Understanding R-Value in Commercial Roofing

When designing a commercial roofing system, insulation plays an important role in overall building performance. One of the most important numbers to understand when evaluating roof insulation is R-value. Knowing what R-value means and having the right information readily available can help roofing professionals make more informed decisions when selecting insulation.

What Is R-Value and Why Does It Matter?

R-value measures a material’s resistance to heat flow. Simply put, the higher the R-value, the greater the insulation’s ability to slow heat transfer through the building envelope.
In commercial roofing, insulation thickness and R-value work together to help a roof assembly achieve its thermal performance goals. Polyiso is commonly specified because it provides high thermal resistance in a relatively thin profile, giving project teams an efficient way to achieve the desired R-value within the roof assembly.

Understanding LTTR

For polyiso insulation, R-value is commonly expressed as Long-Term Thermal Resistance (LTTR), which represents a 15-year, time-weighted average of the insulation’s thermal resistance. LTTR accounts for changes in thermal performance as insulation ages, providing a more representative measure of long-term performance.
LTTR gives the roofing industry a standardized way to evaluate polyiso thermal performance and is incorporated into ASTM C1289. For roofing professionals, it provides a consistent reference when selecting insulation to meet a project’s R-value requirements.

From Specification to Installation

So, how much R-value does a roof actually need? That depends on the project. Minimum insulation requirements are typically defined by applicable energy codes and can vary based on factors such as climate zone, building type, or roof construction. Project specifications may also identify the minimum R-value the insulation package must provide. Once that requirement is established, R-value helps inform the type and thickness of insulation needed for the roof assembly. Because requirements can vary by project and jurisdiction, roofing professionals should reference the project specifications and applicable local building and energy codes when selecting insulation.

R-Value Information, Right in Your Pocket

To make commonly referenced information easier to access, Johns Manville has introduced the R-Value Stocking Card.

Designed as a convenient, pocket-sized reference, the card provides JM polyiso LTTR values across a range of thicknesses, along with pallet information. It also provides dimensions and stocking information for JM Tapered Polyiso panels.

Explore High R-Value JM Polyiso Solutions

Johns Manville offers multiple polyiso products to support the needs of commercial roofing systems.

ENRGY 3® is a closed-cell polyisocyanurate roof insulation board with glass-reinforced facers that offers high R-value to help insulate the building from the elements.

ENRGY 3® CGF also offers high R-value per inch while incorporating inorganic coated glass facers that provide a smooth surface for adhered and self-adhering roofing systems.

These solutions give roofing professionals options for addressing thermal performance alongside the other design and performance requirements of the roof assembly.

Making Informed Insulation Decisions

The right insulation strategy starts with understanding the project’s thermal performance requirements. With high R-value polyiso solutions and a broad commercial roofing portfolio, Johns Manville helps roofing professionals build systems designed to meet the needs of each project.