Direct answer
By board foot (1 sq ft × 1 inch thick). Closed-cell: $1.50-$2.50/bf. Open-cell: $0.50-$1.20/bf. A 1,500 sq ft attic at 6" closed-cell = ~$8K; at 6" open-cell = ~$3K-$4K.
Authoritative sources
- US Department of Energy ↗
DOE Energy Saver guide to insulation types, R-values, and recommended applications.
- IRS Section 25C ↗
Energy Efficient Home Improvement Credit — up to $1,200/yr for qualifying insulation work.
- Energy Star ↗
Energy Star recommendations for attic and wall insulation R-values by climate zone.
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