About Blown-In Insulation Calculator (Bags & Settled Depth)
The blown-in insulation calculator answers the checkout-aisle question — how many bags? — from the manufacturers’ own published coverage charts rather than a generic rule of thumb. Every bag of loose-fill insulation carries a chart listing, for each target R-value, the minimum bags per 1,000 ft² and the minimum depth: this tool encodes the verified Greenfiber INS541LD cellulose chart (19.05 lb bags, R-13 through R-60 with settled and initial installed thickness) and the Owens Corning AttiCat expanding fiberglass chart (27.5 lb bags, where settling is negligible), and interpolates linearly between the published rows.
Because most attics are top-ups rather than bare framing, the calculator first converts what is already on the joists to an R-value using representative R-per-inch figures — fiberglass batt 3.1, loose-fill fiberglass 2.5, cellulose 3.7 per inch — and prices only the shortfall dR = target R − existing R. That is the methodology the charts themselves support: insulation R-values add in series, and the AttiCat card literally records Existing + Installed = Total. If the existing insulation already meets the target, the tool says so instead of selling bags with diminishing returns.
How It Works
- Enter the attic floor area in square feet — length × width of the footprint you will blow, up to 10,000 ft².
- Describe the existing insulation: pick the type and enter its average settled depth in inches (measure a few spots with a ruler and average). The tool converts depth to R with representative R-per-inch values; pick "none" for bare joists.
- Pick the target total R-value — the R-30/38/49/60 presets cover the common code targets, or enter a custom value from 10 to 80. Use the insulation R-value calculator to find your climate zone’s prescriptive target.
- Pick the material to blow: loose-fill cellulose (Greenfiber INS541LD chart, 19.05 lb bags) or expanding loose-fill fiberglass (Owens Corning AttiCat chart, 27.5 lb bags).
- Read the results: whole bags to buy (area × chart bags-per-1,000 ft² at the R shortfall, rounded up), the added settled depth the top-up must reach, the initial installed depth to blow to for cellulose, and the R breakdown. If existing R already meets the target you get an at-target note instead of a bag count.
Worked Example
A 1,000 ft² attic with bare joists is insulated to R-38 with Greenfiber INS541LD cellulose. With nothing existing, the R shortfall is the full dR = 38, which is a published chart row: 61.1 bags per 1,000 ft², minimum settled thickness 10.77″, initial installed thickness 11.97″. Bags = 1,000 × 61.1 / 1,000 = 61.1, rounded up to 62 whole bags (about 62 × 19.05 ≈ 1,181 lb of material). The blower should be set to fill to roughly 12″ initial depth so the pile settles to no less than 10.77″.
Formulas
- Existing insulation R-value
R_existing = depth_in × R_per_inch- R shortfall (top-up delta)
dR = R_target − R_existing- Bags to buy
bags = ceil( area_ft2 × bagsPer1000(dR) / 1000 )- Added depth
depth(dR) interpolated from the chart depth columns
Standards & References
- Greenfiber INS541LD attic card / application coverage chart, PM-6.3-194 Rev F (19.05 lb bag; settled-thickness basis) — verified source of the cellulose rows
- GreenFiber cellulose product guide DM-6.3-342 Rev A 11-16 (Lowe’s) — corroborates the INS541LD rows
- Owens Corning AttiCat Expanding Blown-In PINK FIBERGLAS install card, Pub. No. 10025888 (27.5 lb bag) — verified source of the fiberglass rows
- Existing-insulation R-per-inch values are representative, not product specs — actual products vary; check the label
Frequently Asked Questions
How do manufacturer coverage charts work?
Every bag of loose-fill insulation carries a federally mandated coverage chart: for each target R-value it lists the minimum bags per 1,000 ft² of net area, the maximum coverage per bag, and the minimum thickness. Both numbers are binding — you must install at least the minimum bag count AND reach at least the minimum depth, because R-value depends on both the mass of material and the depth of the pile. This tool encodes the verified Greenfiber INS541LD and Owens Corning AttiCat charts and interpolates linearly between their published rows.
What is the difference between settled and installed depth?
Blown cellulose fluffs up when it leaves the hose and then settles over the following months — Greenfiber’s chart compensates by publishing both an initial installed thickness and a settled thickness about 10% lower (11.97″ installed settles to 10.77″ for R-38). Blow to the installed depth; the stated R is reached at the settled depth. AttiCat expanding fiberglass is different: Owens Corning states settling is negligible, so its chart’s installed and settled thicknesses are the same.
Can I use a coverage chart for a top-up over existing insulation?
Yes — attic R-values add in series, and the manufacturers frame retrofits exactly this way: the AttiCat attic card has Existing, Installed, and Total R-value lines, and Greenfiber’s guides direct you to your target R for the attic. The chart rows are published per target R from bare framing, so this tool reads the chart at the R shortfall dR = target − existing. For a dR below the lowest published row (R-13) it pro-rates the first row linearly through the origin, which slightly over-estimates small top-ups — documented and buyer-friendly.
How do I know the R-value of my existing insulation?
Measure the average settled depth with a ruler in a few spots and identify the material: yellow/pink/white fluffy batts or blankets are fiberglass (about R-3.1 per inch), loose gray-white fluff is blown fiberglass (about R-2.5 per inch), and gray-brown shredded paper is cellulose (about R-3.7 per inch). These are representative values, not product specs — compressed, wet, or wind-washed insulation performs worse. If the old layer is moldy, rodent-infested, or vermiculite (which may contain asbestos), deal with that before topping up.
Should I air-seal the attic before blowing insulation?
Yes — air sealing comes first, always. Blown insulation slows conductive heat flow but does little against air leaking through top plates, wire and pipe penetrations, recessed lights, and the attic hatch, and buried leaks are far harder to find later. Seal those with foam and caulk, install covers over non-IC-rated recessed fixtures with 3″ clearance, and dam around flues and chimneys before the first bag goes in the hopper.
Do I need baffles at the eaves before blowing?
If your roof ventilates through soffit vents, yes: staple baffles (vent chutes) in each rafter bay before blowing so the new pile cannot block the intake air path — a blocked soffit starves ridge and gable exhaust, invites winter moisture, and can void shingle warranties. Keep insulation clear of the vents and use the attic ventilation calculator to check the net free area your attic needs.