Blown-In Insulation Calculator
Calculate bags of blown cellulose or blown fiberglass for any attic or open cavity, with depth, R-value, and cost estimate.
How to Use This Calculator
Select your insulation type using the material buttons at the top of the calculator. Blown cellulose is the best value for most attic projects. Blown fiberglass is faster to install and better in humid environments. Enter the square footage of the area to insulate: for a rectangular attic, multiply length by width. For irregular shapes, divide into rectangles and add the areas. Set your target R-value (R-38 is the standard for most US attics). If you already have insulation, enter the current R-value to calculate only what you need to add. The result shows depth to add, bag count with 10% waste, and a cost estimate. See our Attic Insulation Calculator for attic-specific guidance including depth by climate zone.
How to Calculate Blown-In Insulation
The blown insulation formula: Required Depth = Additional R-Value Needed ÷ R-Value per Inch. Volume = Area (sq ft) × Depth (ft). Weight = Volume × Installed Density (lb/cu ft). Bags = Weight ÷ Bag Weight × 1.10 waste factor.
| Property | Blown cellulose | Blown fiberglass |
| R-value per inch | 3.7 | 2.5 |
| Installed density | 1.5 lb/cu ft | 1.25 lb/cu ft |
| Typical bag weight | 30 lbs | 19 lbs |
| Depth for R-38 | 10.3 in | 15.2 in |
| Settles over time? | Yes (~20% first year) | Minimal |
Example: 800 sq ft area, R-38 target, existing R-0, blown fiberglass. Depth = 38 ÷ 2.5 = 15.2 in = 1.27 ft. Volume = 800 × 1.27 = 1,013 cu ft. Weight = 1,013 × 1.25 = 1,266 lbs. Bags = (1,266 ÷ 19) × 1.10 = 73.3, so order 74 bags.
Bags per 1,000 sq ft (starting from R-0)
| R-value target | Blown cellulose bags (30 lb) | Blown fiberglass bags (19 lb) |
| R-19 | 24 | 46 |
| R-30 | 38 | 73 |
| R-38 | 48 | 92 |
| R-49 | 61 | 119 |
| R-60 | 75 | 145 |
Includes 10% waste factor. Bag counts assume 30 lb cellulose bags and 19 lb fiberglass bags at standard installed density. Verify against the coverage chart on your specific bag before purchasing.
Blown-In Insulation Tips
Mark your target depth on your attic joists before you start blowing. Cut small pieces of wire or use a yardstick as depth gauges and place them at intervals across the attic floor. Without physical markers, it is very easy to under- or over-shoot your target depth. Attic insulation that is 2 inches short of the target loses significant R-value.
Seal attic bypasses before blowing. The top plates of interior partition walls, gaps around plumbing stacks, and gaps around recessed lights are the biggest sources of heat loss in most attics. Close them with rigid foam and acoustic sealant before blowing insulation. Air sealing adds more energy savings per dollar spent than additional insulation thickness in most homes.
Move methodically from the attic perimeter inward toward the access hatch. Blowing from the center toward the hatch means you would be walking on fresh insulation, disrupting depth uniformity. Start at the far end, work back toward the hatch, and finish at the access point. Close off the attic immediately to prevent disturbance from air movement.
What to Buy
The bag's own coverage chart is your most reliable guide to how many bags to buy. Manufacturers print R-value coverage tables directly on the bag: bags per 1,000 sq ft at R-30, R-38, R-49, and R-60. These tables account for the specific product's installed density. Use your calculated bag count as a check against the chart, not as a replacement for it.
Most large hardware stores rent blowing machines free with a minimum purchase of 10 to 20 bags. Ask about the machine when you buy the insulation; some stores will hold the machine for you if you call ahead. Bring the machine back the same day if possible, as daily rental fees apply after the free period. Pick up a respirator rated N95 or better and disposable coveralls; both are available in the same aisle as the insulation.
Frequently Asked Questions
How many bags of blown insulation do I need for 1,000 square feet? +
For 1,000 sq ft to R-30 with blown cellulose: approximately 38 bags (30 lb bags). To R-38: approximately 48 bags. To R-49: approximately 61 bags. Blown fiberglass requires about 40 to 50 percent more bags for the same R-value due to lower density. Check the coverage chart on your specific bag since installed performance varies by brand.
What is blown-in insulation made of? +
The two most common types are blown cellulose and blown fiberglass. Blown cellulose is made from recycled newspaper and cardboard treated with a borate fire retardant. It is dense (1.5 lb/cu ft installed), provides R-3.7 per inch, and is the most environmentally friendly option with the highest recycled content. Blown fiberglass is made from spun glass fibers and provides R-2.5 per inch at a lighter installed density (1.25 lb/cu ft). A third type, spray foam, is applied differently and is not calculated by bag count.
Can I blow insulation into walls? +
Yes, but the process differs from attic installation. Blowing insulation into existing closed walls requires drilling small holes through the exterior or interior wall surface, inserting a fill tube, blowing until the cavity is dense-packed, then patching the holes. This is called dense-pack insulation and is significantly harder than open-cavity attic blowing. For most DIYers, professional installation is recommended for wall cavities. Open attic spaces are well-suited to DIY blown insulation.
How much does blown-in insulation cost? +
DIY blown-in insulation typically costs $0.25 to $0.50 per square foot for materials only, depending on target R-value. For a 1,000 sq ft attic to R-38, expect $200 to $400 in materials. Professional installation adds $1.00 to $1.50 per sq ft for labor, bringing total cost to $1.25 to $2.00 per sq ft installed. Machine rental is often free with minimum bag purchase, so DIY installation is cost-effective for homeowners comfortable working in an attic.
How long does blown-in insulation last? +
Blown cellulose typically lasts 20 to 30 years before settling significantly. It settles about 20% in the first year as it compacts, then stabilizes. Blown fiberglass lasts longer with less settling since glass fibers do not break down. Both materials perform indefinitely in dry conditions. Moisture is the primary enemy: wet insulation loses R-value, promotes mold growth, and must be replaced. Fixing the moisture source before replacing insulation is essential.
Do I need a vapor barrier with blown-in insulation? +
For attic applications in most US climates, no vapor barrier is needed over blown insulation in an unfinished attic. The existing ceiling drywall or plaster acts as the vapor control layer. In very cold climates (zones 6 through 8) with no existing vapor barrier on the ceiling, a vapor retarder may be required by code. Always check your local building code. For crawl space or basement insulation, vapor control requirements differ significantly by application and climate.
Is blown-in insulation better than fiberglass batts? +
For attic floors, yes. Blown insulation fills gaps, covers joists, and achieves a consistent R-value across the entire surface. Fiberglass batts leave thermal bridges at every joist and leave gaps at irregular framing. Blown insulation also conforms to obstructions and is faster to install in large open attics. Fiberglass batts are better suited for new construction walls and floors between joists, where the cavity dimensions are uniform and installation from one face is easy.
