EPS insulation is by far the most popular type of insulation for EIFS in North America-and worldwide. Why is that? This month’s column explains why. The reason is basically because EPS offers a combination of properties that make it especially suitable for use with EIFS.


In North America almost all EIFS uses EPS-the expanded form. There is some EIFS that uses the extruded form of styrene insulation. In other parts of the world, a vast array of insulation types is used in EIFS. This includes unlikely materials such as glass fiber insulation, mineral wool insulation and even cork. Many of the non-EPS insulations are used elsewhere around the globe because of availability and cost issues, and to meet local building ordinances.


Styrene resin was originally made from the sap of a tree found in the Mediterranean area. Styrene is a versatile material. Styrene can be used to make rigid materials (such as molded plastic parts), semi-rigid materials (rubber for tires, etc.), coatings (such as adhesives and paints), and lightweight materials, such as insulation.

To make styrene into an insulation, its density must be reduced. This density reduction is done by injecting a gas into the styrene material. The credit for developing this styrene insulation technology belongs to the Germans and occurred in the 1930s.

The use of styrene-based materials became wide spread during World War II when other resins were in short supply. Styrene can be blended with other resins to create materials with various properties, like rubber for tires. To give you an idea of how prolific this material is, about 15 billion pounds of the basic resin is produced per year, worldwide.


As plastics go, styrene resin is not particularly expensive. But considering the thickness and square footage of insulation used on an EIFS wall, the amount on a wall can add up and make the EIFS expensive.


The term thermoplastic plastic means that the plastic softens when heat is applied. When the heat is removed, the plastic hardens. If enough heat is applied, thermoplastic materials will melt.

Thermosetting plastics do not soften when heat is applied. If enough heat is applied, they char.

EPS stands for expanded polystyrene and is a thermoplastic. It is sometimes called molded expanded polystyrene or MEPS. It is made by fusing together gas-filled beads into a large block, and then cutting the block into boards. The cutting is done with a heated wire. Various thicknesses and sizes of boards can be made. EPS is made by local foam molders of which there are many in North America.

Styrofoam is an extruded type of styrene. It is thus also a thermoplastic material. It is made in a continuous process at a given width and thickness. Styrofoam is a trade mark of the Dow Chemical Co. Styrofoam is sometimes called extruded polystyrene or XEPS. Several other companies make this type of insulation. Styrofoam has a thin skin on both sides that is denser than the core. This can make getting a good bond difficult by the basecoat and attachment adhesive. For this reason, XEPS, when used with EIFS, is often attached with mechanical anchors.

Polyisocyanurate (or polyiso) foam is also used in EIFS. It is used mostly on wood-frame homes. It is a thermosetting type of foam plastic insulation. It is not used on commercial buildings as it fares poorly in the code-required fire tests.

A thermoplastic plastic, like EPS or Styrofoam, can be cut using heat. Thermosetting plastic needs to be cut mechanically. This ability to be cut with heat allows EPS to easily be formed into complex shapes. EPS can also be molded into complex shapes, such as the protective packaging used to ship televisions or to make picnic coolers.


Since extruded polystyrene is made in a continuous process, its flatness can vary. You’ve probably seen extruded polystyrene boards that are warped. This is OK when such insulation is used as cavity insulation and flatness is not an issue. It is not OK for EIFS applications, as the wall needs to be flat for appearance sake.

EPS insulation is aged before it is cut into individual boards. Aging involves letting the insulation block sit undisturbed at room temperature for several weeks, prior to being cut. Aging can be accelerated by putting the block in a warm area. Aging removes residual stresses that occur from the process of molding the EPS block.


This term refers to the bond of the individual EPS beads to each other. When molded into a block, the small granular EPS beads are expanded into round hollow spheres. These spheres are then bonded together under heat and pressure in a mold. The round beads become faceted in this process and the flat faces stick to each other. The degree to which the beads bond together-the fusion is important.

A high degree of fusion makes the resultant EPS block stronger and also decreases the ability of the EPS to allow water to pass through it. Modern molding techniques, using vacuum technology, pull the spheres together and produce a more uniform EPS board.

The degree of fusion can roughly be checked by bending a piece of EPS until it snaps. When looking at the broken edge, at least 50 percent of the beads should split across their diameter, indicating that the bond from bead-to-bead is stronger than the bead itself.


The softness of EPS allows it to “give.” In other words, it can withstand movement without cracking or splitting. EPS used in EIFS is about one pound per cubic foot density. This is about as light as EPS can be molded. This low density makes the EPS softer and also reduces cost. This low density helps reduce the tendency of the EIFS coatings to crack. Styrofoam and polyisocyanurate are stiffer, and require stronger coatings to avoid cracks.


Some types of foam plastic insulation are intended for use in wall cavities. Such types of insulation are not made to high dimensional tolerances. This is OK as the insulation is hidden in the wall and is not an aesthetic material. EIFS is different in the sense that the wall is quite visible. Gaps between the ends of insulation boards can show as lines due to moisture trying to travel from indoors to outdoors through the air gap, if the boards do not fit together tightly due to inconsistent sizing.

EPS boards are made by cutting and can be made to tight dimensional tolerances. The boards can fit together tightly at their edges, eliminating the air gap problem. Close fitting boards also keep the EIFS lamina from moving, which can cause cracks in the coatings. The key dimensional tolerances for insulation boards are length, width, thickness, flatness and squareness, all of which can be controlled to within 1/16 inch. Off-the-shelf EPS boards, such as those found in lumberyards, are not made to such tight tolerances and do not work well in EIFS.


Being a thermoplastic, EPS melts when subjected to the intense heat of a building fire. If a fire goes out through a window, the heat quickly penetrates the thin EIFS lamina and melts the EPS just below the outside surface of the EIFS. This melting creates an air gap, which tends to insulate the EPS within the EIFS somewhat from additional heat. Full scale fire tests show that the EIFS lamina can stay together long enough to prevent it from opening up and allowing the fire to spread. An open lamina would allow air (oxygen) to get at the foam, possibly perpetuating the spread of flame within the EIFS. This performance is the basis for approving the use of EIFS on commercial buildings in the ICC code.


EPS is resistant to the flow of water.This is why EPS is used for flotation. This means that if the EIFS lamina is breached, water is unlikely to reach the supporting wall. This is different from fibrous insulation, which is used occasionally in EIFS.


Permeability refers to the flow of water vapor-a gas-through a material. EPS is partially permeable. This is important when insulation is applied to the outside of an exterior wall. A lack of water vapor permeability can cause moisture to be deposited in the wall in the winter as the water vapor tries to migrate to the outdoors.


ASTM publishes a standard specifically for EPS that is used in EIFS. This standard addresses many of the issues. described above. The ASTM designation for this standard is E2430.