Blown-In Insulation Calculator

About the Blown-In Insulation Calculator

Blown-in (loose-fill) insulation is the most popular choice for attic floors and retrofitting existing walls. It's installed by blowing cellulose or fiberglass fibers through a hose into the space using a blowing machine. The material fills gaps, conforms around obstacles, and creates a continuous thermal barrier without the gaps common in batt installations.

This blown-in insulation calculator estimates the number of bags needed based on your area, desired depth, and the coverage rate per bag. Cellulose insulation (made from recycled paper) and fiberglass loose-fill have different densities and coverage rates, so the calculator accounts for your chosen material.

Most big-box home improvement stores provide free blowing machine rental with a minimum bag purchase (usually 20+ bags), making blown-in insulation one of the most cost-effective DIY insulation projects.

When This Page Helps

Blown insulation coverage per bag varies by manufacturer, material type, and desired depth. This calculator gives you an accurate bag count so you meet the minimum for free machine rental and don't run short mid-project.

How to Use the Inputs

1. Enter the total area to insulate in square feet.
2. Enter the desired insulation depth in inches.
3. Enter the coverage per bag at your desired depth (from the product label).
4. Set a waste factor (5–10% is typical).
5. Review the total number of bags needed.

Example Calculation

Tips & Best Practices

• Coverage per bag decreases as the desired depth increases — always check the bag chart for your target depth.
• Most stores offer free blowing machine rental with 20+ bag purchases.
• Install depth rulers (stakes with marks) across the attic to ensure even depth while blowing.
• Seal all attic air leaks (around pipes, wires, recessed lights) BEFORE insulating.
• Block soffit vents with baffles to maintain airflow while blowing insulation nearby.
• Work from the far end of the attic toward the access point so you don't walk on freshly blown insulation.

Understanding Blown-In Coverage Charts

Every bag of blown insulation has a coverage chart on the label showing square feet per bag at different depths. As depth increases, coverage per bag decreases because more material is used per square foot. Always use the coverage rate for your specific target depth.

Attic Blowing Technique

Start at the far end of the attic and work toward the access opening. Keep the hose at a consistent height and move steadily to build even depth. Use depth markers (ruler stakes) every 4–6 feet to check uniformity.

Dense-Pack Wall Insulation

Wall retroits use dense-pack technique: drill 2–3 inch holes between studs, insert the insulation hose, and blow cellulose at high density (3.5 lbs/cu ft). This fills every void and air-seals the wall simultaneously.

Cost and Savings

Blown cellulose costs about $0.30–$0.50 per sq ft for DIY attic installation (material only). Professional installation runs $1.00–$2.00 per sq ft. Properly insulating an under-insulated attic can save 10–30% on heating and cooling costs.

Frequently Asked Questions

• Cellulose or fiberglass blown-in?

  Cellulose (recycled paper) is denser, cheaper, and has a higher R-value per inch (R-3.5–R-3.8 vs R-2.2–R-2.7 for loose fiberglass). Fiberglass is lighter and doesn't absorb moisture. Cellulose is the more popular choice for attics.

• How many bags per 1,000 sq ft?

  It depends on depth. For R-49 cellulose (about 14" depth): approximately 25–30 bags per 1,000 sq ft. For R-38 (about 10.5"): approximately 18–22 bags. Check the specific product's coverage chart.

• Can I blow insulation into walls?

  Yes, through holes drilled in the exterior siding or interior drywall. Dense-pack cellulose at 3.5 lbs/cu ft is the standard technique for wall retrofits. This requires different machine settings than attic blowing.

• Does blown insulation settle over time?

  Fiberglass settles 10–20% over time unless installed at the proper density. Cellulose settles about 15–20% in attics. Manufacturers account for settlement in their coverage charts — the "installed" depth is higher than the "settled" depth.

• Do I need to remove old insulation first?

  Usually not. Blown-in insulation can be added on top of existing batts or loose-fill. The R-values are additive. However, remove old insulation if it's water-damaged, pest-contaminated, or if you need to air-seal first.

• Can I rent a blowing machine?

  Yes, most Home Depot and Lowe's locations offer free 24-hour blowing machine rental with a minimum insulation purchase (typically 20+ bags). The machine, hose, and basic instructions are included.

• How long does it take to blow an attic?

  A typical 1,000–1,500 sq ft attic takes 2–4 hours with two people (one feeding bags into the machine, one directing the hose). Plan a full day including setup, insulating, and cleanup.

• Is blown insulation safe?

  Modern cellulose is treated with borate fire retardant and is fire-safe. Fiberglass particles can irritate skin and lungs. Wear a dust mask, goggles, and long sleeves during installation regardless of material type.