and started specifying an air leakage rate for their new stores, and the number of tests required increased. A legacy of this is that some companies still specify their air leakage criterion as an air leakage index.

With an increasing demand it was time to build a Mark ll Fan Rover, the trailer was re-designed so much such that it no longer had to be disconnected and turned around to attach to the building; the Land Rover was changed to a 4 litre Land Rover Discovery, which meant that during the winter months overcoats did not have to be worn while driving and refuelling was a more dignified occasion.

In 2002 air leakage testing was included within Approved Document Part L2A for non-domestic buildings, but this resulted in only about 30% of new buildings being tested. In 2006 it was included within the Regulations themselves and it became mandatory for tests to be undertaken on all new non-domestic buildings and a sample of new dwellings. The parameter chosen was air permeability, i.e. including the ground bearing floor area.

The Regulations have certainly had an effect – reducing permeability in non-domestic buildings from 30 – 40 m3/m2/hr to a typical 4-5 m3/m2/hr. Significant carbon and energy have been saved, engineers have the reassurance that their intent is being met in their design and occupants are more comfortable.

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