Polyiso vs. XPS
Thermoset vs. Thermoplastic
XPS is a thermoplastic material which softens or turns to a liquid when heated. Polyiso is a thermoset material meaning it is permanently rigid and cannot be melted back to liquid form. Thermoset materials are generally stronger that thermoplastic due to the network of bonds and are better suited for high-temperature applications.
Fire Test Performance
Not all foam plastic insulations perform equally in fire tests. In roofing applications, polyiso insulation can withstand high temperatures which accounts for its successful performance in FM 4450, ANSI/UL 1256 and CAN/ULC-S126 test. Other foam plastic insulations, like polystyrene, are
thermoplastic materials, which soften at 165°F and melt between 200°F to 210°F, long before the standard 30 minute fire exposure ends. Because of the high temperatures reached in the FM 4450 test, polystyrene melts through the seams of the steel deck, spreads fire on the underside of the deck and liberates flaming particles into the furnace. Accordingly, the FM Approval Guide does not list any Class 1 Roof System Approvals for the use of polystyrene insulation in a direct to steel deck application (i.e., without the use of a thermal barrier). Polyiso is still the only foam plastic insulation product to have direct to steel deck approvals from both FM and UL/ULC. FM Approval for Class 1 Roof Systems was granted to polyiso for passing FM 4450, UL/ULC classification was earned by passing ANSI/UL 1256, and ULC classification by passing CAN/ULC-S126. All of these tests are specifically referenced in many building codes.
In every recommended application, polyiso insulation delivers more R-value per inch of thickness than extruded polystyrene products. For example, when both products are used as wall sheathings, the ASHRAE Design R-value for 1” foil faced polyiso is 6.0 per actual inch versus 5.0 for polystyrene, when tested at 75°F mean temperature.
In roof applications, permeable facers are preferred and the average thickness of foam used is 2 inches. At this thickness, the advantage in thermal performance of polyiso over polystyrene is at least 20%. This benefit produces meaningful savings in energy consumption as well in installation costs.
Moisture can be present in a wall or roof system as liquid water or water vapor — two distinct and separate phases of water that can behave very differently. The real enemy of insulation performance is water vapor. If water vapor passes into and condenses in the insulation, the overall thermal performance will decrease. The questions that should be asked are:
- Will water vapor pass into and condense in insulation used as roof insulation or side wall sheathing?
- How can you determine if insulation is prone to water vapor problems?
A material’s resistance to water vapor is determined by testing the product via ASTM E96, a
measure of water vapor transmission. This method produces permeance ratings, or perms. Typically, polyiso foil-faced sheathings have perm ratings of less than 0.03, or 22 times better than extruded polystyrene. This means extruded polystyrene is more likely to let water vapor penetrate into their cells. If this happens and the dew point temperature is reached, water will condense inside the cells reducing the insulation value.
Liquid water should never be present in a building system. If an insulation, polyiso or polystyrene, is submerged in water, the insulation benefit disappears as the water short circuits around the insulation. Insulations must be kept dry. If minor contact does occur, the foil facings and closed cells of polyiso provide excellent water resistance.
Energy Conservation Management, Inc. (ECM) — an independent energy analysis firm in Baltimore, Maryland has performed a number of case studies comparing the construction costs of polyiso insulation and other materials in roof and wall assemblies. Below are the summaries from those studies.
Polyiso insulation emerged as the decisive winner over extruded and expanded polystyrene
insulations in a recent study that compared the cost benefits of using various insulation products
in commercial roofing systems. The study completed by ECM revealed pertinent information about factors, such as return on investment, installation costs and payback rates, which determine the cost effectiveness of using different insulation products in construction applications. The bottom-line numbers showed why polyiso has become the leading insulation product in the marketplace today, as well as why it is the preferred choice for specifiers, architects, and contractors.
ECM performed a case study comparing exterior wall systems using polyiso wall sheathing to
exterior wall systems using wood composite sheathing. The study demonstrated that the polyiso
wall sheathing system using proper shear bracing out performs the wall system using conventional sheathing, by offering lower construction costs while delivering higher effective R-values. The bottom-line numbers of this study reinforced why polyiso is one of the leading insulation sheathing products in the marketplace today.
Polyiso is the Better Choice
Whether the application is roofing, sheathing, cavity wall, EIFS, metal buildings, commercial, or
residential, polyiso is the product of choice.
Polyiso insulation products are easy to specify and meet the requirements of both ASTM C1289,
Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board and CAN/ ULC-S704 Standard for Thermal Insulation, Polyurethane and Polyisocyanurate, Boards, Faced. By referencing ASTM C1289 or CAN/ULC-S704 in your insulation specifications, you can be assured that you have the best insulation product for the project.