Insulation Calculator
Estimate blown-in insulation from the variables the leading tools use: the area to cover, a climate zone or target R-value, and the material. The calculator suggests the Energy Star R-value for your zone, then derives the settled depth, the installed volume, the number of bags, and an optional project cost, so you do not have to read coverage off a label yourself.
Formula
Worked example
A 1,000 sq ft attic to R-49 with bare joists, blown-in cellulose (R-3.5/in, 1.5 lb/ft³, 25 lb bags), 10% waste: depth = 49 ÷ 3.5 = 14.0 in; volume = 1,000 × 14.0/12 × 1.10 = 1,283 ft³; volume per bag = 25 ÷ 1.5 = 16.7 ft³; bags = 1,283 ÷ 16.7 = 76.9, rounded up to 77 bags. At 12 per bag that is about 924 in material, roughly 0.92 per sq ft DIY. Switch to fiberglass (R-2.5/in) and the depth jumps to 19.6 in for the same R-49.
How the insulation estimate works
The leading blown-in calculators all take the same core inputs and derive the rest, and this tool does too. First it fixes a target R-value: pick your IECC climate zone and it fills in the Energy Star recommendation (R-30 in hot Zone 1 up to R-60 in the coldest zones), or choose to set the R-value yourself. Then it finds the settled depth you need: the R-value shortfall (your target minus any existing insulation) divided by the R-value the material delivers per inch, about 3.5 for cellulose and 2.5 for loose fiberglass. Next it turns that depth into an installed volume by multiplying your floor area by the depth in feet, then padding for waste. Finally it converts volume into bags, because every bag of settled material fills a fixed volume equal to its weight divided by its settled density, so the bag count is the padded volume divided by that per-bag volume, rounded up. You never have to read coverage off a label: coverage per bag is something the calculator computes from your target R-value.
Why R-value and material drive everything
The single variable that moves your bag count the most is the target R-value, because it sets the depth, and depth sets volume. Pushing an attic from R-30 to R-49 raises the cellulose depth from about 8.6 inches to 14 inches, roughly 60% more material over the same floor. Material choice matters just as much: fiberglass needs about 40% more depth than cellulose for the same R-value because it carries fewer R per inch, though its bags fill more volume each. Entering an existing R-value lets the tool fill only the shortfall when you are topping up an attic that already has some insulation, rather than charging you for depth you already have. For dense-pack walls, treat this open-blow figure as a floor, since packed cavities use noticeably more material per square foot. The breakdown table shows bags and depth at every common target so you can see the trade-off at a glance.
Costing the job
Turn on the cost estimate to price the work the way the brand calculators headline it. Material cost is simply the bag count times your retail price per bag, a 25 to 30 pound bag of blown-in cellulose or fiberglass typically runs around 10 to 15 US dollars at the big-box stores. Add an optional labor figure per square foot if you are hiring out the blow; professional installs commonly land between 0.50 and 2.00 per square foot all in, while a DIY blow is usually free of labor because most stores lend the blower with a bag minimum. The calculator reports material cost, the total with labor, and the cost per square foot so you can compare quotes on equal footing. Treat the result as a planning figure: local prices, delivery and access all move the real number.
Settling, depth marks, and waste
Blown cellulose and fiberglass go in slightly fluffed and then settle over the first few weeks, so manufacturers rate coverage at the settled depth this calculator reports. Install to a fluffier initial height so the layer relaxes onto your target rather than below it, and mark the depth on the joists or on rulers stapled across the attic so you can sweep for thin spots. A 10% waste allowance covers uneven coverage, the awkward edges of a typical job, and the fibre that drifts where you do not want it; bump it to 15% for cut-up, heavily obstructed spaces. Rounding up and keeping a spare bag is cheap insurance, an under-filled bay shows up later as a cold strip and lost R-value, not as an immediate problem.
Recommended attic R-value and settled depth by climate zone
| Climate zone | Target R-value | Cellulose depth (in) | Fiberglass depth (in) |
|---|---|---|---|
| Zone 1 (hot) | R-30 | 8.6 | 12.0 |
| Zones 2-6 (mixed to cold) | R-49 | 14.0 | 19.6 |
| Zones 7-8 (very cold) | R-60 | 17.1 | 24.0 |
Energy Star attic targets by IECC zone, with the cellulose (R-3.5/in) and fiberglass (R-2.5/in) settled depth each implies.
Frequently asked questions
How do I find the right R-value for my area?
Pick your IECC climate zone in the calculator and it fills in the Energy Star attic recommendation automatically: roughly R-30 in hot Zone 1, R-49 across most of the country, and R-60 in the coldest northern zones. If you know your exact target or local code value, choose "Pick target R-value myself" to set it directly.
Why does this ask for R-value instead of coverage per bag?
Coverage per bag is the hardest number to know, and it changes with the depth you blow to. The top calculators compute it for you from the target R-value and material, which is exactly what this tool does. There is an optional override if your bag label states coverage at your precise R-value.
How much does blown-in insulation cost?
Turn on the cost estimate and enter your price per bag (commonly 10 to 15 US dollars) to see material cost, and add a per-square-foot labor figure if you are hiring out the blow. As a benchmark, DIY blown-in attic insulation often lands near 0.50 to 1.50 per square foot in materials, while a professional install runs roughly 1 to 4 per square foot installed depending on R-value and region.
How do I top up insulation that is already there?
Enter the existing R-value so the calculator fills only the shortfall. For example, going from an existing R-19 to a target R-49 fills R-30 of depth, not the full R-49, which can roughly halve the bags you buy.