Choosing between spray foam and fiberglass insulation involves balancing upfront cost against long-term performance. Spray foam insulates and air-seals simultaneously, delivering higher real-world performance than its R-value alone suggests. Fiberglass is far cheaper and suitable for most applications where air sealing is handled separately.
R-Value and Air Sealing Performance
Fiberglass batts: R-2.9 to R-3.8 per inch. Fiberglass doesn't air-seal — it leaves gaps around wires, pipes, and framing that allow air infiltration, reducing real-world performance to 60–80% of rated R-value. Open-cell spray foam: R-3.5–3.7/inch. Closed-cell spray foam: R-6–7/inch — the highest of any common insulation material. Both spray foam types air-seal completely as they expand, achieving real-world performance closer to rated R-value.
Cost Comparison
- Fiberglass batts (R-13, 2x4 wall): $0.40–0.65/sq ft installed
- Fiberglass batts (R-38 attic): $1.00–1.50/sq ft installed
- Open-cell spray foam: $0.44–0.65/board foot (1 sq ft × 1 inch)
- Closed-cell spray foam: $1.00–2.00/board foot
For a 2,000 sq ft attic at R-38: fiberglass costs $2,000–3,000; spray foam at equivalent R-value costs $7,000–15,000+.
When Spray Foam Is Worth the Premium
- Crawlspaces and rim joists where moisture and air sealing are both critical
- Cathedral ceilings and tight spaces where depth is limited
- New construction where spray foam's structural contribution matters
- Additions or renovations where eliminating thermal bridging justifies the cost
When Fiberglass Is the Right Choice
For standard attic insulation (where depth isn't constrained), open wall cavities in new construction, and most above-grade wall retrofits: fiberglass delivers excellent R-value at 1/3 to 1/5 the cost. Pair with a separate air-sealing pass using caulk and foam sealant to close the performance gap. Use our Insulation Savings Calculator to estimate payback for your situation.