Thermoplastics are based on linear or slightly branched (non-cross linked) polymers. These foams have a definite melting range and will soften and melt at elevated temperatures. They are also more prone to react and degrade when in contact with some organic solvents as found in some paints, adhesives, and fuels. Therefore it is important to only use manufacturer approved compatible materials when using thermoplastic foams.

Of the thermoplastic foams, EPS and XPS are the most common used in the industry. Both products are based on polystyrene resin and are considered to be closed cell1 rigid foams.

The manufacturing of EPS involves the expanding of polystyrene beads to fill a mold. The densities of EPS foam can be varied if desired. Increased density results in increased thermal resistance and compressive strength. The density of the product also affects the vapor transmission. While EPS is a closed cell foam (slow water vapor and air transmission through the cell walls), the gaps between the cells will still allow for moisture to pass through the matrix. With increased density, these spaces are reduced and the ability of the foam to allow water transmission is reduced.

XPS foams are formed by mixing molten polystyrene with a blowing agent at the correct time, at an elevated temperature, and at an elevated pressure and then extruding the foam through a die to the atmosphere. This creates a more regular cell structure providing for better strength properties and higher water resistance that EPS foams. The density of XPS foams can also be varied, allowing for increased compressive strength, however due to the more regular cell structure, this has little to no effect on the vapor transmission properties.