Expanded polystyrene insulation and polyisocyanurate foam polyiso are the two main insulation types used in EIFS in North America. Overseas, many other types of insulation are used to make EIFS, such a mineral wool and “glass foam.”

Thermoplastic Vs. Thermosetting

EPS is made of styrene. Styrene-based plastics are derived from benzene, originally obtained from trees and used to make tires. Styrene is now synthesized artificially. EPS is not Styrofoam, a trademark of the familiar blue board made by Dow Chemical Co. Styrofoam is extruded foam made in a continuous process and is rarely used anymore in EIFS. EPS is a thermoplastic material and made in large blocks by expanding hollow spheres of styrene plastic beads in a mold. When it is warmed the bead softens, and once the heat is removed it re-hardens. This property is what allows a hot wire cutter to carve curves in EPS. If overheated, EPS melts abruptly and reverts to a resin, which in turn rehardens.

It is possible for EIFS walls made of EPS to get so hot that the foam melts. This leaves the EIFS lamina unsupported and the EIFS coatings can fall off. This melting phenomenon can occur, for instance, by a having a BBQ grill next to a wall, or even on large wall areas due to dark EIFS finish colors and bright sunlight. The maximum service temperature for EPS is around 170 degrees Fahrenheit.

Polyisocyanurate is chemically related to polyurethane, a versatile plastic used for car parts, solid tires, pillows, paints, adhesives, sealants and many other uses. Polyiso is derived from isocyanate in a complex series of reactions. It is mixed with a catalyst to cause a hardening reaction-somewhat like two-part sealants or epoxy glues. The chemicals “boil,” creating tiny bubbles, which turns the resin into a foam. The foam then hardens. Polyiso boards are made on a continuous process at specific thicknesses. Polyiso is a thermosetting type plastic. When overheated it starts to char rather than melt. If the heat is high enough, it can be made to catch on fire. Polyiso is carved using milling/sanding type equipment or by casting the liquid resin in a mold which then rises to fill the mold cavity.

Composition

As the name says, EPS is made of styrene. Styrene is an inexpensive, versatile material and is used for products like model airplanes, ballpoint pen cases, coffee cups, foam coolers and appliance housings. In the case of EPS, it is formed into insulation by making tiny hollow spheres of EPS, which are heated. When heated, they expand and become large spheres. In the case of EIFS, the expanded spheres are about the size of a BB. The spheres are then heated to soften them and pressed together under pressure to form a block. The spheres glue themselves together, forming a light, rigid material. The blocks can be quite large-several feet square and a dozen of more feet long. The block is then sliced into whatever thickness is needed. Before slicing, the block is conditioned by simply letting it rest. This redistributes the stresses and helps keep the board from warping when the boards are sliced out of the block.

R Value

EPS has a thermal resistance (insulation efficiency or R value) of around 3.4. This is the same as fiberglass. Polyiso is higher, at about 6.5. Thus, a thinner piece of polyiso can be used to achieve the same energy savings. Dow’s Thermax sheathing is a common polyiso board.

Chemical Sensitivity

EPS is sensitive to a wide range of solvents, including those used in some paints and adhesives. When exposed, the foam melts. Polyiso is relatively less sensitive. When painting EIFS or applying primers (as in sealant joints), it’s important to be sure the solvent used in the coating be benign toward the EIFS foam, as the solvent might leech through the EIFS lamina and de-bond the lamina from the foam.

Boards Vs. Spray-On

Both EPS and polyiso, when used in EIFS, are used in the form of rigid boards. Polyiso insulation can be spray-applied but the resulting surface is lumpy and requires extensive rasping to make it flat enough to produce a good-looking wall. Thus, spray-on polyiso is not commercially viable.

Fire

Both EPS and polyiso are combustible materials. They can be made to catch fire. The building codes require occupied spaces to be separated by a fire resistant material that will protect the foam for at least 15 minutes. A 1/2 inch thickness of drywall can provide this protection.

The codes require noncombustible materials to be used on commercial buildings. EIFS, although combustible, can be permitted on the outside of exterior walls if the system can pass special fire tests. EPS-based systems can pass these tests; polyiso cannot. This is why polyiso-based EIFS is used on wood framed buildings only, such as a home, and not on tall and large commercial buildings.

Density and Stiffness

The density of both types of insulation affects a range of properties including strength, insulation value and cost. EPS used in EIFS is about a 1-pound per-cubic-foot density. Polyiso is about 1.8 pounds per cubic foot. EPS is “springy” and is also used for packaging for this reason. Polyiso is stiffer and more brittle. This increased stiffness puts extra stress in the EIFS lamina where the polyiso boards abut each other. This, in turn, creates a greater tendency for cracking of the EIFS lamina.

Vapor Barrier

EPS boards are homogenous EPS foam; they have no facings. Polyiso boards have metal (foil) or paper facings, sometimes on one side only. The facing(s) give the board extra strength so they can be handled without cracking. Foil facings can also act as a vapor barrier. Foil facings also make it impossible to use adhesives to bond the boards to the substrate or to apply surface coatings. Thus, polyiso EIFS walls use mechanical anchors (washers plus screws) to attach the boards to the substrate.

Sheet Sizes

EPS boards for EIFS are usually 2-feet by 4-feet. Polyiso board can be up to 4-inches wide and 8-feet long or longer. You’d think this would make installing polyiso faster. In reality, there’s not much difference but there’s a bigger problem. The large sheet size makes it harder to keep the boards flat and having a facing on one side tends to make the board warp because one side expands more than another.

Molded Polyiso Shapes

If a building has a lot of foam shapes that are the same, and polyiso is selected as the foam type, sometimes it’s cost effective to make a mold and make the shape in its final configuration, rather than using milling and sanding to carve the surface. The columns on the building (on page 46) are made this way.

Cost

EPS is less expensive than polyiso on an installed volume basis. The raw materials polyiso is made of is more expensive than styrene and makes a denser foam.

Using EPS and Polyiso Together

EPS is used much more often than polyiso for making foam shapes but the two foam types can be used together. I’ve seen several buildings where the flat base layer of EIFS is polyiso and EPS foam shapes are glued on top of the polyiso layer’s basecoat. The office building shown in the photo on page 46 is EPS and polyiso.

Which to Use

Often the selection is a no-brainer. This is often the case on commercial projects where the only foam type approved by code is EPS. Thus the market for polyiso is smaller and usually limited to homes and lowrise wood framed buildings.