What are the methods to detect leaks of the building envelope?
Detecting the location of leakages in the building envelope is useful in order to reduce the leakage area of the envelope and/or to estimate the leakage distribution of buildings . Blower doors are very useful to find leaks .
The usual procedure consists of creating with the blower door an underpressure from approximately 50 Pa and checking the complete building envelope. This is done for example by walking around from room to room in the house while the blower door is running and feeling the incoming air with a hand. Sometimes it is useful to use a wet hand, because evaporative cooling makes it more sensitive. It is possible to open and close single interior doors in order to get an idea of the leakage from individual rooms. The worst rooms are usually the ones that have plumbing or those with connections to attics, crawl spaces and garages.
Several methods are possible for leak detection, listed below from gross to fine:
- Visual inspection of specific parts of the constructions like wall-roof connections.
- Inspection of the building envelope while the blower door creates 50 Pa negative pressure in the building and locate leaks with your hand.
- Thermal-Anemometry is a very reliable tool to locate air flows at 50 Pa negative pressure. It can be held against places of the building envelope or devices where one might suspect leakages. If the instrument shows an air velocity, it is an indication of leakage .
- During 50 Pa leakage test (depressurization), a thermal camera can be used to localize air infiltration as long as there is a temperature difference between indoor and outdoor. The infrared image shows the heating or cooling of the surface near the leakage caused by the incoming air flow. Sometimes this kind of temperature change on surfaces can also be observed due to the natural pressure difference caused by wind or stack effect.
- Smoke from a smoke generator in combination with a blower door creating a pressurization or depressurization can help to find leaks. The smoke is generated to visualize the airflow through the envelope, devices, etc. and to detect the location of the leakage. In some cases it is also possible to find the way - the leak path - how the air moves through the envelope.
- There are systems available that use ultrasonic sound to get detailed qualitative information on leaks around windows, doors, walls/roof connections. Some research has been done to also use ultrasonic sound to quantify leakages 
 Allison A. Bailes III, PhD, "A House Needs to Breathe … Or Does It? An introduction to Building Science" (page 111: “Using a Blower Door to Find Leaks”), Bright Communications, October 2022.
 International Organization for Standardization, "ISO 9972:2015. Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method", 2015
 Benedikt Kölsch et al., "Quantification of air leakage paths: a comparison of airflow and acoustic measurements", International Journal of Ventilation 22(6), August 2021.