Hsu, Yun-Sheng
(2021)
The impact of wind on the building airtightness measurement using the Pulse Technique.
PhD thesis, University of Nottingham.
Abstract
Ventilation plays a vital role in the energy performance and indoor air quality of buildings, which can also be influenced by the uncontrolled air leakage (i.e., air infiltration) across the building envelope. As a fundamental building property, the building airtightness is a crucial factor that should be addressed in order to improve the building energy performance, indoor air quality and building durability. Minimum building airtightness requirements have been included in building regulations and voluntary standards in more and more regions.
To quantitatively understand the airtightness of the building envelope, different measurement methods for the building envelope leakages have been proposed and evolved, which typically measure the air leakage rate under a certain pressure difference. The steady pressurisation method is most commonly used, while the Pulse technique proposes an alternative solution for providing new metrics. It offers an option of high-quality fast testing, involving a low-pressure pressurisation process, typically around 4Pa. Regarding the building airtightness measurement, its uncertainty has been the key concern in many regions over the past few years. The wind impact on the airtightness measurement has been explored from different perspectives, and solutions to minimise the wind effect have been proposed, whereas the majority of work has been focused on the steady pressurisation method. Therefore, to fill the research gap, the research aims to provide in-depth insights into the wind impact on the building airtightness measurement with the Pulse technique through field testing of two case study buildings under natural wind conditions and Computational Fluid Dynamics (CFD)-based numerical simulations under steady wind conditions.
In this thesis, comparisons between the conventional steady pressurisation method and the Pulse technique were firstly presented, providing fundamental insights into how the practical measurements would be when both test methods were applied to the same house in real scenarios. Based on the experimental airtightness measurements of a five-bedroom dwelling under calm weather conditions, a good overall agreement between the two test methods was observed, with deviations of 3.93% at 7.1Pa and 4.56% at 10.6Pa for the overlapped building pressure range. In order to answer the frequently raised question associated with the dynamic nature of the Pulse test, explorations were conducted to confirm the achievement of the uniform indoor pressure distribution in a five-bedroom dwelling during the Pulse test and therefore prove the applicability and reliability of the method. To assess the wind impact on the Pulse test under natural wind conditions, a short-term experimental work consisting of 423 Pulse tests was completed over seven days in a five-bedroom detached house, and a long-term investigation with 1096 Pulse tests was performed over eight months in an outdoor chamber. The field tests demonstrated the noticeable wind impact on the Pulse measurement, as a higher wind speed led to a significant deviation of the airtightness measurement. Based on the tests in the five-bedroom dwelling, the wind impact was not significant for the wind speed below 3.14m/s at 2.2m above ground level (i.e., approximately equivalent to the meteorological wind speed of 4.95m/s) with RPD <±5%, and the wind speed was below 6.10m/s at 5.0m above the ground level (i.e., approximately equivalent to the meteorological wind speed of 7.51m/s) according to the findings of the outdoor chamber. The observed threshold of wind speed that caused the Pulse measurement of the five-bedroom dwelling with unacceptable errors was 5.02m/s at 2.2m above ground level (i.e., approximately equivalent to the meteorological wind speed of 7.91m/s) with RPD <±10%, while the threshold was 11.65m/s at 5.0m above ground level (i.e., approximately equivalent to meteorological wind speed 14.34m/s) based on the tests in the outdoor chamber, due to the differences in the tested building configurations and airtightness levels. Besides, solutions have been explored to minimise the wind impact, such as performing multi-Pulse tests or multi-step Pulse tests and improving the algorithm to adjust the obtained Pulse measurements. Furthermore, CFD-based numerical simulations were implemented in the ANSYS Fluent based on a full-scale three-dimensional model of the building and the Pulse unit. The numerical work visually and statistically demonstrated how the pressure uniformly distributes across the internal spaces during the Pulse test. Based on the numerical simulations, the meteorological wind speed limit for the Pulse measurement with RPD <±10% was below 6m/s, which is in line with the wind speed limit for the fan pressurisation method stated in the ISO 9972:2015 standard (Thermal performance of buildings-Determination of air permeability of buildings-Fan pressurisation method). The numerical results also indicate that the Pulse measurement is less sensitive to the variation in the wind direction than the wind speed level, as the largest difference of the simulated average air leakage rate among the cases with different wind directions was 3.93%. Additionally, the comparison between the building model with a flat roof and a gable roof reveals the insignificant impact caused by the presence of the gable roof on the Pulse measurement, as the deviation of the air leakage rate was only 1.25%.
The research has been routinely conducted as part of the ongoing Pulse development, providing progress to the knowledge of comprehensive understandings of the wind impact on the Pulse test from practical and numerical perspectives, demonstrating how the Pulse measurement performs under different wind conditions and the reliability of the method. The findings of the wind impact in this research provide valuable supporting information for the improvement of protocols that are used to characterise building airtightness measurement and for the significance of building airtightness on the energy performance of buildings.
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