Part L2A is nonsense
August 1st 2008

Monodraught managing director Professor Terry Payne argues that the potential of natural ventilation as a sustainable alternative to energy-hungry air conditioning is in danger of being stifled by the ambiguous message of Part L2A

In spite of the benefits of Natural Ventilation and the fact that we are all being encouraged to reduce our carbon footprint to meet the Energy Performance of Buildings Directive, the efforts of many are being constrained by the current requirements in Part L2A of the Building Regulations where SBEM calculations favour air conditioning over naturally ventilated buildings.

This is at odds with the recommendations of the Carbon Trust, which has emphasised through a series of documents the need to increase the use of natural ventilation and passive cooling to avoid the high carbon dioxide emissions associated with air conditioning.

In its Technology Guide the Trust states that a typical air conditioned building has double the energy cost and associated CO2 emissions of a naturally ventilated building, and is likely to have higher capital and maintenance costs. It goes on to say: "In the UK there are relatively few days per year where the temperature is very high (over 28°C).Using comfort cooling for this short time can cost as much as a whole year's heating." Surveys have shown that building occupiers prefer an open window or naturally ventilated environment,and even where air conditioned spaces are provided,people often open windows and doors when they can.Once installed air conditioning also tends to be operated well beyond the summer season, often to compensate for poor control of excessive heat gains.

Evidence shows that the implementation of Part L2A 2006 of the UK Building Regulations appears to be biased towards air conditioning rather than natural ventilation and passive cooling.And this applies even when it can be demonstrated that an equivalent naturally ventilated building has a lower carbon footprint than an air conditioned building! The main reason for this is that air conditioned buildings must demonstrate an approximate 28% reduction in CO2 emissions and naturally ventilated buildings a 23% reduction, but this is relative to equivalent 'notional'buildings quoted in Part L2A being a 2002 building.

Although this appears at face value to support natural ventilation, its reduction factor is difficult to achieve because the energy consumption of a 2002 naturally ventilated building was already substantially less than an equivalent air conditioned building. In fact, the 28% reduction applied to an air conditioned building will still result in a CO2 emission greater than that of a notional naturally ventilated building of a current design.

Put simply, this means it is far easier to meet the carbon dioxide reduction target set by air conditioned buildings than it is for naturally ventilated buildings.

This 'notional' approach is also inconsistent with the European Energy Performance of Buildings Directive (EPBD), which requires CO2 reductions to be compared against a single reference value. Indeed, specifiers report that because Part L does not permit the use of passive methods, they cannot achieve the high targets set by the EPBD! None of this bodes well for the UK's 2020 target for zero CO2 emissions in commercial buildings, which is hardly likely to be achieved by increasing the nation's air conditioning load! To resolve the ambiguities of Part L, the CIBSE Natural Ventilation Group, of which Monodraught is a member, is putting forward to government its representations,prepared by Dr Martin Liddament.For all of our sakes we are determined to succeed in resolving this matter because the benefits of wind driven and indeed, solar driven natural ventilation systems are too important to ignore.

It should be agreed that at no time should a naturally ventilated, passive building be rejected when it can be shown to emit less carbon dioxide and be more energy efficient than an equivalent sized mechanically ventilated and air conditioned building that has been deemed to satisfy the current Part L Building Regulations. What doesn't make sense is the ambiguous message implicit in Part L2A, which suggests that energy-hungry air conditioning can save the planet.

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