How the calculator works
Insulation is sold to reach an R-value, a measure of how well it slows heat. So the job is to find the R-value you need, subtract what you already have, and turn the difference into bags, batts or board feet. Here is the math with the calculator's starting values: a 40 x 25 ft attic in Texas with 3.5 inches of old loose fiberglass, topped up with blown cellulose.
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1
Find your target
Most of Texas is climate zone 2, where ENERGY STAR recommends R-49 in the attic. The old layer gives about R-2.5 per inch: 3.5 × 2.5 = R-9 (R-8.75). You need to add about R-40.
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2
Read the bag chart
The maker's chart for a 25 lb cellulose bag calls for 52.6 bags per 1,000 sq ft at R-38 and 63.5 at R-44. Between them, R-40 takes 56.7 bags. The attic is 1,000 sq ft, so buy 57 bags. They settle to about 11 inches.
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3
Price the job
57 bags at $18 is $1,026. Rafter vents for the 40 ft eaves ($60), a respirator and goggles ($35) and air sealing supplies ($90) bring the DIY total to $1,211. Most stores lend the blower free with a 10-bag purchase. A pro would charge about $886 to $1,610.
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4
Estimate the savings
Heat flow through the ceiling drops from 1 ÷ R-10.75 to 1 ÷ R-51 per square foot (each includes about R-2 for the drywall and air films). Over a Houston-like year of 1,500 heating and 3,000 cooling degree days, that saves about $38 of gas and $69 of electricity: $107 a year, paying back the DIY cost in about 11 years.
The savings are an estimate of heat moving through this one surface. Air leaks, ducts in the attic, windows and your thermostat habits all change the real bill, and an attic in summer gets far hotter than the outdoor air, so cooling savings in hot climates are often higher than shown. Open "Energy savings settings" to enter your own fuel prices and degree days.
Recommended R-values by climate zone
ENERGY STAR bases its advice on the 2021 International Energy Conservation Code. The first column is the total for an attic with no insulation. If you already have 3 to 4 inches, add the second number. Tap a zone to load an attic in it.
| Zone | Attic, empty | Add to 3–4 in | Floor | Basement wall |
|---|---|---|---|---|
| 1 | R-30 | R-25 | R-13 | None |
| 2 | R-49 | R-38 | R-13 | None |
| 3 | R-49 | R-38 | R-19 | R-5 foam or R-13 |
| 4 | R-60 | R-49 | R-19* | R-10 foam or R-13* |
| 5–6 | R-60 | R-49 | R-30 | R-15 foam or R-19 |
| 7–8 | R-60 | R-49 | R-38 | R-15 foam or R-19 |
* Zone 4 marine (western Oregon and Washington) follows zone 5 for floors and basement walls. Uninsulated wood walls: fill the cavity, and add R-5 (zone 3) or R-5 to R-10 (zones 4–8) of foam sheathing when you replace the siding.
R-value per inch by material
The same R-value takes very different depths. That matters in a wall, where the studs fix the depth, and in an attic with low eaves.
| Material | R per inch | Inches for R-38 |
|---|---|---|
| Blown fiberglass | 2.2–2.9 | 13.5 (one maker's chart) |
| Blown cellulose | 3.1–3.8 | 10.4 settled |
| Fiberglass batts | 2.5–4 | 12 (R-38 batt) |
| Mineral wool | 3–4.1 | about 9 to 12 |
| Open-cell spray foam | 3.5–3.7 | about 10.5 |
| Closed-cell spray foam | about 6 | about 6.5 |
| EPS / XPS / polyiso board | 3.8–4.4 / 5 / about 6 | 7.5 to 10 |
Common batts: R-13 and R-15 fit a 2x4 wall (3.5 in), R-19 and R-21 fit a 2x6 wall, R-30 is about 8.5 to 10 inches and R-38 about 12 inches. A batt squeezed into a shallower space loses R-value, so don't stuff an R-19 into a 2x4 wall.
How many bags for blown-in insulation
Every bag of loose fill has a coverage chart, and the code-approved way to install it is to hit both the bag count and the minimum depth. These two charts are what the calculator uses. If your bag's chart differs, type its coverage at R-38 into the calculator and it scales the rest.
| R-value added | Cellulose, 25 lb | Fiberglass, 27.5 lb |
|---|---|---|
| R-19 | 22.6 bags, 5.3 in | 9 bags, 7 in |
| R-30 | 38.6 bags, 8.3 in | 14.6 bags, 10.75 in |
| R-38 | 52.6 bags, 10.4 in | 19 bags, 13.5 in |
| R-49 | 70.8 bags, 13.3 in | 25 bags, 17 in |
| R-60 | 86.0 bags, 16.1 in | 31.5 bags, 20.5 in |
Bags per 1,000 sq ft of open attic floor, with settled (cellulose) or minimum (fiberglass) depth. Fiberglass bags hold far more volume, which is why you need fewer of them.
What insulation costs
Typical 2025–2026 prices. Blown-in material runs about $0.30 to $0.60 per sq ft at R-19, and a bag costs roughly $18 to $30 for 25 lb of cellulose and $50 to $68 for 27.5 lb of fiberglass. Fiberglass batts run about $0.30 to $1.30 per sq ft, more for kraft facing or mineral wool. Pros charge by the R-value for attics:
| Pro job | Installed price |
|---|---|
| Blown-in attic, R-30 | $660–$1,200 per 1,000 sq ft |
| Blown-in attic, R-38 | $840–$1,500 per 1,000 sq ft |
| Blown-in attic, R-49 | $1,100–$1,900 per 1,000 sq ft |
| Blown-in attic, R-60 | $1,400–$2,400 per 1,000 sq ft |
| Batts in open framing | $0.80–$2.60 per sq ft |
| Dense-pack cellulose, closed walls | $1.75–$3.80 per sq ft |
| Spray foam, open / closed cell | $1.50–$3.50 / $3.00–$5.00 per sq ft |
Blowing an attic is one of the best DIY jobs in a house: two people, one day, and the store lends the machine. Dense-packing closed walls takes a high-pressure blower and practice, so most people hire it. Spray foam is a pro job: the chemicals need protective gear, exact mixing and time to cure before you go back inside.
Tax credits and rebates in 2026
The federal Energy Efficient Home Improvement Credit (section 25C) used to return 30% of the cost of insulation and air sealing materials, up to $1,200 a year. The budget law signed on July 4, 2025 ended it early. The IRS says the credit "will not be allowed for any property placed in service after December 31, 2025." Work finished in 2025 can still be claimed on the 2025 return with Form 5695; work done in 2026 gets nothing, and labor never counted.
What is left is local. The federally funded state Home Energy Rebate programs can pay for insulation, but as of June 2026 only twelve states and the District of Columbia had launched them, and rules were still changing. Check your state energy office and your utility, which often runs its own insulation rebate or free energy audit.
Tips and safety
- Air seal before you insulate. Loose fill and batts slow heat but not air. Foam the holes around pipes, wires, flues (with fire-rated sealant) and the attic hatch first. ENERGY STAR's estimate of 15% savings assumes both.
- Keep the soffit vents open. Staple rafter vents (baffles) at the eaves so insulation can't block the airflow that keeps the roof deck dry.
- Watch the light fixtures. Only recessed lights rated IC can be buried. Keep insulation away from older fixtures, chimneys and flues, following the clearances on the label.
- Know what is up there. Pebbly gray-brown vermiculite may contain asbestos, and old knob-and-tube wiring should be checked by an electrician before anything covers it.
- Mark the depth. Staple a few attic rulers to the joists so you blow an even layer and can prove the R-value later.
- Cover foam board. Foam on basement walls must sit behind 1/2 inch drywall or another approved thermal barrier in living space.
Common questions
Is there still a tax credit for insulation in 2026?
Not a federal one. The Energy Efficient Home Improvement Credit (25C) does not apply to property placed in service after December 31, 2025. Insulation installed in 2025 can still be claimed on the 2025 return. Some states and utilities offer their own rebates.
What R-value do I need in my attic?
ENERGY STAR recommends R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8 for an attic with no insulation. If you already have 3 to 4 inches, add about R-25, R-38 or R-49 in those zones.
How many bags of blown-in insulation do I need?
It depends on the bag and the R-value. One 25 lb cellulose bag's chart calls for 52.6 bags per 1,000 sq ft to add R-38, while a 27.5 lb fiberglass bag needs 19 bags for the same R-value. Always follow the chart on your bag.
Is blown-in or batt insulation better for an attic?
Blown-in usually wins in an attic. It fills around wires, pipes and odd framing without gaps, costs less per R-value, and goes in fast with a rented blower. Batts work well in open walls and floors, where they fit snugly between the framing.
How much does it cost to insulate an attic?
A pro typically charges about $840 to $1,500 per 1,000 sq ft to blow in R-38, and $1,400 to $2,400 for R-60. Doing it yourself costs mostly the bags, often a few hundred dollars to about $1,500 for an average attic.
How much can insulation save on energy bills?
ENERGY STAR estimates that sealing air leaks and adding insulation saves an average of 15% on heating and cooling costs, or about 11% of total energy costs. Savings are biggest in homes that start with little insulation.
Sources and assumptions
Prices are US ranges checked in October 2026 and vary by store, region and season. Every bag price, coverage and fuel price in the calculator can be edited.
- Recommended R-values by zone: ENERGY STAR, Recommended home insulation R-values, based on the 2021 IECC. State defaults use the zone of each state's largest metro on the IECC climate zone map (PNNL); pick your own zone if you live elsewhere in the state.
- Savings of 15% on heating and cooling (11% of total energy costs), with zone-by-zone results: ENERGY STAR, methodology for estimated savings.
- Federal credit: IRS FS-2025-05, FAQs on the 2025 changes to 25C and 25D and IRS, Energy Efficient Home Improvement Credit. State rebate status: Building Performance Association, state Home Energy Rebates fact sheet (June 2026).
- Bag charts: GreenFiber SANCTUARY attic card (25 lb cellulose; minimum bags and settled depth, no framing correction) and Owens Corning AttiCat (27.5 lb fiberglass). Blower lent free for 24 hours with 10 bags: Owens Corning AttiCat install guide; otherwise we use $125 a day.
- R-value per inch: PNNL Building America Solution Center, maker data and HomeGuide. Wall cavities assume 3.5 in (2x4) and 5.5 in (2x6); dense-pack cellulose at 3.5 lb per cu ft and R-3.7 per inch; open-cell foam R-3.6 and closed-cell R-6 per inch.
- Prices: HomeGuide, insulation cost (pro attic prices by R-value, batts $0.80–$2.60 per sq ft installed, material ranges), HomeGuide, blown-in insulation, HomeGuide, spray foam and Fixr, blown-in insulation (dense-pack cellulose $1.75–$3.80 per sq ft). Bag prices: $17.97 for a 25 lb bag of Greenfiber Sanctuary Defense cellulose at Home Depot (November 2026; other stores listed $19.99 to $28.99), which sets our $18 default. For a 27.5 lb bag of Owens Corning AttiCat fiberglass, Home Depot listings seen in November 2026 showed about $50 to $68; the $50 default is our estimate at the low end, as the store page could not be read directly. Batt prices per sq ft and the spray foam board-foot rates are our estimates inside the ranges above.
- Supplies at estimated big-box prices: rafter vents $1.50 each, protective gear $35, air sealing kit $90, foam adhesive $8, seam tape $12, 1/2 in drywall $16 a sheet.
- Savings model: heat flow through the surface (1 ÷ R) before and after, times degree days × 24 hours. The uninsulated assembly counts as R-2 (attic), R-4 (wall) or R-3 (floor, basement). Floors over crawl spaces and basement walls use half the heating degree days and no cooling. Fuel prices: EIA Winter Fuels Outlook 2026–27 (natural gas $13.44 per thousand cu ft, about $1.29 per therm; propane $2.52 and heating oil $5.26 per gallon; electricity 18.23¢). Efficiency: gas and propane 90%, oil 85%, heat pump COP 2.5, AC SEER 14. Zone degree days are typical values inside each zone's range, not your town's.
- Asbestos in vermiculite: EPA, vermiculite insulation. Foam thermal barrier: IRC section R316.4.