How much insulation (R-value) you need
ENERGY STAR publishes recommended levels by climate zone, based on the 2021 International Energy Conservation Code. For attics:
| Climate zone | Uninsulated attic | Attic with 3–4 in existingAttic with 7.6–10.2 cm existing | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A, 4B | R60 | R49 | R19 |
| 4C, 5, 6 | R60 | R49 | R30 |
| 7, 8 | R60 | R49 | R38 |
Find your climate zone on the ENERGY STAR or DOE map. For walls, the cavity depth limits what fits: batts made for 2×4 cavities are commonly R13–R15, and for 2×6 cavities R19–R21.
Measuring
- Walls: wall length × height for each exterior wall, minus large windows and doors.
- Attic floor: length × width of the attic floor, which is usually the footprint of the conditioned space below.
- Floors over crawlspaces or garages: length × width.
- Sloped ceilings: the area along the slope. The roof pitch calculator gives the multiplier.
How packages are counted
Packages = area × (1 − framing deduction) × (1 + waste) ÷ coverage per package. Batt and roll packages list their coverage in square feet for a given framing width, and that figure already assumes the batts sit between studs or joists. So the framing deduction defaults to 0%. Coverage varies a lot by product and R-value, so the 40 ft² default is just a placeholder. Read the bag. With the defaults: 14 packages for 500 ft².
Blown-in (loose-fill) insulation
Loose-fill bags come with a coverage chart: for each R-value, the chart gives bags per 1,000 ft² and the minimum settled thickness. To estimate blown insulation, look up your target R-value on the chart, then multiply by your attic area ÷ 1,000. You can also enter that coverage per bag into this calculator (for example, if the chart says 1 bag covers 40 ft² at R-38, enter 40). Many stores lend a blowing machine with a minimum bag purchase.
Worked examples
Exterior walls of a room, 40 ft of wall × 8 ft, R13 batts at 40 ft² per bagExterior walls of a room, 12.2 m of wall × 2.44 m, R13 batts at 3.72 m² per bag
Area to insulate 320 ft² · Framing deduction 0 % · Coverage per package 40 ft² · Waste / overage 5 %Area to insulate 29.73 m² · Framing deduction 0 % · Coverage per package 3.72 m² · Waste / overage 5 %
- Packages needed: 9
- Area covered: 336 ft²Area covered: 31.2 m²
Attic floor, 1,200 ft², batts at 50 ft² per packageAttic floor, 111 m², batts at 4.65 m² per package
Area to insulate 1,200 ft² · Framing deduction 0 % · Coverage per package 50 ft² · Waste / overage 5 %Area to insulate 111.48 m² · Framing deduction 0 % · Coverage per package 4.65 m² · Waste / overage 5 %
- Packages needed: 26
- Area covered: 1,260 ft²Area covered: 117.1 m²
Shed, 10 × 12 ft, walls 7 ft (308 ft² gross minus a 21 ft² door)Shed, 3.05 × 3.66 m, walls 2.13 m (28.6 m² gross minus a 1.95 m² door)
Area to insulate 287 ft² · Framing deduction 0 % · Coverage per package 40 ft² · Waste / overage 5 %Area to insulate 26.66 m² · Framing deduction 0 % · Coverage per package 3.72 m² · Waste / overage 5 %
- Packages needed: 8
Installation notes
- Don't compress batts to fit. Squashing insulation lowers its effective R-value (DOE).
- Cut batts to fit snugly around wiring, boxes and pipes, with no gaps or voids.
- Faced batts have a vapor retarder. Which side it faces, and whether you need one at all, depends on your climate, so follow local code and the product instructions.
- Seal air leaks before insulating an attic. Insulation slows heat flow but doesn't stop air.
- Keep insulation away from recessed lights and chimneys unless they're rated for contact, and keep soffit vents clear with baffles.
- Wear gloves, long sleeves, eye protection and a dust mask when handling fiberglass or mineral wool.
Batts, rolls, blown-in or rigid board?
- Batts are pre-cut lengths sized for standard stud and joist spacing. They're the easiest DIY choice for open walls and floors.
- Rolls are the same material in long lengths, handy for long, uninterrupted attic joist bays.
- Blown-in loose fill (fiberglass or cellulose) fills irregular attic floors and works around obstructions. It's usually the easiest way to add depth over existing insulation.
- Rigid foam board is for basement walls, under slabs and continuous exterior insulation. ENERGY STAR's guidance mentions insulative sheathing when siding is replaced.
- Spray foam air-seals and insulates in one step, but it's mostly a professional job.
This calculator estimates packaged products by coverage. For foam board, divide the area by the sheet size (32 ft² for a 4 × 8 sheet).
Common mistakes
- Insulating before sealing attic air leaks around light fixtures, plumbing stacks and top plates.
- Blocking soffit vents. Install baffles first so air can flow from the eaves.
- Buying by "rolls" or "bags" without reading coverage. Two products with the same R-value can cover very different areas per package.
- Compressing thick batts into shallow cavities.
- Putting a second vapor retarder over existing insulation, which can trap moisture between layers.
Seal air leaks before you insulate
ENERGY STAR recommends sealing attic air leaks before adding insulation. Gaps around wiring and plumbing holes, open stud cavities, dropped soffits and flue chases let warm, moist air bypass the insulation entirely. Once a deep layer of loose-fill is down, those leaks are much harder to reach. Sealing them first lets the new insulation perform at its full value.
- Keep soffit vents clear. Rafter baffles hold insulation back from the eaves so air can still flow.
- Keep insulation away from flues and chimneys, using metal flashing and fire-rated sealant, and from recessed lights that aren't rated for insulation contact (IC-rated).
- Fix roof leaks and moisture problems before you start.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 500 ft², 40 ft² packages, 5% waste: packages | 14 | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 46.5 m², 3.72 m² packages, 5% waste: packages | 14 | Hand calculation | Match |
Frequently asked questions
How much insulation do I need in my attic?
ENERGY STAR recommends R30–R60 for an uninsulated attic, depending on climate zone (R49–R60 in most of the US).
Should I subtract studs?
Usually not, because package coverage already accounts for the framing.
How much insulation for a 10 × 12 shed?
Add up the wall areas (perimeter 44 ft × wall height), subtract the door, and add the roof or ceiling area. Then divide by the package coverage.
Can I add new insulation over old?
Generally yes in attics, if the old insulation is dry and not moldy. Unfaced batts or loose-fill go on top.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- ENERGY STAR: Attic Air Sealing Project
- ENERGY STAR: Recommended Home Insulation R-Values
- US DOE Building America Solution Center: Insulation installation (RESNET Grade 1)
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.