Post Occupancy Evaluation (POE) is “the process of obtaining feedback on a building’s performance in use after it has been built and occupied”. POE is intended to investigate how well a building achieves the needs of its occupants, and identify how improvements in the building design can be done. For the avoidance of doubt, this article discusses POE in terms of the energy efficiency of the buildings, a strand within the much wider POE. The merit behind POE in this sense is to evaluate whether a building performs as well as it was predicted to do when it was designed, and to what extent it has achieved this in reality.
POE helps to provide data on how buildings perform in reality, which is often much worse than how it is predicted when designed. This is known as the performance gap, something which has been illustrated clearly in academic research. Research carried out by Johnston et al., (2015) used coheating tests (tests that measure the heat loss coefficient, an assessment of the as-built performance of a building’s fabric) across 25 new build dwellings, built to Part L1A 2006 or better. Their results indicated that the whole-building u-value was just over 1.6 times greater than was predicted, clearly having negative implications for the energy usage and associated emissions for these buildings.
The performance gap between predicted and measured energy use can result from many different causes, such as poor design and technical specification in the design stage, low quality of management and workmanship in the construction and handover phases, and differences between standardised operating conditions in compliance modelling and the actual operation of buildings.
In the UK, the energy performance of buildings is achieved through compliance modelling, whereby the energy use (and associated carbon emissions) is produced through thermal modelling with standardised operating conditions determined by national calculation methodologies. As a result, it is highly likely and, in the views of some, inevitable, that a performance gap emerges; this is defined as a regulatory performance gap, given it comes from the use of software used to demonstrate Building Regulations Compliance (Part L) and its standardised assumptions.
There is a wide variety of POE methods that have been utilised in the past, ranging from questionnaires designed to quantify the views of occupants, use of indoor environmental quality measurements (e.g. thermal comfort, indoor air quality, and lighting), energy auditing and external envelope studies.
External envelope studies help determine the heat loss that is occurring through the fabric of the building, and also identify areas of concern with the construction methods used.
U-values, the thermal transmittance of a building element, are difficult to theoretically calculate in existing dwellings, as materials often used in buildings are not homogenous, and their conductivity values are not available. Furthermore, establishing the exact dimensions of layers of materials requires destructive methods, and as such is undesirable and often not possible. U-values for building elements of existing buildings can be obtained by “measuring the heat flow rate through an element with a heat flow meter, together with monitoring the temperatures on both sides of the element under steady state conditions”.
Thermal imaging can be used to identify areas of concern with the building, and is commonly used to identify thermal bridges and ventilation losses that are greater than would be expected (poorly sealed windows, for example).
Co-heating tests provide a measure of the whole house heat loss in an unoccupied dwelling. They are performed by elevating the internal temperature through a test period and measuring the total heating power input required to do so. The heating power needed to maintain the temperature difference between inside and outside is used as an indicator of the performance of the building envelope. Difficulties with this method arise due to the fact that during the test the building shouldn’t be occupied, and any appliances are switched off so that the energy demand of the heating can be accounted for as the sole user.
If correctly implemented, POE offers a continuous cycle of evaluation and improvement, ultimately resulting in the improvement of the quality of buildings for the end-client. POE should take into account as many areas as is feasibly possible to ensure that the final building meets as many of the occupants demands to the highest level. It is also a crucial step in enabling the UK to meet its goal of achieving Net Zero Carbon by 2050 by calculating more accurate energy usage, and therefore emissions, of our buildings.
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Article published 10/06/21