Emissivity Measurements of Common Construction Materials

ABSTRACT

This paper will review and discuss the determination of the emissivity of common construction materials to aid the investigator during infrared inspections/surveys of building envelope systems. Specifically, this paper will discuss the results of product testing of constructed wall assemblies. The wall assemblies are constructed to mimic the common materials encountered in building envelope surveys. They include materials such as hardboard lap siding, stone veneers (faux and natural), stucco, exterior insulation and finish systems (EIFS), trim accessories, vinyl frames, painted flashings, self-adhering flexible flashing membranes, oriented strand board (OSB), ethylene-propylene-diene monomer (EPDM roofing material), wood framing, and concrete. In order to provide a better analysis and determination of results with thermography measurements it is important to understand the relationships of the products emissivity on multi-surface, multi-material constructed assemblies. As well, the thermographer should understand the effects of the angle of inclination utilized when conducting thermographic surveys. This paper will deal with the determination of the values of emissivity of such common materials and the effect of the angle of view used at the time of the thermographic imaging. Additionally, it will discuss the effects of visible light on the heating of objects and, therefore, its implications when performing emissivity calculations in the presence of visible light such as sunlight. Multiple full-scale mock-ups of building wall assemblies will be constructed and a determination of parameters will be made from these full-scale assemblies. The results will be tabularized and provided for this paper.

INTRODUCTION

The use of infrared measurements during building skin applications involves the process of reviewing single images with multiple emissivities. This paper is being prepared to provide the users with a means of evaluating the differences in emissivities of many common exterior building cladding materials. Mock-up assemblies were constructed of various materials used to commonly clad, cover or protect the building envelope. These mock ups included (as photographed below), stucco, stone, siding, flashing, decking, concrete, coatings, trim, and window frames (vinyl and aluminum). A common laboratory temperature of 70 degrees Fahrenheit was maintained for the testing environment, with low humidity (<40 percent). Application of the infrared was taken at 4 angles of measurement from the surface to develop a means of evaluation of angular effects on the emissivity. The reason for this was the fact that shots distance/angle restrictions are common due to height of buildings and limited viewing angles in more urban or congested areas. This paper should guide the user on the sensitivity of the angular distortion in the measurements using cameras that have a single point emissivity setting.

As known, a perfect blackbody will emit maximum possible radiation at all temperatures and all frequencies, as well at all angles. Conversely, with real or “non-black” bodies the angle of emissivity varies with the angle of interest. As the angle of interest approaches the parallel of the real body, the object of interest becomes almost a specular reflector. Other impacts of the products properties can be seen in the determination of the actual radiation. The emissive power W of a non-black surface for a given temperature (T) radiating to the hemispherical region above it is given by: IMAGE Where IMAGE is the total hemispherical emissivity. The spectral emissive power Wa in BTU/h*ft2 of a non-black surface is given by: IMAGE HERE

Therefore, when IMAGE does not depend on the wave length IMAGE, then IMAGE and the surface of the object is considered a gray body. It is typical that this gray body is assumed when performing calculations. Although this simplicity is desired, it should not be used without thought of the impact of these variables that are relative to the temperature rise. When temperatures are high, this effect of wavelength dependency can be increased substantially.

Many times the object is assumed as a gray body because the information of emissivity variation to wavelength is not available to the engineer. Even with this simplistic approach, the thermographer needs to understand that the emissivity is a function of the material, it’s surface conditions, surface temperature, and also the angle of measurement. Because Kirchoff’s law must be satisfied, that is the relation of the emissivity, absorptivity and reflectivity, the impact of each factor will affect the other factors of the equation. As well the total absorptivity for solar radiation is different from total emissivity for low-temperature radiation, many spectral hemispherical emissivity values will vary for standard building products.

The impact of the field application of infrared photography very often results in application procedures using tight angles or near vertical views of exterior walls as common in the industry. This angular effect on the measurement of temperatures and the overall evaluation of the products performances and interpretation of the data must be therefore considered in the application of the infrared analysis. Each object will have a different emissivity based on the angle of interest. It should be noted that the building materials roughness characteristics will also have some effect on it’s emissivity. It is beyond the scope of this paper to determine the effect of the roughness, but this is another variable that should be considered during field applications. In short, roughening of a surface will typically increase the emittance characteristics of that object.

As mentioned above, the emissivity of any black body is unaffected by the surface conditions, surface temperature, and angle of measurement. A gray body, on the other hand, is greatly affected by these parameters. Specifically, the angle of measurement has a large effect on the emissivity; it will typically decrease as the angle is increased. For the purposes of consistency, a viewing angle perpendicular to the surface will be considered the baseline of zero degrees, and a viewing angle parallel to the surface will be considered 90 (from perpendicular). See the example chart below (Source: Infrared Training Center).

IMAGES

TESTING CONFIGURATIONS

The purpose of this testing was to evaluate the effect of angular viewing on emissivity and, therefore, accurate temperature measurement. In order to properly evaluate an entire range of building envelope systems, several mock-ups were constructed of various materials used to commonly clad, cover or protect the building envelope.

These mock-ups included wood framing, oriented strand board (OSB), building paper, Thermoply sheathing, hardboard lap siding and trim, stone veneers (faux and natural), stucco, exterior insulation and finish systems (EIFS), stainless steel trim accessories, vinyl frames, aluminum frames, painted flashings, selfadhering flexible flashing membranes, ethylene-propylene-diene monomer (EPDM roofing material), and concrete. These materials are commonly encountered during building envelope and roof inspections. The following images show the material or building mock-ups created for testing within the lab.

IMAGES

In order to maintain accuracy while positioning the FLIR Camera for angular readings from 5 feet at 75, 60, 45, and 0 degrees (perpendicular), a system of floor measurements was laid out onto the Laboratory floor. A simple plumb bob was used to center the camera tripod on each of the predetermined points (See Figure 5 below).

IMAGE

The test procedure followed the outline provided in the FLIR training programs to baseline the readings equally. The room temperature was held constant at 70° F for 12 hours prior to the operation of the laboratory experiment. The sample temperature readings were obtained following the use of a directional propane catalytic heater placed approximately 24 inches from the sample surface for approximately 5-10 minutes (this allows the sample product and the Super 88 Tape to reach common steady-state temperatures). The result is a large temperature differential (at least 20° F) between the sample product and the ambient temperature IMAGE.

The procedure used to determine the emissivity of each building material reviewed is as follows:

  1. The camera was set at 0 degrees (perpendicular to the object of interest)
  2. The camera was set to an emissivity of 1.0
  3. The ambient temperature IMAGE was measured (Note: a remote system was used for subsequent readings)
  4. The relative humidity of the room was taken
  5. The background temperature is input to the camera parameters
  6. The camera was set to an emissivity of 0.95 for 3M Scotch “Super 88” electrical tape
  7. The sample was heated using a propane catalytic heater placed approximately 24 inches from the sample surface for approximately 5-10 minutes. (Note: care was taken to ensure that the ambient temperature was not affected by the heat generated from the catalytic heater while taking thermograms)
  8. The image was frozen, and the temperature at the Super 88 tape was measured and recorded
  9. The target product temperature was measured and recorded 10. The image was saved
  10. The image was used to determine the spot emissivity necessary to produce the same temperature as the Scotch Super 88 tape. This adjusted emissivity was recorded as the actual emissivity of the sample product
  11. The parameters were input into the image set up in the camera settings
  12. The target emittance was set in the camera
  13. Steps 7 through 13 were repeated for each angle, as discussed under step 15
  14. The angle of the camera was then reset to 45, 60, and 75 degrees respectively for each reading.

From this testing, the following tabulations were made based on material, angle of reading, and emissivity value determination for each tested material. It should be noted that the emissivity of the Super 88 tape was calibrated for each of the angles prior to testing, and is displayed at the top of the table below. This “calibrated” emissivity of the tape at each angle was necessary in order to maintain a baseline with which to compare the other materials temperatures and calculate accurate emissivities.

IMAGE

As noted above, the angle used when testing the different samples has a significant effect on the outcome of the calculations to determine emissivity. This data is tremendously important to any thermographer who routinely performs building envelope and roofing analysis. Without proper adjustment of the emissivity before taking infrared measurements, the thermographer is unable to obtain an accurate reading. It is for this reason that we are suspicious of smaller, hand-held infrared thermometers. Several of these thermometers do not have the ability to adjust the emissivity. Rather, the emissivity is usually set to some arbitrary value that assumes a near-perfect blackbody (usually between 0.95 and 1.0). Without the ability to adjust the emissivity, the temperature readings obtained by the thermographer from a surface that is not a blackbody, are simply false. As well the spread of these handheld variations of infrared thermometers can range between models. The range of the equipment could indicate an averaging of values of differing emissivity and different temperatures.

FURTHER TESTING

It is commonly said that the visible portion of the electromagnetic spectrum has little effect on infrared thermograms because of the difference in the wavelengths that are sensitive to the typical infrared camera, and that infrared measurements are therefore totally independent of visible light. That is, visible light has little to no effect on the infrared radiation of an object, and therefore can be neglected when calculating temperatures and comparing emissivities. This is only partially true. It is true that an object’s temperature has no effect on the infrared radiation that it produces due to the difference in the wavelengths between visible light and infrared. However, the thermal effects of color simply cannot be ignored while evaluating the temperature and/or emissivity of any object.

The reason for this is simple: In the visible spectrum, the color of an object will affect the absorption of energy and, therefore, the temperature of the object. For example, there are two cars, one black and one white, sitting in an outdoor parking lot in the middle of a sunny day. The black car will be physically hotter than the white car, due to the increased absorption from sunlight. Similarly, the use of 3M Super 88 tape as a baseline for outdoor emissivity and temperature calculations, due to the fact that it is black, can have a significant effect on the calculations when compared to a lighter object. The hotter temperature of the darker object will increase it’s energy radiation, including that energy which is considered to be part of the infrared spectrum. The purpose of the next part of our experimentation was to evaluate the effects of visible light on the calculation of emissivity values.

The test procedure for this further testing followed the outline provided in the FLIR training programs, and followed the exact testing procedure outlined above in steps 1-15. The samples were tested following the use of a pair of 500-watt electric lamps placed approximately 24 inches from the surface. These heating lamps simulate the presence of a visible heat source such as solar energy. From this testing, the following tabulations were made.

IMAGE

As noted above, the use of a visible light as a heat source has a tremendous effect on the angle used when testing any sample. There is a large variation between the emissivities calculated using a non-visible heat source when compared to the use of a visible light source. This has tremendous implications for the thermographer who wishes to use infrared technology as a quantitative temperature evaluation. Because the Scotch Super 88 tape is dark in color, it absorbs more energy from visible light than a subject of lighter color. Although the results displayed in table 2 may be considered extreme due to the proximity of the heat lamps, it clearly depicts the implications of visible light. This data is tremendously important to any thermographer who routinely performs building envelope and roofing analysis in direct sunlight.

The repeatability of the reported emissivities was also evaluated. A random sampling of the products was tested at a later date to ensure that the gathered data was accurate. These were compared with the previously calculated emissivities, and they were all within 3% of the values published in this paper.

SUMMARY

The determined emissivity values in Table 1 indicate the importance of the thermographer’s knowledge base of the product used in the exterior cladding or skin of a building, as well as the need for the operator to provide for both angle inclusion and product emissivity in the reports provided for the purpose of defining proper temperature readings in the analysis of the infrared scan/images. Furthermore, these values verify the wide range of emissivities that can be found in a typical building envelope. It is important that the thermographer be aware of the emissivity of the material that is being viewed and to be analyzed.

The emissivity of the specular, highly reflective materials was determined to be very subject to the camera’s angle of use. However, other items that had less visual mirror effects such as surface roughness were also found to have high variations in the application angle. The importance of the accuracy of the temperatures needed for the interpretation of the results cannot be overstressed in the review of the site parameters used in the application of the infrared thermography. The use of the camera in tight angles, on different materials needs to be evaluated to determine proper temperature measurements.

The angular effects as well as the products both provide for operator input in the evaluation of the surface temperature determination of the buildings cladding and components that are under investigation. Improper evaluation of the infrared readings can lead to improper understanding of the systems performance. The operator must have a basic understanding of the effects of angular effect on the readings, as well as the different emissivity values of each product in relation to the angular readings and the effects of sunlight.

Similarly, the effects of visible light (See Table 2) on the absorption of energy and, therefore, the temperature cannot be ignored when performing a thermal evaluation of an object that is in direct sunlight. The recommendation to use a black tape (Scotch Super 88) as a baseline is somewhat suspicious due to the wide range of subject colors that the thermographer might encounter. The color of the subject simply cannot be ignored, and must be correctly evaluated when performing an emissivity calculation.


REFERENCES

Infrared Training Center, “An Intermediate Course for Thermographers, Level II, Course Manual”, Pub ITC 009 B, 2002-09-11, Copyrighted 2002,

2005 ASHRAE Handbook – Fundamentals, “Heat Transfer, Chapter 3, Heat Transfer, Thermal Radiation, “Pages 3.8-3.9,, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ISBN 1- 931862-70-2, Copyright 2005

ASTM Standard E 1933-99a; Standard Test Methods for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers; Copyright June 26, 2006

ACKNOWLEDGEMENTS

The authors wish to thank the Professional Investigative Engineers, Incorporated for the use of the mock up assemblies and the laboratory facilities and RE Construction Experts, Inc for the actual creation of the mockup assemblies utilized.

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