Rigid-Foam Insulation for Commercial Roofing: Polyiso vs XPS vs EPS

Since typical membrane roofing does not include the insulation factor, as a customer you have more options to choose from to insulate your facility’s roof. There are a few types of insulation options the facility manager/owner can choose from:

  • Polyisocyanurate (Polyiso)
  • Extruded Polystyrene (XPS)
  • Expanded Polystyrene (EPS)



Here at West Roofing Systems, we’ve been installing Membrane Roofing for over 40 years. And in that time, we are asked a lot of questions about not only roofing membranes, but types of insulation as well. Because it’s not only important to keep water and weather out but to keep your warm and cool air in.

What is a Roofing Membrane?

Single Ply Membrane Commercial Roofing
Single Ply Membrane Commercial Roofing

A roofing membrane is the top waterproof layer of roofing. The insulation underneath the membrane insulates the building while the membrane is the layer to keep water and other weather out.

Roofing membranes can be made out of a variety of materials:

  • Synthetic Rubber
  • Thermoplastic Membrane
  • Modified Bitumen

Rigid Foam Insulation Installation

Prepare Roof Deck

The bottom layer of a single-ply membrane roofing system is the roof deck or substrate. The roof deck can be plywood, metal, concrete, or wood planks. The first step with roof installation is to prepare the roof deck by getting rid of all dirt, dust, and contaminants. This can be done with air pressure, vacuuming, or even just sweeping.

Cut to Size

After the roof deck is prepared, the rigid foam boards need to be cut to size to fit snugly together on your roof. Rigid foam is produced in various size panels (4 ft x 8 ft, 4ft x 4ft, etc.). The panels need to fit together tightly to help prevent warm air from leaving the top of the building.

Attach to Deck Board

Rigid Foam Installation
Rigid Foam Installation

After the roof deck is ready, the rigid foam insulation is installed. Depending on the type of insulation you choose and the desired R-Value you want for your facility, you may need to build up a few layers of insulation. The R-Value is a measurement of heat loss and heat gain. The higher the R-Value means there is a higher ability to retain heat or keep heat out of a facility.

Building Energy Codes Program

The National Roofing Contractors Association (NRCA) suggests at least two layers that are installed with staggered seams. The layers are screwed down or adhered to the underlying roof deck and the seams on the top layer of the insulation board are taped to add protection against water.

Types of Roofing Membrane Insulation

NOTE: scroll below if you’d like to view an infographic comparing the three types of insulation


As the most used insulation type for roofing applications, Polyiso is a closed-cell foam that is created by heating chemicals and synthetic materials into a composite material that cools into rigid-foam panels. Closed-cell foam indicates that all the cells are enclosed, not connecting with other cells or combined with air.

The average cost of polyiso insulation is $0.70/sq ft. for a 1” thick panel. The R-Value starts at R-8 when installed, degrades down to R-5.5 over time (Cold-Weather Performance of Polyisocyanurate).

Rigid Foam Installation
Rigid Foam Installation

Extruded Polystyrene (XPS)

Usually defined by the blue, green, or pink color, XPS falls in-between Polyiso and EPS in the range of price and performance. Extruded polystyrene consists of a blended polystyrene polymer heated and ran through an extrusion process. (Facilitiesnet.com)

XPS is semipermeable with a perm rating of 1. The average cost of extruded polystyrene insulation is $0.42/sq ft. for a 1-inch thick panel. The average R-Value is R-5.

Expanded Polystyrene (EPS)

With the highest R-value per dollar, EPS is used for roof, wall, and floor insulation. EPS can be used for ground contact and does not retain water over time.

Expanded polystyrene consists of the polystyrene polymer impregnated with a foaming agent. The material expands when exposed to heat and is molded into a uniform, closed-cell insulating material. (Facilitiesnet.com) Commonly known as beadboard, EPS is also used to manufacture products like coffee cups.

The average cost of expanded polystyrene insulation is $0.26 /sq ft. for a 1-inch thick panel. The average R-Value is R- 3.6 to 4.2, depending on panel density.

Here is an infographic simplifying the data:

Rigid Foam Insulation: EPS vs XPS vs Polyiso

Ready to learn more about commercial roofing? If so, please download our guide below. It will give you the pros and cons about the most popular roofing systems out there, including:

  • Single-ply
  • Metal
  • Spray foam
  • Roof coating systems
  • Bult-ups
  • Shingles
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