How does spray foam compare with fiberglass batts?
Fiberglass batts will not stop air leakage. Spray foam will stop air leakage dead. Spray foam is also mold and moisture resistant unlike fiberglass.
Fiberglass batts have an R-value of about 3.5 (1-inch thickness) but this is not a “true” R-value since there is no air seal. Open Cell Spray foam has a true R-value of about 4.0 per inch and Closed Cell has an R-value over 6 per inch.
Fiberglass batts can sag over time and can leave gaps and small sections uninsulated. Spray foam completely adheres to wood and sheathing; the result is a permanent barrier to heat loss and air entry. In addition, spray foam will add strength to the structure where fiberglass batts will not.