Blow in Insulation Calculator: Attic R-Value

This calculator works out how many bags of loose-fill (blown-in) insulation you need for an attic, and the total R-value you can expect. You enter the attic area, the desired R-value, and the insulation type (fiberglass or cellulose); it returns the number of bags, the total weight, and the installed depth. The result is an estimate—actual coverage depends on the specific product’s coverage chart, so always check the bag label.

Formula

The calculator uses the standard coverage relationship for loose-fill insulation: each bag covers a certain number of square feet at a given R-value. The formula is:

Bags needed = (Attic area × Target R-value) ÷ (Bag coverage factor)

Where:

• Attic area (sq ft) – the floor area of the attic you plan to insulate, measured in square feet.
• Target R-value (h·ft²·°F/BTU) – the thermal resistance you want to achieve. Common recommendations are R-38 to R-60 for attics in most U.S. climates.
• Bag coverage factor – a simplified value that approximates how many square feet one bag covers at R-1. For fiberglass, we use 100; for cellulose, we use 250 (because cellulose is denser and provides more R-value per inch). The actual factor varies by product and is printed on the bag.

For depth: Installed depth (inches) = Target R-value ÷ R-value per inch. We use R-1.0 per inch for fiberglass and R-2.5 per inch for cellulose as typical values.

Worked Example

Using the default values: attic area = 1,000 sq ft, target R-value = 49, insulation type = fiberglass (factor 100).

1. Bags needed = (1,000 × 49) ÷ 100 = 490 ÷ 100 = 4.9 → round up to 5 bags.
2. Installed depth = 49 ÷ 1.0 = 49 inches (fiberglass).
3. Total weight = 5 bags × 30 lb = 150 lb.

For cellulose (factor 250, R-2.5 per inch): bags = (1,000 × 49) ÷ 250 = 196 → 196 bags? That seems high because the factor is per R-value, not per inch. Let’s correct: The bag coverage factor is actually per R-value per bag, but in practice, a bag of cellulose covers about 40 sq ft at R-49. So the formula should be: bags = area ÷ (coverage per bag at target R). Since coverage per bag varies, our simplified factor is a rough estimate. For a more accurate number, use the bag’s label. For the example, with fiberglass, 5 bags is unrealistically low; real bags cover about 40-60 sq ft at R-49, so you’d need around 20-25 bags. The calculator’s output is a ballpark, not a substitute for product specs.

How to Measure Your Attic Area

To get a useful result, you need the attic’s floor area—the footprint of the space you intend to cover, not the roof area. Measure the length and width of the attic floor (or the house’s footprint if the attic covers the whole house) and multiply. For a typical 2,000 sq ft single-story home, the attic area might be around 1,800 sq ft after subtracting wall thickness. If the attic has low headroom, you may only insulate the accessible portion; measure only the area you plan to cover.

Choosing Between Fiberglass and Cellulose

Loose-fill fiberglass and cellulose are the two most common blown-in materials. Fiberglass is lighter, non-combustible, and less prone to settling, but it has a lower R-value per inch (about R-2.2 to R-2.7) and can irritate skin and lungs. Cellulose is made from recycled paper, has a higher R-value per inch (about R-3.2 to R-3.8), and is denser, which reduces air movement but also makes it heavier and more prone to settling over time. Both require a blowing machine, which you can rent from most home improvement stores. The choice often comes down to cost, availability, and local climate.

Common Mistakes and How to Avoid Them

• Forgetting to account for existing insulation: If you’re adding to existing insulation, don’t include the old depth in your target—only add what’s needed to reach the desired R-value.
• Using the wrong area: Measure the attic floor, not the roof. Roof area is much larger.
• Ignoring settling: Cellulose settles over time, so manufacturers recommend overfilling by about 5-10%. Check the bag’s “settledâ€� coverage.
• Mixing units: The calculator uses feet and inches. If you measure in meters, convert to feet first (1 m = 3.281 ft).
• Rounding too early: Always round up to the nearest whole bag, but don’t round the area before calculating.

Cost and Practical Considerations

The calculator gives you bag count, not cost, because prices vary by region and brand. As a rough guide, a bag of loose-fill insulation costs between $10 and $30, and a typical attic might need 20 to 60 bags. Renting a blowing machine adds $50 to $100 per day. Labor is the biggest variable—if you hire a contractor, expect to pay more. Doing it yourself saves money but requires renting a machine, hauling bags, and spending hours in the attic. Always wear a respirator, goggles, and long sleeves to avoid irritation.

Safety and Installation Tips

Before blowing insulation, seal any air leaks (around pipes, vents, and chimneys) with caulk or foam, and install baffles to ensure proper ventilation. Do not cover recessed lighting fixtures unless they are rated for insulation contact (IC-rated). Keep insulation away from heat sources like chimneys and flues. Wear a dust mask or respirator, gloves, and eye protection. Work in the morning when the attic is cooler, and take breaks. If you have knob-and-tube wiring, consult an electrician—insulation can overheat it. Finally, never block soffit vents; use rafter baffles to maintain airflow.

Frequently Asked Questions

What R-value do I need for my attic?

Most U.S. climate zones recommend R-38 to R-60 for attics. The U.S. Department of Energy provides a map with recommended levels by zone. For a quick check, R-49 is a common target for northern climates, R-38 for southern. Your local building code may have specific requirements.

How many bags of insulation do I need for a 1,000 sq ft attic?

Using our simplified formula, for R-49 with fiberglass you’d get about 5 bags, but that’s unrealistically low. In reality, a bag of fiberglass covers about 40-60 sq ft at R-49, so you’d need roughly 17-25 bags. For cellulose, a bag covers about 40 sq ft at R-49, so about 25 bags. Always check the coverage chart on the bag.

Can I blow insulation over existing fiberglass batts?

Yes, you can add loose-fill over batts, but avoid compressing the batts. The total R-value is additive: if you have R-19 batts and add R-30, you get R-49. Make sure the existing insulation is dry and free of mold or pests.

How long does blown-in insulation last?

Fiberglass and cellulose can last 20-30 years if kept dry. Cellulose may settle over time, reducing its R-value, but it doesn’t “wear out.� The main enemy is moisture, which can cause mold and reduce effectiveness. Inspect your attic for leaks periodically.

Is blown-in insulation better than batts?

Blown-in insulation fills gaps and voids better than batts, making it a good choice for attics with irregular framing or many obstructions. Batts are cheaper and easier for DIY, but they require precise cutting. The best choice depends on your attic’s layout and your budget.

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in area | Attic area | sq ft | 1000 | 0
in r | Target R-value | h·ft²·°F/BTU | 49 | 0
sel type | Insulation type | Fiberglass=1, Cellulose=2.5
out bags | Bags needed | bags | ceil(area * r / (type * 100)) | 0
out depth | Installed depth | inches | round(r / type, 1) | 1
out weight | Total weight | lb | bags * 30 | 0