Blown In Insulation Calculator

A 40 × 30 ft attic (1,200 sq ft) with 3½ in of old fiberglass batts has about R-11. To reach R-49 you add R-38: Owens Corning's AttiCat chart gives 19 bags per 1,000 sq ft, so 26 bags of 27.5 lb with 10% extra, blown at least 13½ in deep. In Greenfiber Sanctuary cellulose the same top-up is 70 bags, blown 11.2 in deep and settling to 10.4 in.

How the blown in insulation calculator works

Every bag of loose-fill insulation is printed with a coverage chart: for each R-value it lists the bags per 1,000 sq ft, the thickness to blow, the settled thickness and the minimum weight per square foot. The calculator uses those charts directly.

R to add = target R − existing depth × R per inch; bags = attic area ÷ 1,000 × bags per 1,000 sq ft of the next chart row up × (1 + waste)

Charts come in steps (R-30, R-38, R-49, R-60), so the calculator takes the first row that reaches the R-value you still need. It never interpolates between rows, because the thickness and weight on the chart belong to that row.

Worked example (the default)

• Attic floor 40 × 30 ft = 1,200 sq ft
• Existing: 3.5 in of fiberglass batts × R-3.2 per inch (University of Maryland Extension) = R-11.2
• R-49 − R-11.2 = R-37.8, so the R-38 row: 19 bags per 1,000 sq ft, 13.5 in minimum thickness, 0.523 lb per sq ft
• 1,200 ÷ 1,000 × 19 = 22.8 bags; × 1.1 = 25.1, so 26 bags
• Weight check: 1,200 × 0.523 = 628 lb minimum; 26 bags × 27.5 lb = 715 lb

The weight check matters: blowing too fluffy covers more area at the right depth but not the right R-value. If the bags you bought weigh less than the chart minimum for your area, the calculator warns you.

Bag charts in the calculator

Sources: Owens Corning AttiCat fact sheet (27.5 lb bag), Greenfiber Sanctuary attic card Rev K (25 lb bag) and Johns Manville Climate Pro B-7700 attic card. Climate Pro is a contractor product for truck-mounted machines and its card gives no bag weight, so no weight check is made for it.

Cellulose or fiberglass?

Cellulose is denser: R-3.7 per inch against R-2.2 for loose fiberglass (UMD Extension), so the same R-value is thinner but heavier. R-49 is 13.3 in of settled cellulose at 1.77 lb per sq ft, or 17 in of AttiCat at 0.688 lb per sq ft. For a 1,000 sq ft attic that is 78 bags of cellulose or 28 bags of fiberglass, with 10% extra. Greenfiber's chart lists both the depth to blow and the depth after settling; blow to the first and expect the second. Owens Corning's minimum thickness is its settled thickness.

Weight on the ceiling is worth a thought: USG's Gypsum Construction Handbook limits unsupported insulation over ½ in ceiling board to 1.3 lb per sq ft at 24 in framing and 2.2 lb per sq ft at 16 in. R-60 of cellulose at 2.16 lb per sq ft is near that line on 16 in framing and over it at 24 in.

How much R-value you need

ENERGY STAR recommends R-30 in zone 1, R-49 in zones 2 and 3 and R-60 in zones 4 to 8 for an attic with no insulation. If you already have 3 to 4 in, it recommends adding R-25 (zone 1), R-38 (zones 2 and 3) or R-49 (zones 4 to 8). The insulation calculator finds your zone from your state and covers walls and floors too.

Before you blow

Existing insulation is converted at typical R per inch: fiberglass batts 3.2, loose fiberglass 2.2, cellulose 3.7, rock wool 3.1. Old, compressed or wet insulation performs below that; if in doubt, enter less depth. Keep loose-fill away from recessed lights and flues according to the fixture's rating, and keep soffit vents open.

Next steps

Seal the rim joists and attic hatch with foam first: the spray foam calculator counts kits and the spray foam insulation cost calculator prices the installed job. If the ceiling below needs new board, the drywall calculator counts sheets and screws. More tools for walls and ceilings are in drywall, paint and walls.