Open-plan aircon uncloaked article 02: How building airtightness affects open-plan aircon
EWAN CAMPBELL, technical manager at Air Barrier Technologies, explains how building airtightness affects the conditioning of open-plan spaces.
A permeability rate is the volume of air leakage that a building fabric allows per square metre every hour. Commercial buildings all have some sort of fresh air delivery based on minimum fresh air requirements per person. If delivered through air-handling units, this is hopefully filtered to some degree and mixed with conditioned air before entering the space. The unintended leakage of outside air through the building fabric in an ideal scenario would be zero, as the fresh air requirements are already met by the HVAC system. Every cubic metre of infiltration is an additional, unnecessary load on heating and cooling requirements.
An average shopping centre may have an envelope area of 60,000 square metres, so at ambient building pressures, the volume of excess air requiring conditioning can be around 500,000 cubic metres per hour. You don’t have to be a mechanical engineer to imagine the size of the HVAC plant required. Improving a building’s permeability rate will assist in solving open space conditioning issues.
Another challenge leaky buildings present is the short circuit. The short circuit is when the conditioned air comes in through a register and then goes straight out through a nearby hole in the building fabric. Conditioning a leaky building with short circuits is like trying to fill a bathtub without the plug in. The ‘fresh’ air quickly exits the building without reaching the whole space. This creates a ‘dead zone’: a hot, stuffy corner that has higher than recommended carbon dioxide levels.
In the UK, the maximum permeability rate allowed for a commercial building is 10 and best practice is considered three to five, depending on the end use of the building. Australian buildings typically have permeability rates of around 20 to 40, due to the country’s more relaxed attitude to building airtightness. Suspended tile ceilings have been tested to have permeability rates as high as 80.
Unsealed window frames, skylights and recessed troffer lighting fixtures are other common culprits of short-circuiting. If you are trying to condition an open space with high ceilings with ceiling mounted registers, then the chances of short-circuiting and stratification are even higher.