What is a Combined Blower Door Thermal Imaging Survey?

A combined blower door thermal imaging survey is a method of using a thermal imaging camera and a blower door system together to detect and display the air leakage paths through the building fabric. This can be extremely useful on existing properties that are upgrading to Air Source Heat Pumps, which requires a more air tight building envelope. an A blower door system consists of a large fan that is installed in an external door opening and a manometer that measures the pressure difference between the inside and outside of the building and can ascertain the air leakage rate to a recognised standard, such as ATTMA TSL2.

Blower door and thermal imaging to highlight air leakage 

The fan can either pressurise the building – which works best with smoke testing or depressurise the building – which works best for thermal imaging. By creating a controlled airflow that exaggerates the thermal anomalies caused by air infiltration or exfiltration it provides a much better environment for capturing better thermal images of the resulting air leakage paths.


A thermal imaging camera (or also known as an infrared camera) is a device that captures infrared radiation emitted by objects and converts it into visible images. The images show the temperature differences across the surface of the objects, which can indicate the presence of air leakage through the building envelope. By using a thermal imaging camera during the blower door test, the air leakage paths can be easily seen on the screen as darker or lighter colours, depending on the direction of the airflow, palette settings on the camera, and the temperature difference between the indoor and outdoor air temperature, which should be a minimum of 5C.

 How to Conduct a Combined Blower Door Thermal Imaging Survey?

A typical procedure for conducting a combined blower door thermal imaging survey is as follows:

  • Ensure that there is a sufficient temperature difference (at least 5°C) between the indoor and outdoor air temperature.
  • Seal any intentional openings or ventilation systems in the building, as required by ATTMA and Building Regulations Part L.
  • Set up the blower door system in an external door opening and connect it to the manometer, ensure that the fan is in the correct position for depressurisation.
  • Set up the thermal imaging camera and adjust the settings according to the manufacturer’s instructions.
  • Start the blower door fan and depressurise the building to a target pressure of -50 Pa, which corresponds to an air change rate of 8m3/h/m2 at 50 Pa.
  • Maintain this pressure and walk around the building with the thermal imaging camera, inspecting each room and area for signs of air leakage. Record any defects or anomalies on the camera or on a separate device.
  • Repeat the process with different pressure levels (e.g. -20 Pa, -30 Pa, -40 Pa) to see how the air leakage patterns change with different pressure gradients.
  • Analyse the recorded images and data and prepare a detailed report with recommendations for remedial actions.


What are the Benefits of Combined Blower Door Thermal Imaging Survey?

The main benefits of using this method are:

  • It provides a visual representation of the air leakage in a building, which can help to identify and locate the sources of heat loss, moisture problems, draughts, noise transmission and poor indoor air quality.
  • It allows for a more accurate diagnosis of the building envelope performance and helps to target remedial works more effectively and efficiently.
  • It can help to achieve higher standards of energy efficiency, comfort and health in buildings, as well as complying with regulatory requirements and achieving extra BREEAM credits.

We can help with your combined blower door Thermal Imaging Survey

If you require a combined blower door thermal imaging survey for your building project, please contact us at: info@aptsoundtesting.co.uk. Or call Darren on 07775623464. We have qualified and experienced professionals who can provide you with high-quality service and advice.