Estimating R-values for enclosed reflective air spaces

Images courtesy RIMA International

by David W. Yarbrough, PhD, PE
Reflective insulation systems (RIS) consisting of a reflective insulation material and an adjacent enclosed reflective air space have been used as building thermal insulation in the United States for nearly a century. ( For more, see W. P. Goss and R. G. Miller’s “Literature Review of Measurement and Predictions of Reflective Building Insulation System Performance; 1900−1989,” in the American Society of Heating, Refrigerating, and Air-conditioning Engineers’ ASHRAE Transactions [vol. 95, part 2 (1989)].) Despite this longevity, there continues to be hesitation in many cases to specify and evaluate the thermal performance of these systems for use in building envelope applications.

Well-vetted R-values based on a large number of measurements have been available to practitioners for more than 40 years (This comes from the 1972 edition of the ASHRAE Handbook of Fundamentals—“Thermal Conductances and Resistances of a Plane Air Space,” Chapter 20, Tables 2a, 2b, and 2c.), and the Federal Trade Commission (FTC) Home Insulation Rule contains guidance for labeling and advertising of reflective insulation products. (The rule’s official name is 16 CFR Part 460, Labeling and Advertising of Home Insulation, § 460.5 [3b – d].) The FTC rule contains specific requirements for advertising thermal resistance values. This article’s purpose is to provide for a quick estimate of thermal performance for a specific set of conditions or a well-defined application.

What is reflective insulation?
Reflective insulations include single-sheet products that consist of low-emittance foils or films bonded to a substrate such as paper, plastic, or polyethylene bubblepack and multiple-layer insulations. In most cases, both sides of the single-sheet insulation are faced with low-emittance foil or film. When only one side has a low-emittance surface, it is important to install that facing the enclosed air space. Reflective insulations with low-emittance surfaces on both sides are often used to create two enclosed reflective air spaces in a series.

The reflective insulation products with multiple layers are installed to form two or more enclosed reflective air spaces. The specified number of layers and the spacing must be present for the expected thermal resistance to be achieved.

Enclosed reflective air spaces are labelled with RSI-values that have the same units and same meaning as other building insulations. The thermal performance of reflective assemblies varies with temperature, as is the case with all insulations. R-values for thermal insulation materials typically decrease as the temperature increases. Products are labelled at a particular temperature—for example, 24 C (75 F)—so comparisons of competing products can be made on a uniform basis.

A reflective assembly consists of two parts. The thermal resistance associated with the enclosed air space (i.e. R#-value) plus the R-value of the reflective insulation material.

Estimation of thermal performance for reflective insulation
Detailed calculations and test results of RIS performance have been published in the past (For more, see this author’s co-written paper with Andre O. Desjarlais, “Prediction of the Thermal Performance of Single and Multi-airspace Reflective Insulation Materials,” which appeared in ASTM Special Technical Publication [STP] 1116, Insulation Materials: Testing and Applications, second volume, edited by R.S. Graves and D.C. Wysocki [1991].), with sophisticated computer simulations released more recently. (See Hamed H. Saber et al’s “Numerical Modeling and Experimental Investigations of Thermal Performance of Reflective Insulations,” published in the Journal of Building Physics [36 (2)] in 2012.) In both of these cases, the papers are intended for specialists rather than general design professionals. Fortunately, ISO 6946, Building Components and Building Elements−Thermal Resistance and Thermal Transmittance: Calculation Method, approaches its estimation of thermal resistance values in a simpler manner. (See the ISO standard’s Annex B, “Thermal Resistance of Air Spaces.”)

Since thermal insulations in the United States are labeled for R-value at 24 C (75 F), the estimation procedure that follows has been set up to evaluate on RIS at this mean temperature or 534.69 Rankine—the corresponding absolute temperature. In addition, the model has been extended to include temperatures from  −23 to 71 C (−10 to 160 F). The estimates are for unventilated air spaces of uniform thickness with both the length and width of the air space at least 10 times the thickness. The accuracy of an estimate is reduced when the dimensional restrictions are not satisfied. R#-value estimates include heat transfer across the enclosed air space by conduction, convection, and radiation.

Unlike many well-known building thermal insulations (e.g. expanded polystyrene [EPS] or fiberglass), the thermal resistance for a RIS depends on heat-flow direction and the thermal emittances of the surfaces perpendicular to the heat-flow direction (e1 and e2). The R#-value also depends on the distance (d), in inches, across the air space and the temperature difference, DT in °F, across the air space.

The thermal resistance of an RIS is the sum of R# and the thermal resistance of the reflective insulation material, Rmaterial, which is small in most cases.

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