Building evaluation: the decay method as an evaluation tool for analysing thermal performance
Creators: name: Law, Andrew
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 330204
subject: Building information modelling and management

description: There is growing evidence of a significant gap between the designed thermal performance and the actual thermal performance of residential buildings. As heating and cooling energy accounts for approximately 40% of a building’s total energy consumption, this poses a significant risk to carbon reduction policies and to achieving sustainable housing designs. The performance gap is primarily caused by incorrect assumptions across a range of variables in the predictive modelling, meaning there is a difference between what is designed and what is constructed. Differences in the thermal shell between model and reality can only be detected via in situ testing.<br><br>Evaluating the thermal shell as a whole is possible, but has its drawbacks. According to the literature reviewed, the most common in situ test that evaluates the building performance as a whole system is the co-heating test. However, the co-heating test is highly invasive, requiring between one and three weeks with an empty building. This severely limits its application in field, and means there is currently no sufficiently rapid or non-invasive tool for whole building evaluation of heat loss.<br><br>In response to the need for in situ testing that can be applied on a large scale, this thesis presents the ‘decay method’ as a means of evaluating the heat loss of a building in a non-invasive manner. The principle of the decay method lies in analysing the profile of overnight temperature decay. The use of overnight temperature decay allows the building to be used as normal at all other times, significantly reducing the impact on occupants. Four methods of analysing the temperature decay profile from the decay method have been developed for comparison:<br><br> - the calculated average method, using an application of Newtons Law of Cooling to calculate a decay constant for the building;<br> - the Excel Solver method, combining the Excel Solver add-in with the Law of Cooling to converge on a decay constant that provides the best match to the observed building temperature profile;<br> - the temperature Root Mean Square Error (RMSE) method, combining the experimental observations with a building simulation; and<br> - the measured heat pulse method, combining the temperature RMSE method with measured heat pulses included in both experimental and simulation processes.<br><br>Using a test cell building located on the CSIRO site in Highett, Melbourne, the four methods were compared against the co-heating test. The results of the case study showed that the Excel Solver method provided Heat Transfer Coefficient (HTC) estimates similar to the co-heating test, but with far larger uncertainty in the result. The measured heat pulse method provided similar estimates to the co-heating test with similar levels of uncertainty.<br><br>The case study proves it is possible to evaluate heat loss of the building as a whole, without requiring it to be subjected to a co-heating test. The success of the decay method shows that post-occupancy, in situ testing can be carried out without disrupting the occupants. Further work needs to be done to apply the decay method to multi-zone buildings and buildings of different construction types. Nonetheless, this represents a significant step forward in being able to determine the gap between design performance and actual performance, thereby improving the ability to minimise it.<br>