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Linking Energy Performance Certificates and occupant comfort in homes

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What are energy performance certificates?

In the ever-increasing demand for sustainable buildings, energy performance certificates (EPC) provides an important function for potential occupiers. They provide an energy efficiency rating for both domestic and commercial buildings expressed as numerical values between 1-100; these are represented in alphabetical colour-coded categories G to A. Band A is the most efficient. EPCs are required whenever a property is built, sold or rented.

EPCs evaluate important environmental properties like thermal transmittance through walls and floors, which convey how efficiently a building is performing, and thus could indicate how healthy it is for occupation.

The EPC format

EPCs contain two sections, a graphical representation of the energy rating for the property and the written recommendations. The recommendations provide an improvement rating which could apply to the property if the suggested changes are made.

EPC Assessment criteria

The EPC lists the U-values of various building elements calculated through a visual inspection and assumption of the building services, building fabric and age of the property. It is not an in-situ assessment. Instead, it makes assumptions on occupancy and behaviour from conventional data.

Because of the complexities of thermal transmittance influenced by weather and heterogeneity within materials, U-values can vary considerably. This means that a factory tested insulation U-value will be different in practicality. Therefore the EPC is a theoretical rating because it is not derived using actual energy consumption data or U-values.

While EPCs will either overestimate or underestimate environmental performance, they are a loose indicator of how comfortable a building may be. They could be important for new build homes if the WELL Building Standard or other certification schemes are not used to predict aspects of comfort for potential occupiers.

EPCs as indicators of comfort in buildings

For new build homes, comfort and health certification are not compulsory. Despite the assessment criteria for EPCs not being accurate, they provide a general indication of environmental performance and thus comfort.

The EPC is supposed to be a rating of energy performance, but for the average homebuyer, it may be assumed as a comfort metric. Especially as aspects like low wall U-values indicate to buyers that the property is better insulated and warmer during heating periods.

The health of UK homes and EPCs

The average EPC rating for dwellings in England and Wales is band D. Approximately, 21% of homes were built pre-1919 with solid wall construction. These homes are prone to high infiltration rates and are far below Part L guidance.

The majority of EPCs generated are far below energy efficiency standards, meaning that many homes experience issues such as drafts, thermal bridging and high heat fluxes. In these cases, EPCs are a proxy for poor building performance, and this is often correlated with low levels of occupant comfort and well-being. Inadequate building performance will signify poor building health.

Since EPCs do not properly evaluate environmental features many homes have inaccurate ratings. Due to the variations that can occur in common building materials, assumed U-values can be a lot higher. Overestimation of U-values means that many homes are below the average D rating. Such homes often have draughts, damp and cold bridging; all problems associated with unhealthy buildings.

Whilst EPCs are not intended as a predictor of the comfort of buildings, they do convey important energy efficiency information; they also aim to evaluate building elements which affect the quality and comfort of occupants and thus indicate the health of the building. This means it may be necessary to examine the reliability of EPCs and what they communicate.

Disclaimer: The sole responsibility for any views expressed lies with the author(s). Any opinions shared do not necessarily represent the views of Saint-Gobain.

Bianca Bryan
Bianca Bryan Graduate of Environmental Design and Engi...
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