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Technical Note #105: Incremental R-Value for Cellofoam Poly Shield® EPS Insulation with a Reflective Facer

Monday, January 14, 2019
Cellofoam's Poly Shield® heat laminated sheathing is made of Cellofoam expanded polystyrene (EPS) rigid insulation, faced on both sides with a tough poly laminate.  These facers add considerable strength and durability, enhancing the storing, handling, and installation of this high-efficiency and low-cost insulation.  A popular Poly Shield® option is to replace the clear poly laminate backing with a reflective metalized facer.  In a properly designed wall assembly, the reflective facer retards radiant heat flow and offers a significant increase in R-value, or thermal resistance, relative to non-reflective insulation.  This option is offered for all Poly Shield® EPS insulation products, including standard 4 x 8 ft sheathing as well as Fan-Fold configurations.

When properly installed in an appropriately designed wall system or assembly, expanded polystyrene insulation with a reflective facer can provide a boost in insulative performance due to its enhanced deterrence of radiative heat transfer.  Determining what this incremental R-value actually is compared to plain insulation is not a simple task as it is highly dependent upon a number of factors, including the reflectivity of the facer or its inverse, the effective emittance, the position of the dead air space (vertical, sloped, horizontal, etc.), the mean temperature of the wall assembly, the temperature differential from one side of the wall assembly to the other, the angular direction of the heat transfer through the wall assembly, and the depth of designed-in dead air space that the reflective side faces.  The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook provides tabular data for this incremental R-value for insulation based on variations of these parameters.  These variations are quite extensive, and with different assumptions a wide range in incremental R-values can be found.  The “dead air space” is an enclosed and unventilated air cavity designed to minimize air flow into and out of the cavity.

To simplify determining the incremental R-values for wall sheathing applications, Cellofoam’s technical team recently put together Technical Note 105 - Incremental R-Value for Cellofoam Poly Shield® EPS Insulation with a Reflective Facer and posted it to our website’s library at https://www.cellofoam.com/library.htm.   This effort amounted to acquiring the appropriate information for vertical walls from the ASHRAE 2013 Handbook using Table 3, pp 26.13 - 26.14, Effective Thermal Resistance of Plane Air Spaces, and plotting the data as a function of dead air space and effective emittance of the facer.  The data was then curve-fit with cubic polynomials and appears in Figure 1 of the Tech Note. Since the metalized reflective facer used in Poly Shield® EPS insulation has an effective emittance of about 0.15, we used these curve-fits to acquire and plot values for the R-value increment of Cellofoam Poly Shield® EPS insulation for the different dead air spaces, as shown in Figure 2 of the Tech Note.   

This information shows that a dead air space of about three-fourths of an inch can provide an R-value increment of 2.2 for Cellofoam Poly Shield® EPS insulation when it is manufactured with its reflective facer option and the insulation is correctly installed in a wall system designed with an enclosed and unventilated air cavity.  Similarly, air spaces of even as small as half of an inch can provide an incremental R-value of slightly over 2.0, and larger dead air spaces of 3.5 or 5.5 inches can significantly improve the insulation’s thermal resistance by roughly 2.1 and 2.2, respectively.  This incremental R-value is added to that provided by just the insulation alone.

Dr. Harry Karasopoulos, 14 Jan 2019

VP of  Engineering & Business Development

Cellofoam North America Inc



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