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With energy prices rising year-on-year, there is increasing interest in how these costs can be reduced, especially in the existing social housing sector. If these older properties could be more thermally efficient and have excellent fresh air, the tenants would be cushioned from fuel poverty and have healthier home environments. The Green Deal has been rolled out and should be able to provide finance for this sector. The big question is, will the projected cost-savings actually be realised?

The calculation methodology for estimating the energy use, hence costs, in dwellings is via the Standard Assessment Procedure (SAP), developed by BRE and based on algorithms from the eighties. SAP is used for compliance with Part L and, in a cruder form, for a lower-cost method of assessing the performance of existing dwellings in a reduced form.

Reduced SAP is a blunt tool and therefore does not take into account the actual building location, shading, fabric or airtightness. It also has no capability to consider how people use their buildings or what the weather was like in a given year. Basing a prediction of how much energy a given Green Deal-approved improvement might make is therefore prone to being inaccurate.

Consider for a minute that if the weather is inclement, people tend to spend more time indoors using more energy than when the weather is pleasant. So if a Green Deal assessment is carried out in such a year, the estimated energy savings will be different than when compared to another year. Signing up at this point may mean that the bill payer will end up financially out of pocket.

There is little empirical data on energy use in dwellings and any changes which result from retrofitting. Equally, the skill of the contractors to carry out the works correctly has not been monitored effectively. The most reliable approach to reducing energy consumption in most buildings in the UK is fabric-first – loft insulation, double glazing and draught proofing; then there’s getting the ventilation right.

The Green Deal might be the best approach in certain cases but you need to do quite a lot of maths to see if it is right for a given building.

Rick Wheal is a consultant at Arup