Thermography

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Thermography

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Thermography (infrared; thermal scans) uses specially designed infrared video or still cameras to make images (called thermograms) that show surface heat variations. This technology has a number of applications. Thermograms of electrical systems can detect abnormally hot electrical connections or components. Thermograms of mechanical systems can detect the heat created by excessive friction. Energy auditors use thermography as a tool to help detect heat losses and air leakage in building envelopes. Home energy audits sometimes include a thermographic inspection.

Infrared scanning allows energy auditors to check the effectiveness of insulation in a building's construction. The resulting thermograms help auditors determine whether a building needs insulation, and where in the building it should go. Because wet insulation conducts heat faster than dry insulation, thermographic scans of roofs can often detect roof leaks.

In addition to using thermography during an energy audit, you should have a scan done before purchasing a house; even new houses can have defects in their thermal envelopes. You may wish to include a clause in the contract requiring a thermographic scan of the house. A thermographic scan performed by a certified technician is usually accurate enough to use as documentation in court proceedings.

Thermographic scans can be done inside or outside a structure. Exterior scans, while more convenient for the homeowner, have a number of drawbacks. Warm air escaping from a building does not always move through the walls in a straight line. Heat loss detected in one area of an outside wall might originate at some other hard-to-find location inside the wall. Air movement also affects the thermal image. On windy days, it is harder to detect temperature differences on the outside surface of the building. The reduced air movement and ease of locating air leaks often make interior thermographic scans more effective.

To prepare for an interior thermal scan, the homeowner should take steps to ensure an accurate result. This may include moving furniture away from exterior walls and removing drapes.

The most accurate thermographic images usually occur when there is a large temperature difference (at least 20F [14C]) between inside and outside air temperatures. In northern states, thermographic scans are generally done in the winter. In southern states, however, scans are usually conducted during warm weather with the air conditioner on.

The cost of a thermographic inspection ranges from $200 to $500, depending on the size of your home and the services provided. Copies of the thermographic image, recorded on videotape, may cost extra.

The energy auditor may use one of several types of infrared sensing devices in an on-site inspection. A spot radiometer (also called a point radiometer) is the simplest. It measures radiation one spot at a time, with a simple meter reading showing the temperature of a given spot. The auditor pans the area with the device and notes the differences in temperature. A thermal line scanner shows radiant temperature viewed along a line. The thermogram shows the line scan superimposed over a picture of the panned area. This process shows temperature variations along the line.

The most accurate thermographic inspection device is a thermal imaging camera, which produces a 2-dimensional thermal picture of an area showing heat leakage. These cameras are expensive, with prices ranging from $15,000 to $70,000.

Spot radiometers and thermal line scanners are less expensive ($200 to $400) but do not provide the necessary detail for a complete home energy audit. Infrared film used in a conventional camera is not sensitive enough to detect heat loss.

Credits: US Department of Energy (http://www.eere.energy.gov/consumerinfo/factsheets/cb3.html)

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