New guidance has just been issued to help architects, developers and designers understand more about how the shape and form of a building affects heat loss, so they can reduce the energy consumption of new buildings at little or no extra cost.

This guidance is contained in a new report published by the research arm of the British National House-Building Council, called The challenge of shape and form. The mission of the NHBC is to raise the construction standards of new homes and provide guaranteed protection for homebuyers.

Much of the focus on improving the energy efficiency of buildings has to date been in connection with building fabric, insulation and new technology, but their shape and form can be just as important. By paying attention to this, developers can add value to homes and gain a competitive edge.

Mathematical models are used to predict the energy consumption of buildings. These models correctly reflect the importance of the form factor. The form factor is a measure of the compactness of a building in the form of a ratio of the external area of the building to the floor area. In short:

Heat Loss Form Factor = Heat Loss Area / Treated Floor Area

This ratio can be anything between 0.5 and five. A lower number indicates a more compact, efficient building.


Passivhaus buildings aim to achieve three or less. Once the form factor is over three, achieving the Passivhaus Standard efficiently becomes more challenging.

The early design choices, such as how many storeys a building has, what shape the plan takes, the form and massing, all impact directly on the building energy efficiency.

A building can have a fairly simple massing, but if it has a lot of recesses or protrusions in the thermal envelope, the surface areas soon add up. Less (thermal envelope) surface area means less surface area for heat to escape through. This is shown in the following diagrams.


Form factor and insulation

The form factor has an effect on the amount of insulation needed to achieve the same U-value (measure of heat loss: – the 2013 UK building regulations have limiting U-values of 0.2 W/(m2.K) for a roof and 0.3 W/(m2.K) for walls). If a large compact block of flats had a form factor of 1.0, an average U-value of only 0.28 W/(m2.K) would be required.

The following figures illustrate this:


Heat loss area and U-values have a linear relationship. If the heat loss area of one option is twice that of another option, the insulation will need to be twice as thick!

The problem in Britain

In the UK, the energy and carbon requirements of building regulations do not explicitly give credit for housing designs with lower heat loss areas or more efficient shapes that will reduce heating costs for the building occupants.

The UK’s national calculation methodology, the Standard Assessment Procedure, does give appropriate weight to the form factor in calculating heat loss. But when the basic results from SAP model are fed into the buildings regulations compliance methodology, which follows, the benefits of form factor do not register.

The current Building Regulations in the UK are therefore unable to provide an incentive for industry to design and build homes that have a more efficient type and shape.

The NHBC says designers who focus solely on building regulations compliance may not even realise that they can reduce the energy consumption of homes by changing the form factor.

Doing so can be a low-cost or no-cost measure. The NHBC is calling on the government to consider ways of encouraging designers and developers to take advantage of this effect.

The effect of form and shape

Even though the form may be compact, the building can still be architecturally interesting and provide better comfort conditions for occupants. It need not have to lead to bland or monotonous housing designs.

The guide discusses how the most inefficient design features can often be avoided or replaced by alternatives that are still architecturally interesting. Many designs can provide better comfort conditions for the residents as well.

The NHBC hopes that form factor will gain a “currency” of its own, and will be included among the key parameters that are tracked and discussed as a housing development design evolves.

This is already the case in countries where alternative design approaches are popular; the Passivhaus standard, for example, lists efficient building shape as one of the five key design considerations when planning a new energy-efficient building.

David Thorpe is the author of:

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  1. SO with you there Peter! ‘Sunshine and Shade in Australasia'(1963!) (now in its seventh edition) and a re-evaluation of passive solar design principles is the way to go in Sydney. The ‘Sydney School’ Mid Century Modern homes nailed light shade and capturing cooling breezes, where heating was not the major need for comfortable living. Design done right.

  2. Beware this misleading oversimplification. Compact volumes with minimal external surface are ideal in cold climates, hot climates and climates with diurnal extremes but are not universally desirable. In benign subtropical climates thin,attenuated east west plan forms with tall north and south wall surfaces maximise cooling shaded summer cross ventilation and warming winter solar gain can provide year round passive comfort without energy expenditure on heating or cooling. Only mugs would seal up their houses and rely on British insulated envelope theory on Queensland’s Gold Coast for instance. I write this from Heathrow Airport and am painfully aware that one man’s climate is another man’s misery.