EPS vs. polyiso: A climate change for roof insulation

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ACH Foam Technologies applauded the recent decision by the National Roofing Contractors Association to revise the R-value of polyisocyanurate insulation. In The NRCA Roofing Manual: Membrane Roof Systems – 2011, NRCA updated its design R-value recommendations for polyiso, taking into account polyiso’s loss of R-value as the material voluntarily emits low conductance gas after it is manufactured.

ACH Foam Technologies is a major manufacturer of Expanded Polystyrene roof insulation. According to Tom Huempfner, ACH Foam Technologies’ Vice President of Sales, all rigid foam insulations are made from petroleum derivatives. These products all use trapped air as their insulating medium and have the same actual (in-service) R-value per inch at similar densities. Confusion has existed over the years as to the actual in service R-value of polyiso (ISO) when used as insulation in roof systems. Polyiso manufacturers have advertised a calculated R-value, which may be inflated as explained below. After the initial point of manufacture, polyiso contains some of the low conductance gas used during the extruding process. However, this gas is only present in the material temporarily. Over a short period of time the gas emits from the material (off-gasses) leaving air as the insulating medium. The end result is that polyiso has the same R-value per inch as other rigid foam insulations at similar densities.

The advertised R value of EPS is not inflated which is why EPS retains 100 percent of its published R-value indefinitely and why the R-value warranty for EPS is 100  of published R-value for 20 years. In comparison, the polyiso warranty is only for 80 percent of the published R-value for 10 years. Building owners who are paying the energy bills and design professionals who size HVAC equipment should be aware of this very important information. EPS manufacturers are pleased with the new recommendations from NRCA. “Finally the roofing industry has stated clearly what we’ve known for a long time,” said Huempfner. “The building owners are better served if they know the actual R-value they are getting from the insulation they purchase.”

Mark S. Graham is the NRCA’s Associate Executive Director of Technical Services. Graham makes clear the issue of changing R-values: “In 2009, NRCA conducted R-value testing at various temperatures. NRCA’s testing of polyisocyanurate insulation at 25F, 40F, 75F and 110F showed actual R-values less than LTTR values.”*

Graham continues, “Although the LTTR method of R-value determination and reporting may be appropriate for laboratory analysis, research comparison and procurement purposes, NRCA does not consider LTTR use to be appropriate for roof system design purposes when actual in-service R-value can be an important aspect of roof system performance.”

NRCA’s 2011 manual recommends designers using polyisocyanurate insulation determine thermal insulation requirements using an in-service R-value of 5.0 per inch thickness in heating conditions and 5.6 per inch thickness in cooling conditions. NRCA also recommends designers specify polyisocyuanurate by its desired thickness – not its R-value – to avoid possible confusion during procurement.

Benefits of EPS
According to Huempfner, the increase in use of EPS as a roof insulation over the past several years has to do with its cost and installation advantages, combined with its stable and long-term performance.

Cost & R-Value Warranty Analysis:

Insulation          R-25        R-25 Cost*        R-25 Cost*              Warranty
                       Thickness   Square Foot   (% of Published 
                                                                        R-Value)

Type II EPS      5.50”       $1.10/SF                 100%                     20 Years
Type 2 ISO       5.00”       $1.60/SF                   80%                     10 Years
Type IV XPS    5.00”       $1.60/SF                   90%                     15 Years
*As of March 2011. Subject to current & location pricing.

Huempfner continues, “You can see there is a significant cost savings to use EPS instead of polyiso or XPS. Architects and building owners can then consider spending some of these cost savings on a thicker membrane or a coverboard – both upgrades which would improve the performance and extend the life of the roof.”

                                           Type II                       Type 2
                                           EPS                            Polyisocyanurate

Density                               1.5 pcf                        2.0 pcf

Compressive Strength        15 psi                         20 psi

R-value per inch                 4.6                               5.0

Cost per BF (flat)*             $0.20                          $0.32

Cost per BF (taper)*          $0.215                        $0.43

*As of March 2011. Subject to current & location pricing.

Huempfner explains, “Tapered EPS roof insulation is particularly appealing because the installation is faster due to fewer pieces and less cutting. Also, there is less jobsite waste. Polyiso is manufactured in limited thicknesses, therefore when it is installed in a tapered system, multiple layers of polyiso must be used to achieve the same taper thickness as one piece of EPS.

For the complete NRCA report visit http://www.achfoam.com/media/docs/Roofing/Roofing%203001.pdf. For more information about EPS Roof Insulation visit http://www.achfoam.com/RoofInsulation.aspx

*Taken from an article written by Graham and published in December 2010 on Professionalroofing.net.

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