Extruded Polystyrene (XPS) insulation
Photo: David Hamsley
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Extruded Polystyrene (XPS)

Its closed-cell structure stops water and water vapor, resists compression, and holds its R-value over time. Must be protected from solvents and sunlight. Flammable and must be protected from fire with drywall or plaster.

Good for: In-ground foundation insulation, masonry structures
R-value per inch: 5
Vapor barrier needed: No
Cost: 50¢ per sq. ft. for a 2-in.-thick panel
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