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.
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