Rigid foam insulation is very popular in residential construction. It is used on roofs, exterior walls and concrete basement walls. Although rigid foam insulation has a high R-value packed in relatively thin sheets, not all rigid foam insulation performs in the same way. When choosing which rigid foam insulation to use you must choose wisely and consider the rigid foam insulation variations and the effect each type will have insulating your home.

There are three different types of rigid foam insulation used in home construction; expanded polystyrene, extruded polystyrene and polyisocyanurate. Rigid foam insulation variations are as follows:

Expanded polystyrene (EPS)

EPS is the insulation used most widely in insulated concrete forms, concrete walls and as structural insulated panels. EPS has the lowest average R-value of the three types of rigid-foam insulations which is R4 per inch. It cost the least at approximately twenty one cents per sq. ft. for a one inch thick 4x8 sheet

EPS is acceptable for ground and below ground use. It also can be treated to resist insects. But a drawback is it can absorb water. When applied as wall or roof sheathing, it should be used over house wrap. Most EPS is non-faced, which means it is fragile to work with and is considered semipermeable. It does not create a vapor barrier.

Extruded polystyrene (XPS)

Easily recognized by its blue, green, or pink color, XPS falls in the middle of the three types of rigid-foam insulation in both cost and R-value. At about R-5 per inch, XPS costs approximately 45¢ per square foot for a one inch thick 4x8 panel. XPS comes either non-faced or with a number of other different plastic facings. Non-faced one inch thick XPS has a perm rating of approximately one, making it semi-permeable. Thicker and faced XPS is stronger and can have a lower perm rating, but either way, it is considered a vapor retarder, not a vapor barrier.

Polyisocyanurate (ISO)

ISO panels are expensive, costing approximately eighty cents per square foot for a one inch thick panel, and with a R-values as high as R-6.5. In actuality the R-values start around R-8 and degrade slightly over time. Because ISO starts as a liquid foam and has to be sprayed against a substrate to form a rigid panel, all ISO panels are faced. A few different facings used on ISO affect the performance of the panel in both durability and perm rating.

Foil-faced ISO panels are considered impermeable because applying ISO to sheathing creates an exterior vapor barrier. They should never be used with an interior vapor barrier.

These are the variations of rigid foam insulation. Be careful where you use them so you will get the most insulation effect.

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