Blown In Insulation Calculator

What this blown in insulation calculator does

Blown-in (or blow-in) insulation is sold by the bag, but the target is an R-value, and the bridge between the two is depth. This blown in insulation calculator takes your attic size, your target R-value and the insulation already in place, then works out the depth to blow, the bags to buy, the weight to carry and a cost.

It works as an attic insulation calculator for blown cellulose or blown fiberglass. You can estimate coverage from typical density, or switch to your own bag chart if you have one, which is the better choice for a final order. A cross-section drawing shows the new layer against your joists, and two charts show how the bag count and depth change with the R-value. A second tab converts between depth and R-value for common materials. If you need the attic area first, use our square footage calculator.

How to use the insulation calculator

1. Enter the attic floor. Type the length and width, or a known area, and take off any part you will not cover.
2. Set your target. Pick R-30, R-38, R-49 or R-60, or enter your own.
3. Add what is already there. Choose the existing insulation and its depth, and the calculator counts it toward the target.
4. Choose the material. Blown cellulose and blown fiberglass set different typical values, which you can edit.
5. Use your bag chart if you have it. Switch the coverage option and enter the square feet one bag covers.

Tap the small i beside any field for what to enter. The results update as you type.

In the results, bags to buy is the number to order. Depth to blow is how deep to fill with the blower, and the settled depth is what that layer shrinks to, which is what sets the R-value. The cross-section drawing uses the settled depth against your joists, and the two charts show how the bags and the depth change if you aim for a different R-value.

How much blown-in insulation do I need?

How much insulation you need depends on the R-value you want, and that depends on where you live. The U.S. Department of Energy publishes recommended attic R-values by climate zone, and many fall between R-30 and R-60. Colder climates call for higher numbers. Your local building code sets the minimum, so check it before you decide. The Department of Energy insulation guide explains how to find your zone.

If your attic already has insulation, you only need to add the difference. Measure the depth in several places, multiply by the R-value per inch for that material, and subtract the result from your target. In other words, the insulation you need is the target R-value minus what is already there, spread over the floor area. Insulation that is wet, matted or dirty is worth less than its depth suggests, so treat the estimate as generous.

How to calculate blown in insulation

This blow in insulation calculator follows a few steps, and you can use the same ones to calculate blown in attic insulation by hand:

1. Find the R-value to add. Target R-value minus the existing R-value.
2. Find the settled depth. R-value to add ÷ R-value per inch.
3. Find the weight per square foot. Settled depth in inches ÷ 12 × settled density in pounds per cubic foot.
4. Find the coverage per bag. Bag weight ÷ weight per square foot.
5. Find the bags. Attic area ÷ coverage per bag, plus 5 percent, rounded up.

The weight route is an estimate from typical density. A chart for your exact product is more accurate, which is why the calculator lets you enter it.

R-value, depth and settling

R-value measures resistance to heat flow, and a thicker layer of the same material has a higher R-value. To calculate the R-value of insulation, multiply its depth in inches by its R-value per inch. Blown fiberglass is roughly R-2.2 to R-2.9 per inch, and blown cellulose is roughly R-3.1 to R-3.8. Open-cell spray foam is about R-3.5 to R-3.9 per inch, and closed-cell spray foam is about R-6 to R-7. These are typical ranges, and your product data is the final word. For a metric R-value (RSI), multiply the R-value by 0.1761, or divide the RSI by 0.1761 to get back to R.

Loose-fill insulation settles after it is installed, and cellulose settles more than fiberglass. Manufacturers therefore list an installed thickness and a smaller settled thickness for each R-value. This calculator keeps them apart: the R-value is based on the settled depth, and the depth to blow adds your settling allowance. Settling changes how deep you blow, not how many bags you buy, because the weight of the material does not change.

Cellulose vs fiberglass blown-in

Both materials are installed the same way with a blowing machine, and both work well in an open attic. Cellulose is made mostly from recycled paper and is denser, so it gives more R-value per inch and needs a shallower layer. Fiberglass is lighter and settles less, and it does not absorb moisture the way paper-based cellulose can, although both should be kept dry. It takes a deeper layer for the same R-value. Price, availability and your joist depth often decide the choice.

At the typical values used here, R-38 is about 11 inches of settled cellulose or about 15 inches of settled fiberglass, and a bag covers about 18 square feet for cellulose and about 34 square feet for fiberglass. The fiberglass bag covers more area because each square foot of that layer weighs less. That is why a cellulose insulation calculator and a fiberglass blown in insulation calculator give different bag counts for the same attic. The numbers move with the brand, so use the bag chart option when you have the real chart.

Bags per 1,000 square feet

Bag charts are often written per 1,000 square feet, so this table shows the same view for the typical values in the calculator. It assumes a 25 lb cellulose bag and a 30 lb fiberglass bag, no existing insulation and no waste allowance.

Use it to sanity-check a quote or a bag chart. If your chart is far from these numbers, the product is probably denser or lighter than the typical values, and the chart wins.

Worked examples

Example 1: a bare 20 by 30 foot attic, R-38, cellulose

The attic is 600 square feet and has no insulation. The settled depth is 38 ÷ 3.5 = 10.86 inches, and with 15 percent settling you blow 12.49 inches. The weight is 10.86 ÷ 12 × 1.5 = 1.36 pounds per square foot, so a 25 lb bag covers 18.4 square feet. Bags: 600 ÷ 18.4 = 32.6, and with 5 percent extra 34.2, rounded up to 35 bags. That is about 875 pounds of material, or 54.3 bags per 1,000 square feet.

Example 2: the same attic with 4 inches of fiberglass batts

Four inches of batts at R-3.1 per inch is about R-12.4. To reach R-38 you add R-25.6, which is 7.31 inches of settled cellulose, and you blow 8.4 inches. The coverage per bag is 27.3 square feet, so the order is 24 bags. Counting the old insulation saves a good share of the bags compared with Example 1.

How much does blown-in insulation cost?

The material cost is the bags times the price per bag, plus anything else you buy for the job, such as baffles, air sealing supplies or a blower rental. Some sellers lend a blower when you buy a minimum number of bags, so ask before you budget for one. Enter the price per bag and other costs in the optional section, and the calculator totals them and shows the cost per square foot of attic.

For an example only, if a bag cost $14, the 35 bags in Example 1 would come to $490.00 before other costs. The price is invented to show the math. Local prices vary, so use your own quote. A professional installer often prices by the square foot or by the R-value installed, which includes labor, so compare that against your materials total. The same calculation answers how much blown-in insulation costs for a given attic, and it works as an attic insulation cost calculator once you add your prices.

Before you blow insulation

• Seal air leaks first. Gaps around pipes, wires, chimneys and the attic hatch let warm air into the attic, and insulation does not stop air movement on its own.
• Keep the soffit vents clear. Install baffles at the eaves so the new insulation does not block airflow.
• Keep it away from heat sources. Follow the product instructions and local code for clearances around flues, chimneys and non-IC-rated recessed lights.
• Check for vermiculite. Loose gray-brown pebbles in old attic insulation can contain asbestos, so do not disturb them. See the EPA guidance on vermiculite insulation.
• Protect yourself. Wear a respirator, eye protection, gloves and long sleeves. Blown insulation is dusty and irritating.
• Mark the depth. Place depth rulers or markers across the attic before you start so you can check the layer as you blow.

If you finish walls with drywall as part of the same project, size the boards with our sheetrock calculator, and see our drywall calculators for related tools.

Common mistakes to avoid

• Borrowing another product’s bag chart. Coverage depends on the brand and its density, so use the chart for the bag you are buying.
• Ordering by area alone. The bag count depends on the R-value you want as well as the floor area.
• Ignoring what is already there. Existing insulation counts toward the target, but wet or matted insulation counts for less than its depth suggests.
• Setting depth markers without settling. Blow to the depth that includes settling, or the finished layer will fall short of the R-value.
• Covering the soffit vents. Without baffles, new insulation can block the airflow the attic needs.
• Mixing units. Enter square meters with the unit set to meters, and keep depths in inches.

Blown-in insulation FAQs

These estimates are for planning. Check your product’s bag chart and your local code before you buy. Read our Disclaimer, or contact us if you spot a mistake.