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There is no simple answer to the question 鈥渨hat makes a building sustainable?鈥� That is because sustainability is not a single entity that can be measured and labelled. Engineers like to think in terms of minimising energy consumption, while architects might fix on the embodied energy of building materials, transport planners on reducing car use and planners on ensuring health and education provision and employment.

The fact is that sustainability covers all of these things 鈥� and more. It is probably better to think in terms of sustainable development, rather than sustainable buildings. Sustainable development portrays the bigger picture, in which the buildings are a part of a montage of environmental, social and economic impacts (see 鈥淪ustainability criteria鈥�, below).

Should it be possible to recognise a 鈥渟ustainable building鈥� when you see one? The most obvious characteristics tend to be those that relate to low energy consumption, but these can be deceptive. For example, an office building with small windows and heavily insulated fabric may have a greater energy consumption than one with full-height glazing that reduces lighting use and energy consumption.

There is a tendency to 鈥渂adge鈥� buildings as sustainable by incorporating features such as photovoltaics and green roofs. Unfortunately this can lead to the inappropriate 鈥� and costly 鈥� use of technologies. For example, photovoltaics may be the most expensive method of incorporating renewable technologies into a building 鈥� typically 5-10 times the cost of other renewables per kilogram of carbon saved from the overall emissions. Green roofs, on the other hand, provide ecological niches, storm water attenuation and visual amenity. However, they take up roofspace that could be used for recreation or renewables such as solar panels.

A successful sustainable design will find a compromise between these sometimes conflicting factors, taking into account that a scheme that is not commercially viable can hardly be called sustainable. In today鈥檚 high-density and often high-rise urban developments, sustainability can be achieved.

The 42-storey Castle House development in Elephant & Castle, south London, is the first high-rise scheme in the UK to include integrated wind turbines

High-rise, high-density and sustainable

Because land values in the centre of London are high and planners are urging higher densities, most buildings are heading upwards. Tall buildings can be highly sustainable. By their nature they tend to be close to transport infrastructure, make excellent use of usually previously developed and/or contaminated land, reduce pressure on greenfield or suburban sites and create spaces that optimise use of natural light with potential for natural ventilation. On the negative side, they tend to use larger volumes of concrete than low-rise buildings. Because of their bulk, tall buildings can also have a greater visual impact, create over-shadowing and accelerate winds locally, although a fully integrated design and environmental impact assessment can mitigate these impacts.

Comparing a residential apartment building with a low-density housing development having the same accommodation and U-values, the heat loss for the former will be significantly lower than for the latter because the external fabric area is lower for the same floor area. Lighting use is likely to be greater for flats, but in winter this is likely to offset some of the heating load.

Historically, flats have tended to be heated using electric panel heaters 鈥� which are associated with carbon emissions some 2.5 times higher than those from gas heating. Because of this, carbon emissions from flats have tended to be higher than from a house with gas-fired heating of the same floor area. However, Part L1A of the 好色先生TV Regulations 鈥� which came into force on 6 April and uses carbon emissions to benchmark energy performance rather than U-values 鈥� makes it more difficult to have electric heating in flats.

Incorporation of renewable technologies is a challenge for designers of tall buildings on compact sites. The London mayor wants to double the proportion of energy generated by on-site renewable energy sources to 20% of the energy required to operate major developments. Developers are already finding the 10% requirement a challenge, as well as costly, and the risk is that too little attention and investment will go into low-energy design measures.

As can be seen from the table below (figures extracted from the London Energy Partnership Renewables Toolkit), the most cost-effective renewable technologies may well be biomass heating or wind turbines, while the cost of photovoltaic panels may be far greater. Where a building has a small footprint, there is also a lot of competition for space on the roof. Hence it may be difficult to accommodate roof-mounted photovoltaics or solar panels.

Similarly, although the top of a tall building is one of the best locations for them, wind turbines may suffer from a similar problem. But if done successfully the results can be stunning. Ground coupling, on the other hand, relies on heat exchange underground, providing the potential for carbon savings on both heating and cooling loads via boreholes or energy piles. With compact sites, the energy extracted will be limited by the number of energy piles or boreholes that can be accommodated.