Solar Reflectance for Roof Coatings

November 10th, 2008

Albedo Measurements for Reflective Coatings

INTRODUCTION

Radiation control coatings (RCCs) are described in ASTM Standards to be liquid applied coatings having solar reflectances of at least 0.75 and ambient temperature emittances of at least 0.75 ¹. RCCs are intended to reduce solar heating loads on buidlings by reflecting the incoming solar radiation.

Anderson et al ² have presented an assesment of the use and benefits of using RCCs on exterior surfaces of buildings. Anderson ^3,4 has also discussed the use of RCCs for energy conservation and reduction of air-conditioning loads. The computer modeling ⁵ parts of these studies clearly show significant reductions in daytime roof temperatures and solar loads.

Anderson’s assessment ² included a comparison of solar reflectances, r, from two types of measurements. The data in Table 1 contains r-values measured using ASTM E-9036 and r-values obtained using a reflectometer built by Devices and Services Company. ⁷ The reflectometer gave r-values averaging about 5% higher than those obtained using E-903. This could have due in part to differences in test specimens.

The application of RCCs on exterior surfaces requires specification of the thickness required to achieve the maximum reflectivity and eliminate the influence of the substrate. Table 2 contains r-value results obtained for a single product as a function of coating thickness. ²

Table 1. Solar Reflectances Measured by Two Methods ²

Product	E-903 (R-Values)	D & S (R-Values)
1	0.766	0.797
2	0.723	0.815
3	0.770	0.787
4	0.816	0.817
5	0.789	0.827

Table 2. Measured r-values as a Function of Thickness for a Commercial RCC

Coating Thickness (mm)	Solar Reflectance
0.08	0.59
0.13	0.68
0.15	0.68
0.19	0.72
0.36	0.77
0.51	0.80
0.66	0.79
0.89	0.80
1.14	0.80

The results in Table 1 show a commercial product attaining its maximum solar reflectance of 0.8 when applied at thicknesses of 0.5 mm (20 mils) or greater. The present study was undertaken to develop commercial blends with r-values above 0.8. A parallel effort was undertaken to develop an inexpensive device for evaluating performance. ⁸

CUSTOM BLENDED RCCs

Twenty RCC formulations using readily available components were prepared for this project by Warren Paint and Color Company. ⁹ The compositions of each of the RCCs is given by Nachimuthu. ⁸ An exterior white coating formulation with a solar reflectance near 0.8 was used as a control. Additives such as TiO2, Talc, Mica, SiO2, hollow-glass spheres, or aluminum flakes were added to the basic formulation and tested for solar reflectance at thicknesses greater than 0.5mm. Modest increases in r-values above the 0.8 benchmark previously observed for commercially available RCCs were observed in a few cases. The use of TiO2, Al(OH3), or spheres resulted in increased r-values while SiO2 and aluminum flakes significantly reduced the measured r-values.

The r-values being reported were measured using the solar reflectometer. Twelve test specimens were prepared for each coating and five r measurements were made on each specimen. The results are reported in Table 3 as average r, standard deviation of r, average thickness, and standard deviation of thickness.

The results in Table 3 show only model r-value increases above the nominal 0.80 value for the control. Increasing the amount of TiO2, in the formulation resulted in r-values up to 0.83 but the results are mixed. The use of hollow-glass spheres or alumina also produced r in the range of 0.80 to 0.84. These data suggest that coatings in the range r=0.80 to r=0.85 can be achieved economically by the addition of alumina trihydrate or titanium dioxide.

Table 3. Summary of Results for Twenty RCC Formulations

Coating	Ave. r	Std. Dev. r	Ave. Thickness (mm)	Std. Dev.
Control ( C )	0.799	0.006	0.699	0.046
C + UV Inhibitor	0.787	0.002	0.732	0.084
C + Violet Toner	0.696	0.002	0.765	0.030
C + Talc	0.729	0.002	0.528	0.053
C + UV Inh – CaCO3	0.837	0.003	0.653	0.041
C + MiCA	0.779	0.004	0.831	0.002
C + SiO2 Filler	0.665	0.007	0.820	0.084
C + Hollow Spheres	0.834	0.004	0.894	0.028
C + Embedded Spheres	0.639	1.387	1.387	0.090
C + Aluminum	0.505	0.006	0.681	0.071
C over Al	0.820	0.003	1.100	0.069
Coating over C	0.628	0.001	1.153	0.058
Clear Coating + UV	0.635	0.019	1.173	0.058
Clear + Pearlescent	0.747	0.004	1.168	0.104
C + TiO2 × 2	0.826	0.003	0.790	0.038
C + TiO2 × 4	0.792	0.023	0.732	0.058
C + TiO2 × 6	0.806	0.012	0.668	0.076
C + Alumina Trihydrate	0.806	0.002	0.792	0.056
C + 3 x Alumina Trihydrate	0.814	0.003	0.663	0.048
C + 8 x Alumina Trihydrate	0.801	0.003	0.602	0.064

RCC TESTER

The second objective of this project was the construction of an apparatus to test coated surfaces exposed to solar radiation. The resulting apparatus consisted of 30×30 cm flat surface mounted on 5.08 cm thick extruded polystyrene. The bottom side of the polystyrene boards were in thermal contact with cooling coils used to circulate fluid from an isothermal bath. This stack of materials results in an upward facing exterior surface above a thermal resistance of 1.80 m2 * K/W. The bottom of the thermal resistance was maintained at 15° 3°C by the circulating cool water.

The small RCC testers were used to obtain temperatures of black or white surfaces exposed to the sun. Surface temperatures were obtained for the two surfaces under identical conditions (side by side) in Tennessee in October. Table 4 contains observed surface temperatures as a function of time for the surfaces at angles of inclination from the horizontal of 0°, 15°, 30°, 45° and 60°. The surfaces were oriented to face South as the angle was increased. The temperature differences (black-white) at noon ranged from 17 to 30°C with an average value of 22.4°C. The maximum observed temperature differences in July were 38°C for Las Vegas and 31°C for Tampa. ² The reflectance values used in the calculations were 0.10 for the black surface and 0.85 for the white surface. Maximum temperature differences of 30-32°C between black and white surfaces were observed in August at the Roof Testing Research Apparatus at the Oak Ridge National Laboratory. ² Data from several sources, therefore, indicate significant reduction in roof temperatures due to high solar reflectance of the surface. The point of interest here is that measurements to compare surface temperature can be made with a very simple apparatus. The apparatus can also be used to measure heat flux into the system.

SUMMARY

A study of paints produced using commercially available additives and fillers shows that RCC with solar reflectances in the range of 0.80 to 0.85 can readily be achieved. A previous survey of commercially available RCC reported solar reflectances from 0.78 to 0.83 using the same measurement technique.
An inexpensive apparatus for comparative testing has been described. The appratus could be useful for comparaing candidate RCCs in a given geographical location. The apparatus showed a white surface operating 20-30°C cooler than a black surface. This reduction in surface temperature is consistent with published data from other sources.

REFERENCES

ASTM C 16.21 draft standard dealing with radiation control coaitngs (1994).
R.W. Anderson, D.W. Yarbrough, R.S. Graves, and R.L. Wendt, “Preliminary Assessment of Radiation Control Coatings for Buildings,” ASTM STP 1116, Insulation Materials: Testing and Applications, Vol.2, Edited by R.S. Graves and D.C. Wysocki, American Society of Testing and Materials, PA (1991) pp. 7-23.
R.W. Anderson, “Radiation Control Coatings: An Underutilized Energy Conservation Technology for Buildings,” ASHRAE Transactions 95 (2) (1989).
R.W. Anderson and D.W. Yarbrough, “Use of Radiation Control Coatings to Reduce Building Air-Conditioning Loads,” Energy Sources 15 59-66 (1993).
BLAST – Building Loads Analysis and System Thermodynamics. A building energy simulation software program. U.S. Army Corps. of Engineers, BLAST Support Office, Urbana, Illinois.
ASTM E 903, “Test Method for Solar Absorptance, Reflectance and Transmittance of Materials using Integrating Spheres,” 1993 Annual Book of ASTM Standards, Vol. 12.02, American Society of Testing and Materials, Philadelphia, PA (1993).
Devices and Services Co., Dallas, TX, Solar Spectrum Reflectometer Model SSE-ER, Version 5.0.
R. Nachimuthu, “Assessment of Radiation Control Coatings,” Master of Science Thesis, Tennessee Technological University (1993).
J. Smythe, Warren Paint and Color Company, Nashville, TN.

Tags: Air Conditioning, Albedo, ASTM, R-Value, Radiation Control Coatings, Reflective Coatings, Roof Coatings, Solar Reflectance

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