Direct answer
Spray polyurethane foam (SPF) is a two-component liquid that mixes at the spray gun, expanding 30-100x to fill cavities completely. It hardens into a rigid (closed-cell) or flexible (open-cell) insulating mass.
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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