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Demystifying metrics

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Because sustainability metrics are often difficult to grasp, the AJ brings you a guide to promote better understanding, says Hattie Hartman

One aim of the AJ Bridge the Gap campaign is to promote a better understanding of sustainability metrics. To that end, the AJ has asked architects and engineers who are leading the way in sustainability to help clarify the metrics designers need to understand. The results are presented on the following pages. While the metrics may be familiar to many, unpicking them reveals gaps in understanding and helps establish a common vocabulary for discussion.

The experts unanimously cited an understanding of a building’s primary energy demand as critical. Primary energy demand includes both ‘regulated’ energy loads included in the SBEM and SAP calculations required for Building Regulations, as well as the ‘unregulated’ loads, such as IT, catering and plugs. While some argue that unregulated loads are beyond a design team’s control, since they are not part of the base building, they are a significant part of the performance gap. Early discussions with clients can highlight where the major unregulated loads in a project are likely to be and identify strategies to address them.

The need for common metrics for understanding the environmental impacts of buildings is globally acknowledged. Both the 2010 protocol developed by the United Nations Environment Programme’s Sustainable Buildings and Climate Initiative and the Sustainable Building Alliance’s Framework for Common Metrics of Buildings 2010 are great resources. Initiatives on the ground are needed to bring common sustainability metrics into daily practice in the UK. The CarbonBuzz platform, relaunching at City Hall next week and the RIBA awards programme’s requirement for sustainability metrics are steps in the right direction.

It’s easy to become obsessed with metrics and some, such as robustness in a changing climate, are difficult to quantify. While the AJ advocates greater fluency in metrics, they are not a substitute for informed passive design at the earliest project stage and throughout the design process.

Bridging the terminology gap

Bridge the Gap

Source: Hanna Melin

Energy versus carbon emissions

KWh//year or kgCO2//year

KWh/m² is the measure of a building’s energy use measured by floor area. To be transparent about building performance, kgCO2/m²/year must be reported with any renewable energy produced on site. The Passivhaus primary energy requirements are120KWh/m²/year. KWh/m²/year can be converted into kgCO2/m²/year, but this may hide an inefficient building fabric offset with on-site energy production. Architype’s Jonathan Hines observes that the UK is ‘obsessed’ with carbon: ‘You can reduce kgCO2/m² of a building by changing the heat source or adding renewables, but you end up with a building that consumes too much energy and just offsets carbon.’

Bridge the Gap

Source: Hanna Melin

Embodied carbon


The embodied energy of a building is the total carbon released over its whole life cycle. This includes the carbon included in the extraction of raw materials, as well as their manufacture and transportation to the site. To calculate the embodied energy of a building, it must be broken down into all its constituent parts: the foundations, structure, cladding, roofing and internal fitout, and so on. Whole life cycle analysis also includes maintenance through a building’s lifetime and its eventual dismantling and disposal. Benchmark figures for embodied carbon are rare. An Atkins study in 2010 suggested a best practice figure of 845kgCO2/m² for a university building.

Bridge the Gap

Source: Hanna Melin


ach@50Pa or m³/hr/m²@50Pa

Airtightness affects both energy consumption and the comfort of occupants. Building Regulations define airtightness based on the permeability of the external envelope: the resistance of the building fabric to inward or outward leakage in terms of air moving through 1m² of fabric at a default air pressure standard. The Passivhaus Planning Package (PHPP) measures airtightness in terms of air changes per hour, taking into account the internal volume of air to be conditioned and allows buildings of different sizes to be more easily compared. The Part L requirement is 10m³/hr/m²@50Pa. This is approximately a factor of 10 poorer than the Passivhaus standard.

Bridge the Gap

Source: Hanna Melin

Indoor air quality


Indoor air quality refers to the air quality inside buildings and relates directly to the health and well-being of building occupants. As buildings become more airtight, indoor air quality becomes increasingly important.

Filtration and ventilation are the primary methods for controlling indoor air quality. Lynne Sullivan of Sustainable BY Design explains that CO2 concentrations are useful as a proxy for good or bad internal conditions. Also the recommended upper limit of 1,000ppm is easy to remember!’

Bridge the Gap

Source: Hanna Melin


Litres/person/day or Litres//yr

Water consumption is generally measured in litres/m²/day. Building Regulations Part G recommends 125litre/person/day for new-build domestic use. Poor building design squanders water as much as occupants; an alternative is to look at consumption in litres/m²/year.

Architects specifying water-efficient fittings need to examine the associated carbon uses of other water-saving technologies. Water harvesting and recycling technologies may save water, but need to be looked at holisticly, because they increase energy use and have their own embodied carbon.

Bridge the Gap

Source: Hanna Melin

Occupant satisfaction

Post-occupancy surveys

The best way to understand how a building works is to engage with its occupants. This can be done informally or more formally using established or bespoke techniques. The BUS Methodology, developed by Adrian Leaman and Bill Bordass and later acquired by Arup, has an established track record and is one approach to quantifying occupant feedback. Surveys are measured on a sliding scale of 1-7 by users of the building and cover topics from thermal comfort and lighting to noise, personal control and perception. Results are benchmarked against 700 surveys in the BUS Methodology database.


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