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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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This Old House To Go









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