Rigid Foam Board Insulation

Foam board, rigid panels of insulation, can be used to insulate just about any area of your home, from the roof down to the foundation. They offer good thermal resistance and often add structural strength to your home. Foam board insulation sheathing reduces heat conduction through structural elements, like wood and steel studs.

The most common types of materials used in making foam board include polystyrene, polyisocyanurate or polyiso, and polyurethane.

Molded Expanded Polystyrene Foam Board

Molded expanded polystyrene (MEPS) is a closed-cell material that can be molded into many everyday items, such as coffee cups and shipping materials, or into large sheets of foam board insulation. MEPS foam board insulation is commonly known as beadboard.

To make beadboard, loose, unexpanded polystyrene beads containing liquid pentane are mixed with a blowing agent and poured into an enclosed container. The mixture is heated to expand the beads many times their original size. The beads are then injected into a mold. Under more heat and pressure, they expand to become foam blocks, which are shaped as needed.

The physical properties of MEPS foam board vary with the type of bead used. It's manufactured at various densities, depending on the application. Beadboard for roofing materials has to be dense enough to walk on without damage; wall insulation foam boards are several times less dense than roof boards. R-values range from 3.8 to 4.4 per inch (2.54 cm) of thickness.

MEPS foam board is available with a variety of facings. Since spaces between the foam beads can absorb water, a vapor diffusion retarder is necessary if water transmission through the insulation might become a problem.

MEPS foam board also is often used as the insulation for structural insulated panels (SIPs) and insulating concrete forms (ICFs).

Polyisocyanurate and Polyurethane Foam Board

Polyisocyanurate or polyiso and polyurethane are very similar, closed-cell foam insulation materials. Because both materials offer high R-values (R 5.6 to R 8) per inch of thickness, you can use a thinner foam board to achieve the required thermal resistance. This can be an advantage if you have space limitations.

Polyiso foam board insulation is available in a variety of compressive strengths. Compressive strength refers to the ability of a rigid foam board to resist deformation and maintain its shape when subjected to a force or load. Also, polyiso remains stable over a wide temperature range (-100ºF to +250ºF). This makes it good as roofing insulation. And when used with a laminated aluminum foil facing, polyiso foam board provides an effective moisture or vapor barrier.

These foam boards can also be used to make SIPs.

Extruded Expanded Polystyrene Foam Board

Extruded expanded polystyrene (XEPS) is a closed-cell foam insulation similar to MEPS. To make it, the polystyrene pellets are mixed with various chemicals to liquefy them. A blowing agent is then injected into the mixture, forming gas bubbles. The foaming, thick liquid is then forced through a shaping die. When cooled, the panel is cut as required. Foam densities are typically 1.5 pounds per cubic foot (0.21 kilograms per cubic meter).

XEPS is more expensive than MEPS. Like MEPS, the R-value depends upon the density of the material and is generally about R-5 per inch. It's also much more consistent in density and has a higher compressive strength than MEPS, making it better suited for use on roofs or for wall panels. Extruded polystyrene also has excellent resistance to moisture absorption.

Like MEPS, XEPS is available with a variety of facings and is also often used as the insulation for SIPs and ICFs.

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