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Preparing for an Infrared Thermographic Scan

The best conditions for a scan

The infrared scan is most effective when the inside and outside temperatures are as far apart as possible. Typically, in cold climates, the outdoor temperature should be below freezing and the inside of the building over 60 degrees. The latest equipment is capable of detecting much lower temperature differences, so scans can be done under less favorable conditions; however, conditions close to those that are causing the problems (usually colder weather) provide the best time to do the test. The scan is normally done at night when the sun will not impact the thermal image. At times when the ambient daytime temperatures are warm, early morning is better than the evening as it is coldest just before dawn. If the sun is warming the building (solar gain), the insulation and air infiltration patterns will not be accurate representations.

Daytime scans must be limited to days when the weather is very cloudy; if not, only the sides of the building that are in the shade can be analyzed correctly.  Summer scans can only be done if the inside of the building can be cooled to give a significant temperature difference.  Note that most types of scans are done from the inside or from both inside and outside.  Scans can be done without noise or lights on if sleeping children make late night or early morning work difficult.

The following information will help you to prepare for the infrared survey. Some of the items may not apply to your project. (For example, if you want us to only scan the roof, you do not have to do any preparation regarding the walls).  Please feel free to contact us prior to your scan if you have questions about any of this information.

How the scan is done

  1. A visual inspection can be done before or after the scan.  Usually the inspection will be done after the scan to limit it to the areas where problems are found.

  2. The infrared camera is focused on the areas to be tested. Some of our cameras must be kept moving in a slow panning movement to maintain a clear image.  During the scan the image is viewed through the viewfinder and the image can be interpreted as the scan proceeds.  In some cases, the building may be depressurized with the aid of a blower-door fan to reverse the flow of stack-effect air leakage and enhance the infrared images.  Large or multi-family residences and commercial buildings may require the using or adjusting the building’s mechanical systems to perform pressurized or depressurized tests.

  3. The scan is recorded on a video tape.  Viewing the scan on a television provides additional opportunity to study the images.  The tape includes an audio track on which conversation will be recorded to identify the project, the time of day, the environmental conditions, and the areas being viewed.  Observations and preliminary interpretations will be included to help the viewers of the tape  understand what is being shown and how to use the information.

  4. The scan may include a test of both sides of the building envelope.  Some areas may be concealed from view by the structure on either side.  For example, an inside scan may not "see" uninsulated areas between the floors. Cold air coming in can be seen from the inside; warm air exiting can best be seen from the outside.  Snow and other weather conditions may limit the work to the inside, especially where roof insulation is being analyzed.  Again, depressurizing the building can augment an inside test by reversing air movement where stack-effect pressures cause upward or outward flow.  Note: When depressurizing a building, gases from wood stoves and other combustion appliances can be drawn back into the house.  Fires should be out and other heaters turned off to prevent back drafting.  Units that are of the "sealed-combustion" type should not cause a problem.

Pre-Test Preparation

Before conducting the blower door test, we need to put the house in its normal heating or cooling configuration.  This includes closing operable openings and preparing combustion appliances. The building that is to be tested should be prepared in the following ways:

  • The building should be wired with at least one working 20-amp circuit available to power the blower door fan and usually, the accompanying fog testing equipment.

  • The forces created by the blower door fan for this test (up to at least 50 Pascals) are quite powerful and will tear material from walls that is not securely fastened. Sheetrock and rigid foam will hold, Tyvek, poly, and aluminum vapor and air barrier materials probably will not.

 Doors and Windows

  • A doorway must be provided that can be sealed and off limits for the duration of the test. For the blower door test, this will be up to one hour, but the fog test and on-the-spot air sealing work this could be a half day or longer.

  • All interior doors must be open.

  • Close and latch all storm and prime windows.

Combustion Appliance/Exhaust Devices

  • Adjust any combustion appliances so they do not turn on during the test.  This is usually done by turning off power to the heating system and turning the water heater to the “Pilot” setting.  NOTE:  If vented combustion appliances turn on during a depressurization test, it is possible for flames to be sucked out of the combustion air inlet (flame rollout). This is a fire hazard.

  • Be sure that fires in fireplaces and woodstoves are completely out.  Take precautions to prevent ashes from being blown into the house during the test.  In most cases, closing dampers and doors is sufficient, but when they are leaky or absent, it will be necessary to either tape doors shut, clean out the ashes, or cover the ashes with  wet newspaper.

  • Turn off any exhaust fans, vented dryers, air conditioners, and HVAC fans.

How to Prepare

  1. Radiant heaters should be turned off about an hour before the scan if the areas behind and above the heaters need to be viewed.  If the heaters or the materials behind them are still hot, they will prevent a true reading of the thermal envelope's performance in those areas.  Temporary heaters located to the interior may need to be used to keep the building warm without running the central heating system.  If only the roof is to be scanned, the heat can be kept on. Night-time scans can be performed with as little as 15 degrees temperature difference from inside to outside with our equipment.  The greater the difference, the clearer the images.  Depressurization during the scan is usually required when temperatures are marginal, especially for roof slopes and ceilings.

  2. If drapes, pictures, or other furnishings are covering areas which are to be scanned, they should be moved away from the walls and windows well in advance of the survey so that the temperatures of the surfaces behind them can stabilize.  Cupboards and closets should be opened and emptied if these areas are to be included in the study.  This should be done well ahead of the scheduled scan so that surfaces can reach the indoor ambient temperatures.

  3. When available, detailed architectural drawings or photos of the building under construction are often useful in interpreting the thermal image.  Also, photos or other information about the symptoms of problems that led you to have a scan done are helpful in corroborating the information available.  For example, patterns of snow melt and ice formation can provide significant clues or corroborating evidence about the causes of heat loss problems.  Exterior photos and the information from the interior scan can be cross-referenced to arrive at the best understanding of heat flow into a roof or attic.  Keeping track of room and outdoor temperatures when discomfort or freeze ups occur can help to prioritize heat loss and air leakage problems that the scan will locate.

  4. Visual inspections of the building's construction are also helpful in explaining the causes of the images revealed by the scan.  Access panels, unfinished basements, attics, and current renovation work can provide more information about the thermal envelope and how it works.  If access to these areas will require special equipment, (such as a ladder or electric screw gun) please advise us in advance of what equipment will be needed.

The Results

  1. The scan is usually a black and white image.  Color images can be used where required, but the maximum contrast is available in the black and white mode.  The camera senses the temperature of the surfaces viewed.  The lighter the image, the warmer the surface; the dark areas are cooler.  Color scans include a bar code showing the temperature range from hot to cold on the recorded image.  The interpretation of the scan involves assessing the meaning of the contrast of adjacent surface temperatures.  For example, if the studs are darker (cooler) than the area between the studs, the wall areas have a higher insulation value than the studs.  Another contrast which is used to evaluate the insulation is the difference between the interior and exterior walls or surfaces. Exterior walls which are well insulated should not be dramatically different than the value or "color" of the adjoining interior surfaces.  A lack of contrast usually means that the surfaces are of a consistent temperature.  This would indicate an exceptionally well-insulated building, or an inadequate temperature difference between the inside and outside.

  2. Infiltrating cold air itself cannot be seen; however, infiltration can be located by the cooler surfaces which are washed by the cold air flowing by them. Moving air patterns are normally not well defined and are irregular in shape. Fan shaped patterns or semicircular dark areas originating along seams or penetrations reveal the location of holes or linear cracks. A gradation in the contrast typically indicates the direction of flow to the thermographer.

  3. The duplicate video tape of the scan can be viewed on a standard VCR.  Other formats are available, including printed still images captured from the tapes.  The tapes are normally recorded at standard speed (SP), but other speed settings should be tried if adjusting the tracking does not give a steady image without lines.

  4. The results of the scan can be subtle and customarily require the interpretation of the experienced thermographer. After reviewing the scan, a visual site inspection of problem areas, and a review of photos and detail drawings may be necessary to understand how the building works and to completely understand the causes of its problems.  In some cases other types of testing will be used to corroborate or pinpoint air-leakage sites in remote locations.

We encourage the owner or someone else closely involved with the operation  of the building to be present during the scan.  This person will probably be able to provide insight about the problems which will help us interpret the data collected.  This may also save report writing time as it enables a better understanding of the video and, if necessary, the report.  If a report is required, one will be provided at the same rate as the scan plus any unusual out-of-pocket costs such as detail drawings, production of stills from the video, etc.