Today, the fan pressurization method is the most frequently used method to evaluate a building's airtightness. However, the localization and quantification of leaks remain difficult. In this paper, an acoustic method is introduced to estimate the leakage size of single leaks. Acoustic and airflow measurements were conducted and compared in laboratory tests within the same boundary conditions. This work aims to investigate if various leak sizes can be predicted using acoustic measurement methods. The test apparatus consists of two chambers, separated by a test wall. This wall represents a single characteristic air leakage path. Various types of wall structures with different slit geometries, wall thicknesses, and insulation materials were investigated. The acoustic measurements were performed with a sound source placed in one chamber and ultrasonic microphones located in both chambers. These results were compared to measured airflows through the test wall to provide estimates of uncertainty in the acoustic approach, which indicate a linear trend. Finally, these laboratory measurements were compared to the same measurements at a real office building. Although the acoustic measurement uncertainty is still significant (greater than +/- 50%), the acoustic method has the potential to give an order of magnitude of single leak sizes.

Air leakage in building envelopes is responsible for a large portion of the building’s heating and cooling requirements. Therefore, fast and reliable detection of leaks is crucial for improving energy efficiency. This paper presents a new approach to determining air leakages in a building’s envelope from the outside, combining lock-in thermography and thermal excitation by a blower door system. The blower creates a periodic overpressure within the building, inducing periodic temperature variations of the surfaces near the leaks on the outside surface, the façade. With the temperature variations excited at a known frequency, Fourier transforms of the time-series of the thermal images at the excitation frequency result in amplitude and phase images highlighting the areas affected by leaks. Periodic excitation and detection by an IR camera is known as lock-in thermography and is widely used to characterise semiconductor devices and in non-destructive testing. Excitation is usually achieved by optical, electrical, or mechanical energy input. For this work, measurements of outside façades have been performed with three excitation cycles of a period of 40 s at a 75 Pa pressure difference, leading to a total measurement time of only 2 min. Measurements have been performed with air temperature differences of 5 to 7 K at highly variable conditions of irradiance, wind, and cloud cover. The measurements show higher detection quality and less impact from changing ambient conditions than the state-of-the-art differential infrared thermography measurements. With the method highlighting the variations in the amplitude image only at the excitation frequency, variations caused by environmental effects are filtered out. A temperature difference as low as a few Kelvin is therefore sufficient, and large façades can be examined from the outside. This amplitude image is already clearer than an image created with differential thermography. A further reduction of unwanted artefacts in the image is demonstrated using phase-weighing of the amplitude by scalar product.