Thermal Performance Evaluation

Ideal vs. Actual Situations

The 2001 ASHRAE Handbook of Fundamentals shows that the thermal performance of reflective insulations is highly dependent upon having low-emittance facing materials and the presence of a smooth, parallel sealed air space where air exchange and movement are inhibited in the construction where they are applied.7 Under the right conditions – with heat flow down and a 3.5" air space at typical application temperatures of 90°F and an average emittance of .03 – R-values of up to 10 can be achieved.8 However, the R-value can be 85% lower9 if:

The heat flow direction changes

The emissivity of the facing is degraded

The air space is less than 3.5"

The air space is not thoroughly sealed

The ASHRAE Handbook of Fundamentals states,“Values for foil insulation products supplied by manufacturers must also be used with caution because they apply only to systems that are identical to the configuration in which the product was tested.”10 Typical installation instructions from reflective insulation manufacturers discuss the value of “dead air space,” in thermal performance, but do not emphasize that in order to be effective, these dead air spaces must be sealed to prevent any air movement due to convection. Typical installation instructions also depict draping the insulation11 to achieve dead air spaces, but neglect to state that the draping can result in non-uniform air spaces which impact the thermal performance. In actual application, heat transfer across an air space involves conduction, convection and radiation and is usually reported as one combined value. In sealed air spaces, the R-value is substantially reduced when the temperature between the surfaces is increased.12 If the air space is not thoroughly sealed, the resistance is also reduced due to convection currents. Having a true, leak-free uniform air space is a nearly impossible situation for most constructions. Tests of metal building roof assemblies containing reflective insulation products were conducted in a hot box apparatus conforming to ASTM C 1363.13 The construction of the test assemblies involved draping the reflective insulation material over zpurlins spaced 60" apart. Sheet metal roofing panels were screwed to the purlins to simulate, as closely as possible, the actual construction details typical of a screwed-down metal roof. For summertime conditions (heat flow down) measured overall (air-toair) R-values averaged 5.9, or less than 50% of the R-value calculated using the 2001 ASHRAE Handbook of Fundamentals values. For wintertime conditions, the measured R-value averaged 3.8, or 25% below the calculated value. (See Table 2). These tests illustrate the importance of accounting for real-world effects when comparing insulation systems for metal buildings.

Surface Emissivity Value

The emissivity value of the surface plays an important role in insulation performance. Data sheets from reflective insulation manufacturers base their claims on new materials that have a bright foil surface. But, normal deterioration due to aging, dust contamination, surface oxidation, or exposure to polluted environments can result in “rapid and severe”14 performance losses of up to 46% or more over time.15 Bright aluminum foil has an average emissivity of 0.05. Once installed, the “brightness”may begin to deteriorate quickly. Minor deterioration can increase the emittance to over 0.2, which, in turn, decreases the thermal resistance. Even the presence of light condensation can increase the surface emittance to 0.30.16

Are Reflective Layers The Answer?

Many marketers of reflective insulations claim high thermal performance based on multiple reflective layers. However, 2001 ASHRAE Fundamentals Handbook warns that the performance of these layers may not be additive.17

Energy Code Compliance

Because the performance of reflective insulation systems depends on specific application conditions, builders may need to use worstcase conditions in order to comply with energy code requirements. There are currently no known third-party programs for certification of reflective insulations. This means builders must rely on the manufacturers’ claims.

Summary

Under ideal conditions, the thermal performance of reflective insulation systems can be predicted using Chapter 25 in the 2001 ASHRAE Fundamentals Handbook.However, if the manufacturers’ claims do not reasonably agree with the ASHRAE calculation results, specific R-values can not be assured.18 In addition, the fire safety characteristics of these products should be carefully evaluated before using them exposed in occupied spaces of buildings.

References

1. Johns Manville test report E436-T-1580 dated 9/30/81, pg. 12.

2. Reflective insulation manufacturer’s packaging.

3. 1999 Annual Book of ASTM Standards, vol. 04.07, ASTM E 84-00a, Section x3.7.8, p. 499.

4. Omega Point Laboratories, 2001, UL1715 Fire Test of Interior Finish Materials, Project No. 13220-109410, pg. 3 and Project 13220-109402, pg. 3.

5. Reflective insulation manufacturer’s packaging. 6. 2001 ASHRAE Fundamentals Handbook, Chapter 25. 7. 2001 ASHRAE Fundamentals Handbook, pg. 23.3.

8. 2001 ASHRAE Fundamentals Handbook, pg. 25.4.

9. ibid.

10. 2001 ASHRAE Fundamentals Handbook, pg. 25.2.

11. Reflective insulation manufacturer’s instructions

12. The Thermal Performance of Reflective Insulation Materials and Systems with Horizontal Heat Flow:A Parametric Study,A.O. Desjarlais & R.P.Tye, pg. 279.

13. Johns Manville Test report CHB-03-001 dated 9/05/03.

14. 2001 ASHRAE Fundamentals Handbook, pg. 25.2.

15. Contamination of Reflective Foils in Horizontal Applications and the Effect on Thermal Performance, J.C. Cook, Jr., D.W.Yarbrough,PhD., P.E., K.E.Wilkes, PhD., P.E.,VA-89-8-4.

16. 2001 ASHRAE Fundamentals Handbook, pg. 25.2.

17. 2001 ASHRAE Fundamentals Handbook, pg. 25.1.

18. 2001 ASHRAE Fundamentals Handbook, pg. 25.2.

19. Omega Point Laboratories, 2001,ASTM E84-00a Surface Burning Characteristics, Report No. 13220-109417, pg. 3 and Report No. 13220-109405, pg. 3.

20. Omega Point Laboratories, 2001, UL1715 Fire Test of Interior Finish Materials, Project No. 13220-109410, pg. 3 and Project 13220-109402, pg. 3.

21. 2001 ASHRAE Fundamentals Handbook, pg. 25.2.

22. ibid. Printed on Recycled Paper PUB. NO. MB313 11/04

 

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