Comparisons
Open-Cell vs Closed-Cell Spray Foam
Both are polyurethane; the difference is whether the cells are broken open (light, spongy, vapor-open) or sealed (dense, rigid, vapor-closed). They are not interchangeable.
Updated January 2026 · about 6 min read
Quick comparison
Open-cell: about 0.5 lb/ft³, R-3.5 to R-3.9 per inch, vapor permeable, $0.45–$0.75 per board foot installed, expands about 100x so it fills irregular cavities generously.
Closed-cell: about 2 lb/ft³, R-6 to R-7 per inch, a vapor retarder at 1.5–2 inches, $1.00–$1.75 per board foot, adds racking strength and resists bulk water.
Where each belongs
Open-cell suits interior walls, sound control, and vented-to-unvented attic conversions in mixed climates where the roof deck must stay able to dry inward.
Closed-cell suits rim joists, crawl spaces, below-grade walls, flood-prone areas, metal buildings and any place where you need maximum R in a thin cavity.
Gear for this job
Great Stuff Gaps and Cracks Insulating Foam Sealant,12 Ounce - Case of 12
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BEEST FullStop Spray Foam Insulation Kit - 240 Board ft/sq.ft. Fire Rated Spray Insulation Foam for Walls & Attics with Thermal & Soundproofing - Includes 12x Cans, 1 Cleaner, Pro X Gun, Safety Gear
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BEEST FullStop Fire-Rated Spray Foam Insulation for Walls & Attics - 240 Board ft/sq.ft - High Density Expanding Spray Insulation Foam Sealant for Thermal & Soundproofing - 12 Pack (12x26.8 oz)
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Magic Chems Spray Foam Insulation, Expanding Sealant, 2 Pack-25.4 oz | Closed Cell; High Density; Covers up to 40 sq ft; Cans Only, No Gun Included
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Cost per R, not per board foot
At equal R-value, open-cell is usually 25–40% cheaper — but only if you have the cavity depth to spend. In a 2x4 wall, closed-cell delivers R-13 in 2 inches while open-cell needs the whole bay to get to R-13.
Sound and moisture
Open-cell is markedly better at absorbing airborne sound. Closed-cell is better at blocking bulk water and vapor. Neither replaces a drainage plane or flashing.
Run the numbers
Frequently asked questions
- Which spray foam is better for an attic roof deck?
- Both are used; open-cell is common in mixed climates so the deck can dry inward, closed-cell in cold or hurricane zones. Follow local code for ignition barriers and venting.
- Is closed-cell foam a vapor barrier?
- At about 1.5–2 inches it becomes a Class II vapor retarder, which is enough for most assemblies.
- Does open-cell foam absorb water?
- It can hold water temporarily but dries out. It should not be used where bulk water contact is likely.
Insulation gear worth owning
The tools and materials that show up on almost every insulation job.
BEEST FullStop Spray Foam Insulation Kit - 240 Board ft/sq.ft. Fire Rated Spray Insulation Foam for Walls & Attics with Thermal & Soundproofing - Includes 12x Cans, 1 Cleaner, Pro X Gun, Safety Gear
Check price on Amazon
Great Stuff Gaps and Cracks Insulating Foam Sealant,12 Ounce - Case of 12
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4 Pack High-Density Core with Fabric Cover Acoustic Panels(Pebble Gray) | 24 × 12 × 2in self-adhesive soundproofing and sound-absorbing wall panels for echo reduction in studio, office, and more
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Respirator Mask with Filters set - Reusable Respirator Mask for Chemicals, Painting, Spraying, Epoxy Resin, Dust, DIY etc
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