Cellulose Insulation Calculator: How Many Bags?

This calculator tells you how many bags of cellulose insulation you need for a given area. You enter the length, width, and desired R-value (or depth), and it returns the number of bags required, along with the total volume and weight. The result is based on the standard coverage data printed on most cellulose insulation bags, which assumes a settled density of about 1.5 to 1.8 pounds per cubic foot.

Formula

The calculator uses the standard coverage formula for loose-fill cellulose insulation:

Bags = (Length × Width × Depth in feet × Settled Density) ÷ Weight per Bag

Where:

• Length (ft) – the length of the attic floor or wall cavity.
• Width (ft) – the width of the area.
• Depth (in) – the installed thickness of the insulation (after settling).
• Settled Density (lb/ft³) – typically 1.5 lb/ft³ for cellulose, per industry standards.
• Weight per Bag (lb) – usually 30 lb per bag (common for cellulose).

The result is rounded up to the nearest whole bag because you can’t buy a fraction of a bag.

Worked Example

Using the default values from the calculator: length = 40 ft, width = 25 ft, depth = 10 in.

1. Convert depth to feet: 10 in ÷ 12 = 0.833 ft
2. Calculate volume: 40 × 25 × 0.833 = 833.3 cu ft
3. Calculate weight: 833.3 × 1.5 = 1,250 lb
4. Divide by bag weight: 1,250 ÷ 30 = 41.67 bags
5. Round up: 42 bags

The calculator shows the same: 42 bags, 833 cu ft, and 1,250 lb.

How to Read the Result

The number of bags is what you’d buy for a new installation. If you’re topping up existing insulation, subtract the current depth from the desired depth before entering it. The volume and weight are useful for planning delivery and handling—cellulose is heavy, and a typical attic may require hundreds of pounds.

Note that the result is an estimate. Actual coverage varies by manufacturer, application method (blown vs. poured), and how much the insulation settles. Always check the coverage chart on the bag you plan to buy.

How to Choose the Right Depth

Your target depth depends on your climate and the R-value you want. Here’s a quick guide:

• R-30 (about 8–9 in) – mild climates
• R-38 (about 10–11 in) – mixed climates
• R-49 (about 13–14 in) – cold climates
• R-60 (about 16–17 in) – very cold climates

These are typical values; check local building codes for minimum requirements. The calculator assumes you’re using the settled depth, not the initial blown depth, because cellulose settles over time.

Common Mistakes and How to Avoid Them

• Using the wrong depth: Don’t enter the depth you blow initially; use the settled depth (usually about 10–15% less).
• Ignoring obstructions: Attics have joists, ducts, and other obstructions that reduce the area. Measure the actual floor area, not the entire footprint.
• Mixing units: The calculator uses feet for length/width and inches for depth. Convert everything to the same unit system before calculating.
• Rounding too early: Always round up to the nearest bag, but don’t round intermediate numbers—it can cause a shortage.

Limitations of the Formula

This formula gives a close estimate, but real-world factors can change the number of bags:

• Bag weight: Some bags are 25 lb, not 30 lb. Adjust the formula accordingly.
• Density: The 1.5 lb/ft³ is a typical settled density, but some products settle to 1.8 lb/ft³.
• Waste: You’ll lose some material to overspray, wind, and uneven application. Add 5–10% to your total.
• Manufacturer variations: Coverage charts differ slightly between brands. Use the one on the bag for the most accurate result.

For a precise estimate, it’s best to consult a professional or use the coverage chart on the specific product you intend to buy.

Cost and Environmental Considerations

Cellulose insulation is made from recycled paper (mostly newsprint) treated with fire retardants. It’s one of the most eco-friendly insulation options because it uses recycled content and has a lower embodied energy than fiberglass or foam. The cost per bag is typically lower than fiberglass, but you’ll need more material for the same R-value because cellulose is denser. However, the higher density also means better soundproofing and air sealing.

Safety and Installation Tips

• Wear a dust mask and goggles when handling cellulose—it’s dusty and can irritate skin.
• Use a blowing machine for large areas; hand-pouring is only for small spaces.
• Keep insulation at least 3 inches away from recessed lighting fixtures and other heat sources to prevent fire risk.
• Don’t block soffit vents—use baffles to maintain airflow.
• Check for moisture issues before installing; cellulose can hold water and promote mold if wet.

Frequently Asked Questions

How many bags of cellulose do I need for a 1000 sq ft attic?

For a 1000 sq ft attic at R-38 (about 10.5 inches settled), you’d need roughly 1,312.5 cu ft of cellulose. At 30 lb per bag and 1.5 lb/ft³, that’s about 66 bags. Always add 5–10% for waste.

What is the settled density of cellulose insulation?

Most cellulose insulation settles to a density between 1.5 and 1.8 lb/ft³. The calculator uses 1.5 lb/ft³, which is the most common value cited by manufacturers.

Can I install cellulose insulation myself?

Yes, you can rent a blowing machine from many home improvement stores. However, for large areas, it’s physically demanding and requires careful attention to even coverage. Many people hire professionals for attic insulation.

How long does cellulose insulation last?

Cellulose insulation can last 20–30 years or more if it stays dry. It doesn’t settle significantly after the first year, and it doesn’t degrade like some other materials. However, it can lose R-value if it gets wet.

Does cellulose insulation settle?

Yes, cellulose settles about 10–15% after installation. That’s why the calculator uses settled depth, not the initial blown depth. Manufacturers’ coverage charts account for this.

Is cellulose insulation fire-resistant?

Cellulose is treated with borates or other fire retardants, making it resistant to ignition. It’s not fireproof, but it meets building codes for fire safety. The treatment also deters pests.

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in l | Length | ft | 40 | 0
in w | Width | ft | 25 | 0
in d | Depth | in | 10 | 1
out bags | Bags needed | bags | (l * w * (d / 12) * 1.5) / 30 | 1
out volume | Loose-fill volume | cu ft | l * w * (d / 12) | 0
out weight | Total weight | lb | l * w * (d / 12) * 1.5 | 0