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Rigid Foam Update Over the past two decades, the use of rigid foam insulation in residential construction has grown enormously. From below-grade foundation insulation to structural insulated panel systems, from vinyl siding backers to cathedral ceiling insulation, rigid foam has proved to be an effective means of reducing air infiltration, adding Rvalue, and eliminating conductive heat loss, particularly where conditions limit the allowable thickness of the insulation. Although several foam recipes have come and gone, three main types of rigid foam boards are still readily available today: expanded and extruded polystyrene, and polyisocyanurate (see Figure 1, next page). Expanded polystyrene (EPS), commonly referred to as "beadboard," is a closed-cell foam made of dense polystyrene crystals that are steam-expanded to 40 times their original volume. Although the resulting beads