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Depressurised Thermal Survey – Bigger and Better Results?

Friday, 29th October

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A depressurised thermal survey uses a large fan fitted within a fabric-covered frame to induce a negative pressure within a building. Doing so exaggerates some thermal anomalies, such as air infiltration, which may not be clear under normal operating conditions.

The depressurising kit is known as a blower door and is more commonly used to measure the air permeability of new builds. This is one of a series of compliance tests that are required under Building Regulations. For a thermal imaging application, we are not interested in a numerical air leakage value; we are simply using the kit to provide a controlled way to depressurise the building.

How To Conduct a Depressurised Thermal Survey

A typical blower door-assisted thermal imaging survey of the interior of a building, or single room, would be conducted in the following way:

  1. Confirm acceptable conditions for a thermographic survey (refer to Annex D of BS 13187).
  2. Carry out a thermal survey under normal operating conditions, i.e. without an induced negative pressure. This step is important to identify areas of interest. The surveyor will return to these locations at each pressure step.
  3. Carry out temporary sealing of mechanical ventilation etc. as required. Although this is not an ATTMA air test, their guidance with regards to temporary sealing is helpful.
  4. Set up the blower door in the opening of an external door.
  5. The fan speed should be increased incrementally. Always start low at -5 PA. Maintain this pressure and revisit the areas of interest identified in step 2 and take comparison images.
  6. Continue to depressurise the buildings in steps and inspect at every hold: -5 / -10 / -25 / -50 Pa. Ensure that there is a clear record of which images were taken at which pressure.
  7. Results are analysed by an accredited thermographer.

Depressurised thermal survey

What are the Advantages of this Method?

The principal benefit is that you can see a thermal defect ‘develop’ as the pressure increases. In the case of air infiltration where the thermal pattern commonly appears as fingers or branches of lower temperatures extending behind the plasterboard, these patterns will become more pronounced. As the negative pressure increases, more air will try to force its way in from outside. 

The significant point to note is the pressure at which the anomaly increases in size. If the anomaly deteriorates markedly for each pressure step, it is more severe than an area of interest that only shows a change later in the test. Care should be taken when interpreting ‘anomalies’ that only appear at the highest fan pressures. Inevitably there will be some air leakage through parts of the fabric that have been well constructed if these parts are tested beyond their design capabilities.

In summary: a blower door-assisted thermal survey can be helpful to gauge the severity of an anomaly by charting its development at different negative pressures. The blower door acts as an amplifier for the IR camera.

What are the Disadvantages of this Method?

Surveying a depressurised house is not inspecting it under normal operating conditions. And normal operating conditions are those we should care about.

50 Pa is the equivalent of a 20-mph wind blowing against all sides of the property simultaneously. The suction from an induced negative pressure will artificially stress parts of the construction that will arguably never experience these types of conditions in-service. Judgement is required when analysing the results and presenting them to the client. In some circumstances, a thermal anomaly that is identified at an induced pressure will have limited or no perceptible impact on thermal comfort and energy usage at the standard operating pressure.

A blower door-assisted thermal imaging survey is a way of supercharging and exaggerating results and it has its place, however, easily achieved results aren’t always reliable ones. There is a reason why thermographers survey before dawn; often in freezing temperatures; and spend some time understanding the construction and the orientation of the building they are surveying. 

Achieving good results takes time and effort. That’s why at Thermalume, if you notice a room is cold when a wind from a particular direction is blowing, our team will schedule a survey that allows us to inspect your site during these conditions. The results will not be the same if we shortcut the process by installing a blower door.

 

Contact us today at Thermalume if you require a thermal imaging survey on your home or commercial property. Get in contact with us via our contact form.