What You May Not Know About Your Roof Insulation

By: Jim Nugent, Regional Sales Manager

In Recent years, building codes increased minimum R-Values for commercial buildings which also increased the cost for new and replacement roofs. The insulation in the roof assembly is the most expensive component in the assembly. For this reason owners, architects, consultants, and contractors should be aware of the cost and performance characteristics of this component of their roof assembly.


Since the late 1980’s Polyisocyanurate (polyiso) has been the dominate rigid foam insulation used in commercial roofs. The growth in Polyiso can be tied to the time frame when membrane manufacturers started producing Polyiso and offering their roof membranes with the insulation, along with their full system warranty.

Expanded Polystyrene (EPS) roof insulation has been used in commercial roof assemblies for over 50 years providing long-term, stable R-values. As the membrane manufacturers invested in producing Polyiso the use of EPS roof insulation became more prevalent in tapered systems, retrofit metal roofs, green Roofs, plazas decks, and other specialty applications.

From 2011 to today the roofing industry has begun to learn more about the long-term R-value of Polyiso, specifically how temperatures above or below 75 degrees can negatively impact the R-value of Polyiso. In 2011 Mark Graham the technical director of the National Roofing Contractors Association (NRCA) published a technical bulletin about the R-value testing of Polyiso. Graham recommended to owners and architects that R-5 per inch was appropriate for parts of the United States where there are more heating days than cooling days, such as the northern part of the US. Whereas, R-5.7 per inch is recommended for areas with more cooling days with warmer climates.

In 2012, Building Science Corporation, a well-respected building envelop consulting firm, published their own findings on the temperature sensitivity of Polyiso. In this article written by Joe Lstiburk, the findings were consistent with Mark Graham’s NRCA technical bulletin. The article confirmed that the R-value of Polyiso drops below R-5 per inch as the mean temperature dips below 40 degrees Fahrenheit.

In 2015 Mark Graham published another technical bulletin on this same topic based on new sampling and testing of Polyiso R-values at 75°, 40°, and 25° degree Fahrenheit mean temperatures. The results of the testing were compelling: average Polyiso R-value at 75° is R-5.55, 40° is R-4.99, and 25° is R-4.05 per inch.


Why is all of this so important and why should building owners be concerned about the findings? Historically Polyiso has published one R-value. In the 1980’s Polyiso’s R-value was as high as R-7 per inch. Today Polyiso’s published R-value is 5.7 per inch. EPS has always published R-values at 3 mean temperatures; 75°, 40°, and 25°. These published R-values have never changed. As a matter of fact, the thermal value of EPS increases as weather conditions get colder. For example, Type II EPS roof insulation insulation has an R-value of 4.2 at 75°, 4.6 at 40°, and 4.8 at 25°. When temperatures fall below 40° it becomes more crucial for your insulation to be effective. The R-values of EPS are equal to or higher than Polyiso! Wait, did I just say EPS R-values are higher than Polyiso….? Yes!

Each year in the US billions of dollars are spent on Polyiso roof insulation, often without any consideration of its effectiveness. Articles published by industry respected organizations confirm that the insulation in those roofs, especially in certain regions of the US, are most likely under-performing. What does this all mean? That there is a large percentage of commercial buildings in the cooler, northern climates that would benefit from a better performing insulation such as EPS. The added benefit of its lower cost per R-value also saves building owners significant material costs. It’s a winning combination: effective, stable performance at a lower cost.

Learn more about Foam-Control® EPS roof insulation.

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One Response to What You May Not Know About Your Roof Insulation

  1. Scott says:

    It took me a minute to realize just how massive the roof is in that first picture.

    I live in a house that was built in 1948, I would love to have more insulation in the roof, and hopefully I can add some when I have it re-roofed in about 10 years.

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