The following information is provided to aid in preparing for an infrared survey/scan. Some of the items may not apply to the environmental conditions at the time of the scan, or 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, cabinets, or basement). Please feel free to contact me if you have any questions.
What is an Infrared Scan?
Infrared (meaning far red) is the name given to the part of the electromagnetic spectrum that occurs just beyond the red end of the visible spectrum. Infrared (IR) radiation is emitted by all objects warmer than –273°C. IR travels through space in similar fashion to visible light, but at longer wavelengths, approximately 0.7 microns to 1,000 microns. The amount and wavelength of infrared emissions generally varies with temperature. Usually, the amount of radiation emitted by an object increases with temperature; therefore, thermography allows variations in temperature to be “seen.”
Although modern infrared cameras can measure heat emission very precisely, infrared thermography must be considered as a qualitative tool, and it is best to employ other methods to quantify heat losses from a building. Quantitative infrared surveys can only be conducted under very precise thermal and environmental conditions, which rarely, if ever, occur in the field. So, while there may be temperatures shown in the infrared images, these should be considered approximations, not precise values. When viewed through a thermal imaging camera, warm objects stand out well in contrast to cooler surrounding areas or backgrounds, providing a way to translate the heat waves radiating from an object into a visual image that can be interpreted into information about the thermal performance of a building enclosure. The hotter an object is, the lighter it appears; consequently, the colder an object is, the blacker or darker it appears. Maximizing the difference between the inside air temperature and the outside air temperature provides the greatest contrast between the hot and cold areas on the video image. For this reason, it is best to do an infrared scan on a cold day when the inside of the building is well heated. In hot climates, this would apply in reverse, requiring air-conditioned interior spaces to contrast with the hot outdoor conditions.
In building applications, infrared thermography is a non-destructive diagnostic tool used by building construction technicians to “see” thermal signatures that indicate where heat leaks are occurring in faulty thermal insulation or air barrier systems. The results are used to improve the efficiency of building enclosures. Like an X-ray technician, a thermographer must interpret what the images mean in the context of how a building is constructed and how environmental conditions impact thermal performance. In many cases the infrared imaging can be enhanced by strategic pressurization or depressurization of the structure. Ultimately, a thermographer can usually determine the following:
Which areas have excessive conductive heat loss due to inadequate or compromised insulation?
Which areas have excessive air leakage/infiltration?
In some cases, the direction of the air leakage flow.
Which areas represent the most energy loss?
Which building materials are wet?
Thermographic inspections for building enclosures generally follow procedures outlined in ASTM C1060 – 11a Standard Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings for insulation analysis, and ASTM E1186 – 03(2009) Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems for air leakage diagnostics. Flat and near-flat roof moisture investigations follow ASTM 1153. C1371-04a – Determination of Emittance of Material is used to aid in adjusting for the emissivity of various materials and surface conditions.
With this information we can make recommendations about the remedial actions necessary to solve thermal and moisture problems.
Infrared scans can be recorded onto a videotape, DVD, or into a digital file for reports. Scanning can be combined with blower door depressurization to exaggerate leakage or to reverse stack-effect air flow direction so that exfiltration can be located from the inside. Thermal scans can also be used during blower door compliance tests to verify building envelope performance and meet energy code requirements. These scans are used to locate and prioritize air barrier defects that impact air barrier standards compliance.
Ambient light and infrared images of the same roof area taken from inside
The Best Conditions for a Scan:
An infrared scan is most effective when the inside and outside temperatures are as far apart as possible. Ideally, in cold climates, the outdoor temperature should be below freezing, and the inside of the building should be over 65 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. (If the outdoor temperature is predicted to be too warm, i.e., about the same as the temperature indoors, the scan may need to be rescheduled.)
Scans are normally done at night when the sun will not impact the thermal image (ASTM C1060, X2.3.1 Avoid Solar Radiation). At times when the ambient daytime temperatures are warm, early morning is better than the evening as it is usually coldest just before dawn. If the sun is warming the building surfaces (solar gain or radiation), the thermal patterns will not be accurate representations of the thermal envelope’s performance in those areas.
Daytime scans must be limited to days when the weather is very cloudy; if this is not possible, only the sides of the building that are in the shade can be analyzed correctly. Summer scans can only be conducted if the inside of the building can be cooled to give a significant temperature difference. Note that most types of scans are performed from the inside or from both inside and outside. Scans can be done generally without having the lights on if sleeping children make late night or early morning work difficult to schedule.
How to Prepare:
Radiant heaters should be turned off about an hour before the scan if the areas behind and above the heaters are to be surveyed. 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 inside rooms can be used temporarily 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 my 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.
If drapes, pictures, or other furnishings are covering areas that 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 when contents conceal the walls) if these areas are to be included in the study. This should be done well ahead of the scheduled scan so that surface temperatures can stabilize.
When available, detailed architectural drawings or photos of the building under construction are often useful in interpreting the thermal images. Also, exterior 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.
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 driver), please advise me in advance.
How The Scan Is Done:
- 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.
- The infrared camera is focused on the areas to be tested. 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. See the How to prepare for a Blower Door Test section of HCF Blower Door Test Information. Large or multi-family residences and commercial buildings may require using or adjusting the building’s mechanical systems to perform pressurized or depressurized tests.
- The scan is recorded on a digital video recorder. Viewing the scan on a computer or television provides additional opportunity to study the images. The video includes audio which will 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 to understand what is being shown and how to use the information.
- 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” un-insulated 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, ice, and other inclement 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. See How to prepare for a Blower Door Test section of HCF Blower Door Test Information.
- 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 with my equipment. The camera senses the temperature of the surfaces viewed. The lighter the image, the warmer the surface; the dark areas are cooler. Gray-scale and 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 that are well insulated should not be dramatically different than the “gray 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.
- infiltrating cold air itself cannot be seen; however, infiltration can be located because the surfaces are cooled 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.
- A video recording of the scan can be viewed on a standard DVD player or computer. Other formats are available, including printed still images captured from the videos.
- 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.
I encourage 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 that will help me interpret the data collected, which may also save report writing time as it enables a better understanding of the video, and if necessary, the report. If a written report is required, one will be provided at the same rate as the scan plus any unusual out-of-pocket costs.
After the Test:
- Return all HVAC and water heating systems to normal operation at the end of the test.
- After the scan is completed, any of the following presentables can be provided in accordance with the contract requirements
- The digital record of the scan.
- A summary report describing the scan process and a general overview of the findings.
- A full report with data and graphics supporting the findings and the recommendations.
- A remediation plan.
To learn about Infrared thermography as quality assurance for foamed-in-place (i.e. cavity fil) insulation read this article by H C Fennell as published in SPIE Vol. 1094 Thermosense XI (1989) click here.