Thermal imaging

A number of local Community Action Groups (CAGs), along with several councils, including OCC, now own thermal imaging cameras which they are willing to lend out to groups who want to offer advice to local residents on improving the energy efficiency of their homes. These cameras record infrared emissions, turning whatever they see into a palette of colours depending on the heat emitted by the surfaces. Although absolute heat loss is hard to calculate and depends on the different thermal properties of the structural elements of the building, thermal images can highlight significant areas of loss. Carried out in conjunction with a more detailed discussion about the building and the ways it is used, thermal imaging can help inhabitants to plan effective eco-renovation projects, or to adopt simpler measures which may increase their thermal comfort, lower energy bills and decrease carbon footprints.

Summary

External surveys

Imaging the outside of a house will highlight both heat absorbed from the sun or from external lights, as well as any heat lost through poor insulation or from areas of the fabric of the house which could benefit from draught proofing. Because the effects of solar load cannot be easily differentiated from heat loss from within the building, external images must be recorded at least 3 hours after sunset, and surveys cannot be carried out during or for several hours after rain or in high winds. Surveyors should also document the weather conditions and note which images are taken from which direction. Differences due to north and south facing falls after sunny days will be very apparent.

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Front and rear of a property- north (left) and south (right) facing imaged after a sunny day

When recording and interpreting images, be alert to the temperature scale shown on the right of each image: the information is relative to what else is in the frame and the scale used, i.e. ‘cold’ could be 8oC in one frame and 2oC in another. In addition to the temperature scale, the camera also allows the user to select one or more spots for an absolute temperature reading- these are indicated by a crosshair mark, the temperature of which is shown on the image as the ‘spot’ figure on the top left of the image.

This feature can be switched off to keep the images less cluttered.

In order to show heat loss from within the building, the images depend on the presence of a thermal gradient. Home owners should be contacted in advance and asked to set their heating system to operate at its full winter setting for at least 4 hours before the agreed session time to achieve a minimum internal temperature of 18oC throughout the house). Under these conditions, external images will show up areas of heat loss on a palette ranging from blue through green, yellow, orange and red to white (as indicated by the scale on the right of the image below).

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Typical thermal image of a brick house without insulation: the red patch under the left hand window is a radiator, the wall is south facing and the yellow/green patches are likely a combination of heat loss due to lack of insulation, and residual heat from the sun. The windows are all double glazed, but the one on the top right is much newer, while the one on the top left has been refurbished.

In addition to residual heating from the sun, there are other features which are not directly connected with heat loss through insufficient insulation and to which users should also be alert. Heat (from within the house or from the sun) can be trapped under the eaves of the roof, showing up as a characteristic red line:

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Different building materials also display different thermal properties which cannot be compensated for in a simple image, and may cause, for example, lintels and frames of windows to be highlighted. Some of these kinds of thermal bridges are inherent and cannot be remedied.

For these reasons care must be taken in interpreting the images, and it can be helpful to try to image a number of neighbouring properties during a single session and without changing the camera settings. Together with knowledge of the differences between the properties, this allows for more informative analysis of the images.

Internal surveys

Alternatively, or additionally, the camera can also be used to pick out cold spots inside properties, typically areas of leakage or thermal bridging which may coincide with visual evidence of condensation (see images below). Internal images are ‘reversed’, i.e. areas of heat loss show up as dark blue (colder) rather than red (hotter). Internal surveys are easier to carry out, since they do not depend on clear/dry weather, and the camera (and operators) is more comfortable! However the images can be more difficult to interpret, as air currents within the room and other building features can throw up anomalous patterns in the images.

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