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?
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
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 the 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.
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 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 if 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).
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 in 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 in thick panel. The average R-Value is R- 3.6 to 4.2, depending on panel density.
Here is an infographic simplifying the data:
NOTE: did you know, all these types of insulations will be used on almost every type of roofing system. And all these types of insulations need a roofing membrane over top, such as TPO, EPDM, or Built-up roofs.
There’s a roofing system out there that acts as a type of insulation and a membrane.
What we’re talking about is spray polyurethane foam roofing.
If you’d like to learn more about spray foam roofing, please give our spray foam cheat sheet a try,
It’s easy to read, easy to understand, and can be read in about 5 minutes:
Author: Rufus West
Rufus West is a commercial roofing expert! You may see him on the West Roofing website and also on the West Roofing social media pages, giving fun facts and tips about the company and roofing industry.
Just like West Roofing System’s roof solutions, he isn’t bothered by water—it simply rolls off of his back!