PIMA Technical Bulletin # 118
Recommended Polyiso Insulation Thicknesses for Meeting Commercial Roof Prescriptive R-value Requirements
Polyiso roof insulation is manufactured in a wide range of thicknesses offering flexibility to design professionals when specifying R-values for commercial roof systems. Under the International Energy Conservation Code, continuous roof
insulation is generally required to be installed in two or more layers (see C402.2.1). In light of the minimum code requirements for insulation across all compliance pathways and industry best practices, all roof system designs for
new and replacement roofs should incorporate a multi-layered, staggered joint continuous insulation system. This technical bulletin provides guidance consistent with this recommendation.
Meeting Prescriptive R-values with Polyiso Roof Insulation Thickness Options
Minimum prescriptive insulation requirements for low-sloped roofs in the United States (insulation entirely above deck - IEAD) and Canada typically range from R-20 in warmer climate zones to greater than R-40 in colder climate zones (an equivalent U-factor may be specified for the roof assembly). Polyiso roof insulation is typically manufactured in incremental thicknesses from 0.5” to 4.5” with versatile options for meeting energy code requirements. Construction professionals should consult polyiso manufacturers for specific information regarding the relationship between product thickness and system R-values.
A multi-layered, staggered joint polyiso insulation assembly provides the benefits of improved thermal performance and condensation control in roof systems. See PIMA Technical Bulletin #113 on “Multi-layering of Polyiso Roof Insulation” for additional details on energy efficiency benefits. The high R-value per inch of polyiso roof products and the availability of tapered systems provides additional design flexibility for assemblies that incorporate polyiso roof insulation.
The examples below provide various, multi-layered configurations for polyiso roof insulation that satisfy minimum R-value requirements ranging from R-20 to R-40. This range of R-values represents typical minimum requirements for the majority of climate zones in the United States and Canada. These examples are common for new and replacement roof project specifications. Polyiso roof insulation boards in various thicknesses also can be combined to address project requirements that exceed R-40.
Example 1: Recommended Polyiso Roof Insulation Thicknesses for Meeting Commercial Prescriptive R-value Requirements.
Figure 1. Roof assembly with two layers of polyiso insulation installed with staggered joints.
Example 2: Recommended Polyiso Roof Insulation Thicknesses with HD Polyiso Cover Board for Meeting Commercial Prescriptive R-value Requirements.
The high R-value and light weight of high-density (HD) polyiso cover boards provide important contributions to the design of a durable roof system. A typical 0.5” thick HD polyiso cover board adds R-2.5 to the total R-value of the roof insulation system.
Figure 2. Roof assembly with HD polyiso cover board and two layers of polyiso insulation installed with staggered joints.
