My previous blog on the basics of thermography was becoming very long on the tooth so I decided to omit a few things including the below section on external surveys.
As my line of work focusses primarily on diagnosing building defects and not heat loss or external insulation assessments I do not carry out external thermal surveys. Also the two cameras I have access to do not have a sufficient thermal resolution to carry out the type of detailed surveys I would like to so I would steer clear of this type of survey.
If you are considering conducting external thermal surveys I would recommend considering some training to avoid misinterpretation of the results. The tips and recommendations shown below have been gathered from a variety of sources from case studies to academic research papers. Hopefully, this gives an idea of some of the key things to consider when preparing for external thermal surveys.
Tips for External Surveys
There are a number of thermal influences that can alter the results of a heat loss survey. When looking at the external building envelope some of the things that will need to be considered to prevent the misinterpretation of results are as follows:
The area being surveyed must be clearly visible with a direct line of sight. Knowledge of the materials being inspected is important to prevent misdiagnosis of thermal results.
Irregular shaped buildings with areas sheltered from wind should be taken into account.
The weather conditions. The weather must be dry and the facade must not be visibly wet (difficult in Scotland). Rain, snow, thick mist and high winds are all a no go.
Best conditions are usually cool, overcast days are optimum as direct sunlight is also a no go. For this reason, a good plan of action might be to conduct external surveys in the morning before sunrise.
In order to produce clearly defined thermal images, there must be a temperature difference between the inside and outside of the building of at least 10°C for a period of 12 hours so winter months are preferable.
An important aspect when interpreting thermal images is to discount heat loss through lintels and soffits as this is not due to poor insulation but rather usually an example of thermal bridging. Air vents can also give the false impression of failed insulation.
Have a look to see where the boiler and radiators are situated as the heat from a these could lead to misinterpretation of heat loss. Are radiators upstairs all turned off? If so this might show different external thermal patterns.
Stand back an appropriate distance to capture the picture keeping in mind your IR resolution. The higher the resolution, the further away you can take pictures without losing accuracy.
Thermal tuning will be important as you will want to adjust the thermal range of the camera to cut out things you don’t want to see and focus on things you do.
If you are conducting a large scale survey it would be sensible to complete the thermal imaging from October to March with the analysis of the images then taking place over the spring and summer.
Careful consideration of the results is key, for example, a wall showing broad patchy heat loss could suggest deterioration in the cavity wall insulation. a follow-up borescope survey could confirm these findings.
The ideal scenario would be to have control over the internal environment of the building being surveyed. In some cases where large scale surveys are being conducted on hundreds of properties, this is not going to be feasible. So in these cases, it would be prudent to understand as much of the pitfalls as possible whilst liaising with trained thermographers.
In the social housing context, staff will already have a good understanding of the construction type, heating arrangements, and historic alterations so this information will all come into use to gather an understanding of the thermal images obtained.