Plastic Foams

Frequently Asked Questions

1. What exactly are plastic foams?
Plastic foams are plastics that have been “frothed up” while molten, and then

cooled. The process fills them with countless tiny bubbles, much like a sponge.

Manufacturers create many different foams by varying ingredients and

manufacturing methods.

2. What do plastic foams have to do with ICFs and concrete

buildings?

Many of the benefits of concrete structures built with insulating concrete forms

(ICFs) come from the plastic foams of which the ICFs are made. Plastic foams

are lightweight, easily cut, excellent insulators, and good barriers to air and

moisture. They are also strong enough to hold concrete and serve as the backer

for interior and exterior finishes.

3. What foams are used for ICFs?
The ICFs sold in North America are made of either expanded polystyrene (EPS),

extruded polystyrene (XPS), polyurethane, or a cement-foam composite.

EPS and XPS are made from the same plastic (polystyrene), but the process is

different. EPS begins as small plastic “beads” that are expanded and fused

together. It is the same as the white foam in many disposable coffee cups. XPS

begins as a continuous mass of molten material. It is familiar as the yellow foam

used in trays for fresh cuts of meat at the supermarket.

Polyurethane foam results when two separate ingredients (an isocyanate and a

polyol) are mixed and react with one another. Cement-foam composites are not

actually a pure foam, but a mixture of Portland cement and loose EPS beads.

Most ICF manufacturers mold their form units out of the foam they judge best for

the job. But in several ICF systems, the formwork is assembled from standard

flat sheets of foam, available from building suppliers. With these systems, the

buyer may have a choice of different types of foam.

4. What's the difference between the types of foam?
Evidence indicates that all of the foams used in ICFs produce excellent homes.

However, each has its own unique profile of properties. Note also that any one of

these foam materials may vary. By changing manufacturing, producers can get

somewhat different properties with the same plastic, so any numbers given here

for a type of foam are approximate. Consult specific ICF manufacturers for exact

specifications.

Typical Properties of Plastic Foams

	EPS	XPS	Polyurethane	Composite
Density (lbs/cu ft)	1.35-1.80	1.60-1.80	2.00	21
R-value per inch	4.17-4.35	5.00	5.90	3.00
Compressive strength (psi)	15-33	25-40	30	72
Tensile strength (psi)	18-27	45-75	30	42
Water absorption (%)	< 3.0	< 0.3	2.0	NA
Retail cost ($/bd ft)	.17	.35	.70	NA

EPS is usually the least expensive of the foams. It also insulates well, is

resistant to air and moisture infiltration, and is moderately strong. It is available

in either molded or sheet form. XPS is usually more expensive compared to

EPS, but it has about 25% higher insulating value, somewhat greater resistance

to water, and higher strength. It is ordinarily available in sheet form only.

Polyurethane has a somewhat higher insulating value than either of the

polystyrenes and comparable strength, but may cost somewhat more. It is

available in some molded ICF products. A closely related foam, called

polyisocyanurate, is available in sheets. But polyisocyanurate sheet is not

usually recommended for use in ICFs.

Since they contain cement, the composite materials tend to be strong, heavier

than the pure foams, possibly more durable, and require somewhat more effort

to cut and shape. Their insulating value is a bit lower. Cost estimates are

unavailable since the material comes only in completed, molded units.

5. Are these materials safe?
All of the plastic foams are highly inert. None is considered toxic by U.S.

government agencies. None contains any formaldehyde. None has been linked

to respiratory problems or skin irritation, as have some fiber-based insulations.

Attempts to find any emissions from polystyrene foams used in walls failed. No

emissions could be detected by the most sensitive of instruments. The plastic

foams used in ICFs are also formulated so that they do not burn by themselves.

Extensive research on the polystyrenes concluded that when they were forced

to burn, their emissions were “no more toxic” than the emissions of burning

wood.

6. How do I choose the right one?
Most important is to choose the right ICF, not a particular foam. The properties

of an ICF are only partly determined by the foam it contains. For example, the

cost of ICF units might be lower for a product that uses a more expensive foam,

because the manufacturer has achieved production efficiencies. However,

understanding the properties of the different foam materials helps in

understanding the differences among the various ICF systems.

7. What's the bottom line?
The plastic foams used in ICFs help to create a superior house. There is no one

“best” material to use, but they have variances that help to explain the

differences among ICF products.

More Information?

Concrete Homes Hotline: 1-888-333-4840

Concrete Homes Online: www.concretehomes.com