Insulation Calculator - Area, Packages, R-Value & Cost
Insulation estimating starts with the area to cover and the package coverage at the selected R-value. Batts, rolls, and blown-in products must follow the manufacturer coverage chart.
Use the product coverage in the unit shown by this label.
Cost uses rounded package count.
Change any value and the results, formula, and diagram update immediately. Use the same unit system throughout one estimate.
Insulation R-Value Targets
US Department of Energy recommendations by climate zone. Use these as targets when picking insulation type and thickness.
| Location | Climate zone 1-2 (FL, southern TX) | Zone 3-4 (Mid-Atlantic, CA) | Zone 5-7 (Midwest, NY) | Zone 7-8 (MN, AK) |
|---|---|---|---|---|
| Attic / ceiling | R-30 to R-49 | R-38 to R-49 | R-49 to R-60 | R-49 to R-60 |
| Wood-frame walls | R-13 to R-15 | R-13 to R-21 | R-19 to R-21+ | R-21+ |
| Floor (over crawl) | R-13 | R-19 | R-25 to R-30 | R-30 to R-38 |
| Basement walls (interior) | n/a or R-5 | R-10 to R-15 | R-15 to R-19 | R-15 to R-21 |
UK / EU follows the U-value scale (lower is better). Walls 0.18-0.30 W/m²K, roof 0.13-0.18 W/m²K (Passivhaus target). India NBC: walls 0.4-0.5 W/m²K, roof 0.4 W/m²K for ECBC compliance.
Insulation Types
Fiberglass batt
The US residential default. R-3.1 to R-4.3 per inch. Sold pre-cut for 16 or 24 in stud bays. Cheap, DIY-friendly.
Mineral wool (Rockwool, Roxul)
R-3.0 to R-3.3 per inch. Fire-resistant, sound-deadening, moisture-resistant. Slightly higher cost than fiberglass.
Blown-in cellulose
Recycled paper treated with borate. R-3.2 to R-3.8 per inch. Best for attic top-up - fills gaps that batts cannot.
Blown-in fiberglass
Loose-fill for attics. R-2.5 to R-2.7 per inch. Lighter than cellulose, less settling.
Spray foam (open-cell)
R-3.5 to R-3.8 per inch. Expands to fill gaps. Air-sealing built in. $1-2 / sq ft installed.
Spray foam (closed-cell)
R-6 to R-7 per inch - the highest R-value per inch. Vapor barrier and structural. $2-4 / sq ft installed.
Rigid foam board (XPS, EPS, polyiso)
R-4 to R-6.5 per inch. Used on basement walls, foundation exterior, roof decks. 4x8 sheets, 1-4 in thick.
Reflective / radiant barrier
Foil on substrate. Reflects radiant heat; rated emissivity rather than R-value. Used in attics in hot climates.
Coverage and Sizing
- Wall batts: 15 in wide for 16 in OC studs; 23 in wide for 24 in OC. Length matches stud height (typically 92-5/8 or 96 in).
- Attic batts / rolls: typically 16 or 24 in wide, 8-9 ft long, R-13 to R-49.
- Blown-in coverage: per bag varies. A 25 lb bag covers about 30-50 sq ft at R-30, 17-20 sq ft at R-49. Check the bag chart.
- Rigid foam: 4x8 sheets cover 32 sq ft. Order full sheets; cuts go to waste.
- Spray foam: calculated by board feet (1 sq ft x 1 in deep = 1 bd ft). Professional install only for whole-home.
Where Insulation Goes
- Attic: #1 priority. Heat rises; an uninsulated attic loses 25-35% of total home heat.
- Walls: in stud bays. Air seal at top and bottom plates, around penetrations.
- Rim joists: small but critical - the band joist around the floor edge is a major air leak.
- Basement: rigid foam on the interior (or exterior) of foundation walls.
- Crawl space: either rim and ceiling insulated, or unvented with foam on walls.
- Floor over garage: R-19 to R-30 minimum.
- Cathedral ceiling: needs venting + insulation; closed-cell foam often used.
Air Sealing - Half The Battle
Insulation slows conductive heat loss. Air sealing stops convective heat loss. A house with R-49 attic but unsealed wire and pipe penetrations still leaks badly.
- Caulk and seal: around windows, doors, attic hatches, plumbing penetrations.
- Weatherstripping: on door bottoms and window sashes.
- Foam at penetrations: spray foam wires, pipes, ducts through walls and ceiling plate.
- Vapor barrier: 6 mil polyethylene on the warm side (cold climates), or follow regional best practice.
Cost Estimates Around The World
2026 retail prices for common insulation materials installed.
| Region | Fiberglass batt (R-13) | Blown cellulose attic | Closed-cell spray foam | Rigid foam (1 in XPS) |
|---|---|---|---|---|
| United States (USD) | $0.50-1.20 / sq ft installed | $1.00-1.80 / sq ft | $2.00-4.00 / sq ft | $0.70-1.50 / sq ft |
| Canada (CAD) | C$0.70-1.60 / sq ft | C$1.30-2.40 / sq ft | C$2.80-5.50 / sq ft | C$1.00-2.00 / sq ft |
| United Kingdom (GBP) | £6-14 / m² (mineral wool) | £10-22 / m² | £30-60 / m² | £15-30 / m² (Celotex) |
| Eurozone (EUR) | €7-15 / m² | €12-25 / m² | €35-70 / m² | €15-35 / m² |
| Australia (AUD) | A$12-24 / m² (R2.5 batt) | A$18-35 / m² | A$45-90 / m² | A$22-45 / m² |
| India (INR) | ₹40-100 / sq ft (Rockwool) | ₹60-150 / sq ft | ₹200-450 / sq ft | ₹80-200 / sq ft (XPS) |
Brand reference: US - Owens Corning, CertainTeed, Johns Manville (fiberglass); Greenfiber (cellulose); Lapolla, Demilec (spray foam); UK / EU - Kingspan, Celotex, Knauf, Rockwool; India - Rockwool India, U.P. Twiga, Owens Corning India; Australia - Bradford, Earthwool, Pink Batts.
Common Mistakes
- Compressing batts into thin spots - kills R-value.
- Skipping air sealing before insulating - the wind blows through.
- Stuffing batts behind wires/pipes without splitting - leaves gaps.
- Vapor barrier on the wrong side - traps moisture, rots framing.
- Blown insulation in unventilated attics without baffles - blocks soffit airflow, causes ice dams.
- Using fibreglass in damp areas - gets wet, loses R-value, grows mould.
- Calculating only wall area without including rim joists, attic hatches, and basement walls.
- Treating R-13 + R-15 as R-28 - they need separate layers, and air gaps between them ruin the math.
Insulation Calculator FAQ
How much insulation do I need for a 1500 sq ft attic?
For R-49 (cold climate): about 17 inches of blown cellulose, or 14 inches of fiberglass. Bagged: about 30-40 of the 25 lb cellulose bags, or 75-100 sq ft of pre-rolled R-49 fiberglass. Always check the bag chart.
What R-value do I need for walls?
R-13 to R-15 for 2x4 walls in mild climates. R-19 to R-21 for 2x6 walls in moderate to cold climates. R-21+ in zones 6-8 (northern US, Canada).
Fiberglass vs cellulose vs spray foam?
Fiberglass: cheapest, DIY-friendly, batts for wall stud bays. Cellulose: best attic top-up, blown over existing insulation. Spray foam: highest R per inch, air-seals at the same time, professional install.
Should I insulate basement walls?
Yes in cold climates. R-10 to R-15 rigid foam on the interior face of foundation walls. Saves heating energy and prevents condensation/mold on cold walls.
How thick is R-30 insulation?
About 10 inches of fiberglass batt, 9 inches of cellulose, or 5 inches of closed-cell spray foam. R-49 is about 15 inches fiberglass or 8 inches closed-cell foam.
How much does insulation cost?
US installed: $0.50-1.20 per sq ft for fiberglass batt. $1.00-1.80 for blown attic. $2-4 for spray foam. UK: £6-14 per m² mineral wool. India: ₹40-100 per sq ft.
Do I need vapor barrier?
Cold climates: yes, on the warm side of insulation (typically the room-facing face). Hot/humid climates: often skipped or installed differently. Always check local code and best practice.
Can I add insulation over existing?
Yes for attic top-ups - blown cellulose over old fiberglass batts works fine. For walls, opening the cavity is usually required. Some products like injection foam or blown-in dense pack go into existing walls without demo.
How does insulation save money?
R-13 walls + R-30 attic can cut heating/cooling bills 20-40% compared to no insulation. Air sealing alone often saves another 10-15%. ENERGY STAR estimates 11% average savings on home upgrades.
How long does insulation last?
Fiberglass and mineral wool: 50+ years if not wetted. Cellulose: 20-30 years before settlement matters. Spray foam: 80+ years. Replace any insulation that has been water-damaged or contaminated.
Related Construction Calculators
For heating equipment sizing once insulation is set, see the BTU Heater Sizing Calculator. For wall framing area, the Drywall Calculator. For attic floor area, the Square Footage Calculator. More tools live on the Construction Calculators hub.
Sources
- U.S. Department of Energy - Types of Insulation
- ENERGY STAR - insulation and air sealing checks
- FTC home insulation consumer guidance
- North American Insulation Manufacturers Association (NAIMA)
- UK EPC and insulation guidance
- Bureau of Indian Standards - ECBC building energy code
- NIST Handbook 44 unit conversion tables
This calculator is for planning and ordering conversations. Local code, project drawings, engineered design, and manufacturer instructions control the final work.