Is blown-in insulation better than spray foam insulation?

Blown-in insulation and spray foam insulation are better for different situations, so neither is universally better. Blown-in insulation, which includes fiberglass and cellulose, is loose material blown with a machine to fill attics and wall cavities. It is much cheaper, typically $1.00 to $2.50 per square foot installed versus $3.50 to $7.50 for spray foam. It is excellent for attics because you can easily add it over existing insulation to reach R49 to R60, it is DIY-friendly, and it covers irregular joists well. However, blown-in does not air seal. Air can still move through it, so you must separately caulk and foam all air leaks. Spray foam, especially closed-cell spray foam, is both insulation and an air barrier in one step. Closed-cell gives about R6.5 to R7 per inch, adds structural strength, acts as a vapor retarder, and completely stops air leaks and drafts. Open-cell at R3.7 per inch is great for soundproofing. Spray foam is better for new construction walls, rim joists, crawl spaces, and cathedral ceilings where air sealing is critical and space is limited. The downsides of spray foam are high cost, need for professional installation with protective equipment, and potential off-gassing if mixed incorrectly. For attics on a budget, blown-in is usually the best value. For walls, basements, and air sealing, spray foam performs better.