Ronald D. Lucier FLIR Systems N. Bellerica, MA ABSTRACT Infrared thermography is a proven technology that has been used for years to locate areas of wet insulation, thus aiding roof consultants and building owners. This broad scanning technique has advantages over other methods in that it is non-destructive, non-intrusive, and presents a live visualization of the wet areas. The most commonly used standard to conduct these inspections is ASTM-C1153, “Standard Practice for Locating Wet Insulation Using Infrared Imaging.” This standard only addresses inspections of roofs at night. This paper will discuss the techniques under which these inspections may be performed during the day, both from inside the building and on the rooftop. Case histories will be presented as well as a matrix of the required roof material and environmental conditions. These daytime inspections offer the same sensitivity as those at night, but are done with an additional margin of safe¬ ty, as the potential for trips and falls is reduced. Ronald Lucier Ronald Lucier is a mechanical engineer with over 20 years of experience in the development and application of infrared thermography. He conducts over 30 one-week training courses each year through¬ out North America and has lectured in Europe and China. He is a member of RCI. Lucier— 63
(You Can Get a Good Night’s Sleep!) INTRODUCTION Infrared thermography is a proven technology used to help locate wet insulation in roofing systems. As an application, roofing is second only to electrical inspections in popularity. The technology has been applied to roofing since the early 1970s, though the equipment available back then was expensive, suffered from poor resolu¬ tion, and was very heavy. The modem thermal imagers available on the mar¬ ketplace today offer an inexpensive, lightweight, and high resolution solu¬ tion to many of today’s inspection missions. Unfortunately while the imagers have improved dra¬ matically, the approaches to roof inspections have not. The default standard practice used by most infrared thermographers is ASTM Cl 153, which addresses only night-time surveys. It is clear from many years of experi¬ ence that the laws and principles of heat transfer and physics apply during the day too! Radiation Physics and Heat Transfer Every object above absolute zero ( 0 Kelvins, – 459.72°F) emits infrared radiation, according to the laws described by Planck, Stefan-Boltzman, and Kirchhoff. Infrared imagers sense this radiation and display the results as a live video image. The intensity of the radia¬ tion coupled with the physics model inside the camera can yield temperature patterns on the surface of the inspected object. The current infrared imagers have sensitivity in the neighborhood of +/- 0.2°F. Typical wet insulation areas can yield temperature differences between 0.5°F and as high as 30°F or more (depending on latitude and time of year). Therefore, there is adequate infrared camera sensi¬ tivity to see some of the smallest temperature indications on roofs. So why do wet roof areas even exhibit temperature differences? It is due to a material property called “Heat Capacity.” The technical definition is, “the amount of heat required to raise a unit mass one degree in tempera¬ ture.” The chart above provides a graphical representa¬ tion of heat capacity of various materials. It is evident that water has the highest heat capacity (also known as “Specific Heat”). A careful interpretation of these data reveals that it takes over ten times as much heat to raise the tempera¬ ture of water one degree than it does copper. Also, once the water has been heated, it remains warmer longer! When it comes to roofing materials, the data are much harder to obtain. An exhaustive search of the Internet, manufacturer’s product sheets, and quite a few phone calls yielded a very small list of the specific heat values for roofing materials. Specific Heat – Roof Materials Material Specific Heat Cp BTU/lb-°F) Bitumen 0.59 “Rubber” Membranes 0.48 Fiberboard 0.32 Gypsum 0.26 Stone 0.20 Fiberglass 0.16 Concrete 0.16 It is clear from these data that it takes 6.25 times (1 .0 divided by 0.16) as much heat to heat up water than it does fiberglass. In fact, experience tells us that those roofs constructed with fiberglass insulation exhibit the Lucier— 65 characteristic thermal signature much sooner (but for a shorter duration) than those of other insulating materials. Also, from these data, the difference in heat capacity between the bitumen and “rubber” materials (an average of several sources) shows that the temperature change necessary to cause a discernible thermal pattern favors the “rubber” over the bitumen. The net result of this is that when the sun radiates and warms the roof, the wet areas and dry areas heat at different rates. Similarly, at night, the wet and dry areas cool at different rates. Graphically: It is evident from this representative graph that at two points during a day, the wet and dry roof areas are at the same temperature; the infrared inspections would not be worth it. This may occur for two or three hours, depending on the weather and amount of solar insolation. However, for a significant part of the day, there exist sig¬ nificant temperature differences. These are the windows of opportunity in which to conduct the infrared inspec¬ tions. In the morning, the roof can be inspected from the underside if the deck is: • Visible • Painted, coated and/or fireproofed Also, in the morning, the roof may be inspected from the top if: • There is an unballasted cap sheet. • The tester can stand the heat. ASTM C1153 Most roof thermographers are familiar with this stan¬ dard. ASTM Cl 153 was developed in the 1980s to pro¬ vide guidelines and recommendations for conducting infrared roof surveys. It addresses the walk over method and the fly over method. The entire procedure is focused on nighttime surveys, the state of the art at the time it was developed. Over the past 20 years of infrared thermography experience, the author has been able to easily provide complete inspections dur¬ ing the day using the basic princi¬ ples of physics and heat transfer. But it is evident, though not noted in ASTM Cl 153, that these basic laws and principles apply 24 hours a day, not just at night! Daytime Roof Top and Underdeck Inspections The daytime inspection process is very similar to one at night. On the rooftop, if it is inspectable (typ¬ ically limited to adhered, unballast¬ ed membranes), the inspector sys¬ tematically walks the roof looking for cooler areas typical of wet insu¬ lation (refer to the time vs. temperature chart). The two images on the adjacent page present results the author located during the night and then during the day. The images were taken at an electronics manufactur¬ ing facility from different perspectives, but the results are clear and easy to determine. The temperature difference observed at night was 19°F and 14°F during the day – very similar! Both were taken with a FLIR P60 thermal imager on subsequent days. A follow-up core cut verified wet insulation. One image looks like the negative of the other. During the night, the wet insulation was warmer than the dry insulation. This causes a temperature difference on the cap sheet (fully adhered). During the day, the wet insulation is heating up slower than the cap sheet, thus it looks (and really is!) cooler. Temperature Response Dry vs Wet Roof Lucier— 66 9:54 PM 10:31 AM An underneath survey at a different facility (large paper mill) yielded different, though dramatic, results (see photo, below). This is a modified bitumen, built-up roof. The area was marked on a plan view of the section (impractical to mark from the floor; 75-foot-long paint sticks are hard to handle). As in the previous picture, the wet area is darker because it is cooler than the dry area. Environmental Parameters The ASTM Cl 153 standard is well written, though a bit dated. The environmental parameters apply when inspecting roofs during the day as well. These are: • No appreciable precipitation for the previous 48 hours. • Wind less than 15 miles per hour. • Air temperature above 50°F. • Roof free of standing water. • Direct sunshine on the roof during the day. • At least 18°F between the inside and outside of the roof if there is little sun. Through experience, several other physical parame¬ ters may be considered: • Underneath daytime surveys require direct view¬ ing of the roof underdeck. • Best results obtained if the underdeck is coated, painted or fireproofed. • Worst results if the underdeck is bare steel (low emissivity) • Heavy ballast or pavers probably prohibit success¬ ful inspection. • There is no advantage to choosing a short-wave vs. a long-wave infrared imager. CONCLUSION You can get a good night’s sleep! Not every roof will be inspectable from the inside. However, for those roofs in plain sight, give it a try. Lucier— 67
