By: Cameron Stoddard, Sales Representative – Architectural Products
The “Curtain is Falling” the truth is coming out regarding the actual performances of Polystyrene (EPS) and Polyisocyanurate (ISO) insulation and Polystyrene is shining. Some of the myths of these foam insulation’s are as follows:
• Thermal Resistance Value
• Polystyrene (EPS) needs a thermal barrier for “Direct to Metal Deck” applications
• Fire resistance, “EPS passes ASTM E 84 but that is misleading because Polystyrene can drip into the interior of the building and could cause fires to spread.”
Thermal Resistance Value
Most polyisocyanurate (ISO) manufacturers market their roofing insulation products based on a “conditioned” R-value of 6.0 per inch. While this figure is impressive on paper, it doesn’t tell the entire story. Due to thermal drift, the R-value of polyiso may drop dramatically to 5.6 within 180 days of manufacture.2 So, the questions for polyiso buyers are: “What R-value did you buy?” and, “What R-value will you still have in 15 years, 20 years?”
Ask your Polyiso manufacture for their R-value warranty and compare it to the Polystyrene warranty. The fact is that Polyiso loses R-value over time, Polystyrene does not. Due to the fact that Polyisocyanurate (ISO) loses R-value as it ages, the NRCA recommends designers use R-5.6, or a R-5 depending on average temperatures.
The bottom line is that even with the higher reported R-value of R-6, Polyiso is the highest cost per R-value gained, with the proper reported R-value of R-5.6, or R-5, the cost of polyiso’s R-value compared to Polystyrene is even larger. For more information on the diminishing return of Polyiso check out the article from EPS Roof Insulation Tech Bulletin 3001, “NRCA Polyiso R-value recommendation.”
Direct to Metal Deck
Expanded Polystyrene (EPS) can be applied direct to metal deck without a thermal barrier and be in compliance with the code. The requirements for thermal barrier application with foam plastics are covered by the 2006 International Building Code (IBC). IBC Section 2603.4 requires that foam plastic shall be separated from the interior of a building by an approved thermal barrier unless approved for in sections 2603.4.1 or 2603.9.
Thus, most installations of foam plastics in building construction require a thermal barrier. However, the code recognizes that roof deck constructions may not require a thermal barrier when evaluated under recognized test methods “A thermal barrier is not required for foam plastic insulation that is part of a Class A, B, or C roof-covering assembly, provided the assembly with the foam plastic insulation satisfactorily passes FM4450 or UL1256.” UL has investigated EPS insulation as part of an assembly in accordance with UL1256 full scale fire test. ICC-ES has recognized EPS Insulation as part of an assembly in accordance with UL1256 large scale fire test.
One myth is that expanded polystyrene (EPS) will melt and spread the existing fire throughout the building causing potential fires. The ASTM E-84 test is used to test for flame spread when a component is placed in direct contact with a flame. It is not an indicator of spread of flame on the underside of a roof deck. A UL 1256 test is a test that measures the performance of a plastic insulation when installed direct to metal deck. There are 2 variations of the UL 1256 test: the small scale test and the large scale test known as the “White House” test.
In Summary, when comparing Polystyrene to Polyiso you need to look at where your information is coming from and use common sense to determine what is fact and what is fiction. Polystyrene has passed and proven it’s required performance, and is the clear choice for roofing insulation when considering, thermal resistance value, long term stability, recyclability, dimensional stability, and versatility. I would advise the designer, consultants and owners to look at the facts of long term performance and consider where you are getting your information from when choosing what insulation is best for you.
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