Insulation Calculator
Insulation calculator for an attic or walls: area and target R-value to blown-in bags or batt packages, plus settled depth.
Your estimate
27 bags
Bags to add R-49 (incl. 5% extra)
- Attic area
- 1,000 sq ft
- Minimum settled depth to add
- 17 in
- Bags per 1,000 sq ft
- 25
- Total weight
- 743 lb
- Final R / RSI
- R-49 · RSI 8.6
Show the math
- R to add = target R-49 − existing R-0 = R-49
- Coverage chart: 25 bags per 1,000 sq ft at ≥ 17 in settled
- 1,000 sq ft × 25 ÷ 1,000 × (1 + 5%) = 26.25 → 27 bags
Shopping list & cost
| Item | Spec | Qty | Cost |
|---|---|---|---|
| Blown fiberglass (27.5 lb bag) | R-49 added · 25 bags per 1,000 sq ft | 27 bags | — |
| Attic baffles / rafter vents | one per rafter bay at the eaves | — | — |
| Depth rulers | one per 300 sq ft (code requires markers) | 4 | — |
Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.
Quick answer A 1,000 sq ft attic with no existing insulation needs 27 bags of blown fiberglass (27.5 lb) to reach R-49 with 5% extra — at least 17 in deep. In cellulose it is about 66 × 25 lb bags; a 12 × 8 m attic takes 28 bags of blown fiberglass. For walls, 2×4 cavities take R-13 or R-15 batts and 2×6 cavities R-19 or R-21. Bags = area × bags per 1,000 sq ft from the bag's coverage chart.
Method & assumptions
Blown: Bags = Area × Bags per 1,000 sq ft (at target R − existing R) ÷ 1,000 × (1 + waste %) Batts: Cavity area = Area × (1 − 1.5 in ÷ framing spacing) Packages = Cavity area × (1 + waste %) ÷ sq ft per package RSI (metric) = R × 0.176
- R-value
- resistance to heat flow (US units, ft²·°F·h/BTU)
- Bags per 1,000 sq ft
- from the manufacturer's coverage chart, at the stated settled thickness
- Framing fraction
- share of the surface taken by 1½ in studs or joists
Assumptions & limits
- Blown-in figures use published coverage charts: Owens Corning AttiCat (27.5 lb) for fiberglass and National Fiber (25 lb) for cellulose. Other brands differ — use your bag's chart.
- Attic area is the flat floor area; joists are not deducted for blown-in.
- Batt package sizes are typical fact-sheet values; rolls and other brands vary.
- Air sealing, baffles and ventilation are separate jobs and matter as much as R-value.
Rounding: purchase quantities are always rounded up to whole units as sold. See our methodology.
Diagram
How to use this calculator
- Choose blown-in for an open attic floor, or batts/rolls for walls, floors and open attic joists.
- Enter the attic floor length and width — or, for walls, the total wall length and the wall height.
- Set the target R-value for your climate zone, then subtract what you already have. Measure existing depth and multiply by about R-2.5 to R-3 per inch for fiberglass.
- For batts, choose the R-value that fits your framing depth and the stud or joist spacing.
- Buy to the coverage chart on the bag: the label gives both the minimum bags per 1,000 sq ft and the minimum thickness.
Worked examples
Imperial: 1,200 sq ft attic, R-19 existing, top up to R-49 with cellulose
- R to add = 49 − 19 = 30 → 35.4 bags per 1,000 sq ft
- 1,200 × 35.4 ÷ 1,000 = 42.5; +5% = 44.6 → 45 bags
- Minimum added settled depth ≈ 8.1 in
Imperial: 2×4 walls, 140 ft × 8 ft, R-13 batts at 16 in o.c.
- Gross = 1,120 sq ft; framing = 1.5 ÷ 16 = 9.4% → 1,015 sq ft
- +5% = 1,066 sq ft ÷ 106.56 = 10.0 → 11 packages
- (subtract windows and doors for a tighter count)
In depth: insulation planning notes
Reading a blown-in coverage chart
Every bag of loose-fill insulation carries a coverage chart, and it is the legal basis for the R-value you get. For each R-value it lists the minimum number of bags per 1,000 sq ft, the maximum coverage per bag and the minimum settled thickness. You need both: blowing to the right depth with too few bags gives fluffy, low-density insulation that settles and underperforms, and the right number of bags spread too thin misses the depth. This calculator uses published charts for a common fiberglass and a common cellulose product and interpolates between rows, but brands differ — use the chart on the bag you buy.
Cellulose is denser than blown fiberglass, so it takes more bags (and more weight on the ceiling) for the same R-value but a shallower depth. Fiberglass needs more depth: R-49 is about 17 in of AttiCat but about 13 in of cellulose. If your attic has limited height at the eaves, that matters — baffles keep the soffit vents open and stop insulation spilling into them.
Batts, framing and topping up
Batts are sized to fit between framing: 15 in wide for studs at 16 in on center and 23 in for 24 in spacing. The framing itself is not insulated by the batt, so the calculator deducts the share of the wall taken by 1½ in studs — about 9% at 16 in and 6% at 24 in — before dividing by the coverage printed on the package. A 2×4 wall takes R-13 or R-15 (3½ in) batts and a 2×6 wall R-19 or R-21; squeezing a thicker batt into a shallower cavity lowers its R-value.
When topping up an attic, measure the existing depth in several places and estimate its R-value — roughly R-2.5 to R-3 per inch for old fiberglass and R-3.5 for cellulose — then add enough to reach the target. Fix moisture problems and seal air leaks first: top plates, wiring holes, plumbing stacks, recessed lights and the hatch. The 2021 IECC calls for R-30 in the warmest zones, R-49 in zones 2–3 and R-60 in zones 4–8, and requires depth markers about every 300 sq ft so the installed thickness can be checked.
Reference table
| R-value | Fiberglass 27.5 lb · Cellulose 25 lb |
|---|---|
| R-19 | 9.0 (7 in) · 19.8 (5.3 in) |
| R-30 | 14.6 (10.75 in) · 35.4 (8.1 in) |
| R-38 | 19.0 (13.5 in) · 46.8 (10.2 in) |
| R-49 | 25.0 (17 in) · 62.6 (13.1 in) |
| R-60 | 31.5 (20.5 in) · 78.4 (16 in) |
Key facts
- 2021 IECC attic R-value
- R-30 (zones 0–1) · R-49 (2–3) · R-60 (4–8)
- 2×4 / 2×6 wall batts
- R-13 or R-15 / R-19 or R-21
- Depth markers
- one per 300 sq ft of attic
- RSI conversion
- R-49 ≈ RSI 8.6
Tips & common mistakes
- Air-seal top plates, light fixtures, plumbing stacks and the attic hatch before adding insulation — it is the cheapest energy fix in most houses.
- Install baffles at the eaves so blown insulation doesn't block soffit vents.
- Keep insulation 3 in away from non-IC-rated recessed lights and chimneys, and box around them.
- Don't compress batts: an R-19 batt squeezed into a 3½ in wall performs worse than an R-15 batt made for it.
Frequently asked questions
How many bags of blown-in insulation do I need for 1,000 sq ft?
To reach R-38 from nothing: about 19 bags of 27.5 lb blown fiberglass or 47 bags of 25 lb cellulose. For R-49: about 25 fiberglass or 63 cellulose bags. Always check the chart on your bag.
What R-value should my attic be?
The 2021 IECC calls for R-30 in climate zones 0–1, R-49 in zones 2–3 and R-60 in zones 4–8. ENERGY STAR publishes similar recommendations by zone.
How thick is R-38 insulation?
About 13.5 in of blown fiberglass, 10–11 in of blown cellulose, or 12 in for an R-38 fiberglass batt.
Can I put new insulation over old insulation?
Yes, if the old insulation is dry and not moldy. Blown-in can go straight over old batts or loose fill; new batts should be unfaced and laid perpendicular to the joists.
Blown-in or batts for an attic?
Blown-in fills around wires and framing better and is faster for open attics; batts are easier to handle in small or floored areas.
Sources
- ENERGY STAR – Recommended home insulation R-values — accessed : “Attic, if uninsulated: zone 1 R30; zones 2–3 R49; zones 4–8 R60”
- Owens Corning – AttiCat coverage chart; Johns Manville batt fact sheet — general reference — not cited for a specific figure
- National Fiber – cellulose bag coverage chart — general reference — not cited for a specific figure
Default values (yields, coverage rates and densities) are typical figures from manufacturer literature. Always check the label on the product you buy.
Related calculators
- Drywall CalculatorCount drywall or plasterboard sheets for walls and ceilings, plus joint compound, tape and screws. 4×8, 4×12 and 1200×2400 mm.
- Square Footage CalculatorFind the area of a room or yard in square feet, square meters and square yards — rectangles, triangles, circles and trapezoids.
- Roofing CalculatorRoof area from building size, overhang and pitch for gable, hip or shed roofs — squares, shingles, underlayment, drip edge, ice shield and ridge cap.
Learn more in our guides
Method, waste and worked examples: Drywall sheets, finishing materials and insulation coverage.
Embed this calculator
Run a blog, forum or contractor site? Add this free insulation calculator to your page. It resizes itself, has no ads and sets no tracking cookies.
Free for any site. Keep the small “Calculator by DIYCalcLab” credit visible; it is rel="nofollow" by design. No ads or tracking cookies inside the widget. Widget terms & all calculators.
Cite this calculator
DIYCalcLab. “Insulation Calculator.” Nov 9, 2026. https://diycalclab.com/drywall/insulation-calculator/
Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.