Fiberglass vs Cellulose Insulation

Cellulose delivers about R-3.6 per inch; blown fiberglass about R-2.5. To reach R-49 you need roughly 14 inches of cellulose or 19 inches of fiberglass — a real consideration in a shallow attic or where you want to keep storage space.

Dense cellulose (about 1.5 lb/ft³ blown loose) resists airflow far better than light fiberglass. In very cold weather low-density fiberglass allows convective looping, which can cut its real-world performance noticeably. Cellulose holds its rating better in the deep cold.

Material is close: roughly $0.60–$1.10 per square foot at R-49 for cellulose and $0.75–$1.30 for fiberglass. Installed prices run $1.60–$2.80 per square foot for either. Cellulose usually wins slightly on material, fiberglass on bag handling weight.

Cellulose absorbs and releases moisture; if it gets soaked it mats and must be replaced. Fiberglass sheds water and dries out. Cellulose settles 10–20% (labels account for this); fiberglass barely settles.

Both are treated against pests and fire — cellulose with borates, which also deter insects and mice, fiberglass because it is inorganic and non-food.

Cold climate, deep-enough attic, want the best air resistance for the money: cellulose. Shallow attic, humid or leak-prone roof, or you want the lightest load on the ceiling: fiberglass. Either one is only as good as the air sealing underneath it.