Glenn Massey advises engineers where to focus their energy

A building’s environmental performance is now high on the checklist of potential tenants. Lower energy bills are a powerful attraction. Recent work by Hoare Lea has shown that focusing on key elements can make a big difference to performance.

EPCs were introduced by the government to inform potential buyers and tenants about the energy performance of a building. Ratings are from A to G: A is very efficient and G the least; the better the rating, the lower fuel bills are likely to be.

An EPC is based on elements including the building fabric, its air-tightness and the building services installed – the types of heating, cooling and hot water services, for example, and the lighting and lighting controls, plus any renewable energy systems or low- to zero-carbon technologies. Some of these have more influence on the energy banding than others.

Knowledge of how the banding is weighted is useful for those carrying out EPC surveys as more time can be allocated to extracting data on the factors that have the greatest influence. It is also useful for property owners that want improved energy performance ratings because it will allow them to focus resources on the elements with the biggest impact.

In the past year, Hoare Lea has completed more than 1000 EPCs in the UK. Most of the properties assessed were existing buildings and the largest proportion fell into the D-rating band. This is in line with the government’s predicted energy performance of the existing building stock (Figure 1 shows the ratings for a sample of 200 existing commercial buildings).

The influences of different types of data on the EPC rating can be demonstrated by overlaying changes on the government’s simplified building energy model (SBEM). This software tool analyses compliance with the Building Regulations by calculating the annual energy use for a proposed building and comparing it with a notional building. This system is familiar to most engineers who undertake EPC calculations. The following examples demonstrate which elements of construction have the greatest influence on the EPC rating.


The date when a property was constructed is important because it identifies which version of the Building Regulations it had to be in compliance with and hence the U-values and thermal capacity of the different elements. The SBEM has links to a database with Building Regulations compliance values back to 1965.

How does the EPC rating change when comparing the SBEM example building for the 2002 Building Regs with a 1965 property? The SBEM example has a score of 63, which puts it in the C band rating of 51 to 75. Changing the fabric values to Building Regulations 1965 compliant gives a score of 69. So, even though the building is 37 years older, it still gets a C rating EPC.

The difference stems from the thermal transmittance and thermal capacity of the fabric. It illustrates a small step change. So, if the information on the original drawings or lease documents gives only an approximate indication of the build date, the assessor can be satisfied that the EPC result should not be adversely affected by using a conservative estimate on the age.

Air infiltration

It is rare for existing buildings to have an air-tightness certificate. In the current version of the SBEM, the default setting has been increased from 10 to 25m3/h/m2 at 50Pa. This rise appears fair since older buildings are far more likely to be leaky.

Figure 2 is based on the SBEM example building, with just the air leakage results changed. A leakage rate of 8m3/h/m2 puts the building into the C band with an EPC rating of 63. If air-tightness is improved and leakage reduced to 4m3/h/m2,, the EPC result of 62 still falls within band C. Even if the rate is changed to 25m3/h/m2, the rating only moves to 64. This shows that reduced air leakage has little impact on EPC banding.

Even if an assessor suspects an older building is likely to be more airtight than the default value of 25m3/h/m2 at 50pPa the improvement in EPC rating may not justify paying to have an air test.

Heating and cooling

Engineers undertaking EPC calculations need to spend time looking up efficiencies of chillers and boilers. They should gather data from nameplates and O&M manuals and then search the Carbon Trust Energy Technologies List for efficiencies of chillders and boilers. Listed items have been tested and the data is reliable. For unlisted items, engineers should contact the manufacturers, which can often provide documented evidence of their products’ efficiencies.

In 1998 there was a big step change in boiler efficiencies in the UK. In the absence of data from the Energy Technologies List or manufacturers, engineers can use a boiler efficiency appropriate for boilers manufactured after 1998.

The SBEM building has a chiller with a seasonal energy efficiency rating (SEER) of 2.5. For a chiller with a SEER of 4, the rating would improve from 63 to 56, but remain a C. If the SEER were 2, the EPC rating would slide to 68 – still a C.

It is important to note that the EPC calculation tool imposes a penalty on air- conditioned buildings to discourage its use. So if the existing building has air- conditioning, and the internal environment would be compromised by its removal, the only option is to optimise the SEER of the chiller. However, the building would still be at a disadvantage in EPC rating compared with a non-air-conditioned building because of this penalty.

The results are similar for changes to the heating system. If an older boiler with a seasonal efficiency of 65% is replaced by one with a seasonal efficiency of 94%, the rating improves from 67 to 59. Quite a dramatic change, but the EPC rating remains a C.

Improving the efficiency of a single item of plant such the chiller is, then, unlikely to change a building’s EPC band. Replacing several plant items should enable a move to the next EPC band. It is, though, fortunate that upgrading the boiler and chiller can have a significant impact as a landlord expects to replace them several times during a building’s lifetime, and will have budgeted for this. The landlord is not so likely to have made the same provision for major work on the building fabric.


This has an important role to play in the EPC rating. Using the SBEM example building and changing the lighting to T5 from T8 standard ballast fittings has a positive effect by reducing the EPC score from 63 to 56. In contrast, changing the lighting to tungsten halogen sees the building’s score rise to 220, which means a G rating.

Control technologies such as daylight and occupancy sensors also influence a building’s energy score but not as heavily as the type of light fitting.


Few existing buildings have integrated renewable or low- to zero-carbon technologies. These can have a positive influence on EPC calculations if they produce a significant reduction in energy demand. Token “greening” will not alter an EPC rating because the energy produced will not offset sufficient grid energy.

dec sample

Display energy certificates show how much energy a building actually uses. They assess consumption as an operational rating on a sliding scale from a meagre A all the way to an energy profligate G.

The benchmarks in the DEC calculation tool are challenging. This is a good thing as it puts everyone on an equal footing and underpins the aspirations of the Energy Performance of Buildings Directive (EPBD) to reduce CO2 emissions attributable to buildings. 

The chart illustrates the results from a selection of 50 public sector buildings assessed by Hoare Lea. The organisation that operates the buildings has spent a number of years monitoring energy use and implementing incremental energy saving projects. This enabled 50% of the ratings in the sample set to be better than the typical 100 DEC rating.

However, there is room for improvement in the DEC scheme. One anomaly is that it is possible to get a good DEC rating in a building that has inefficient fixed building services by reducing occupation levels. A half-empty inefficient building can, for example, get a very good rating. It is also possible to get a poor DEC rating in a building that has efficient fixed building services but is occupied very densely.