Can U Values be measured using a thermal imaging camera?
One of the questions that I am commonly asked is: Is it possible to measure the U value of a wall using thermography?
A material's U Value (heat transfer coeeficient) can be defined as the rate of heat loss through an element and can be calculated using the following formula:
U = Q/(Ti - Te)
Where:
- Q is the heat loss (watts) through the element
- Ti is the internal ambient temperature
- Te is external ambient temperature
There are number of key elements to consider in trying to answer this question:
The first point is the term 'measure'.
A thermal imaging camera can only be used to measure surface temperature. It is possible to use the measured surface temperatures using the camera to calculate the heat loss (Q) which can then be substituted in the formula above to calculate the U Value.
Therefore it is not possible to measure the U-Value using a thermal imaging camera, it is only calculated using the surface temperatures measured by the camera.
The second point, and this is fundamental in answering the question. The formula above relies on the heat flow under steady state conditions (i.e. where the heat flow through the building fabric has met an equilibrium for the temperature difference from internal to external). Achieving steady state conditions is near on impossible; whereas it is relatively easy to control the temperature internally, externally the temperature is constantly changing, particularly when you take into account other factors that will affect the surface temperature of the external walls (i.e. solar gain, clear night skies, winds, precipitation/rain).
A thermal image is only showing temperatures recorded at a snap shot in time. Due to the changeable environmental conditions the rate of heat flow is constantly changing, therefore the use of a thermal imaging camera along will give unreliable results when calculating U Values.
There is other equipment that can be used to measure in-situ U-Values. Heat flux probes connected to a temperature data logger can be mounted to the internal wall. The data logger should also be measuring internal and external temperature in parallel to the measurements from the heat flux probes. This equipment is typically left in place for up to 10 days and enables ‘a line of best fit' to be drawn through the results that best matches steady state conditions.
