Insulation Calculator

Insulation looks passive — a layer of fluff in the attic — but it quietly does the largest single job in your energy bill. A home that meets its climate-zone R-value can cut heating and cooling loads by a third or more, and the material is cheap compared to the windows, doors and equipment it enables you to downsize. Getting the quantity right matters: too little leaves cold spots and condensation, too much spends money that never returns.

Insulation Calculator tells you three things at once: the R-value you still need (your target minus anything already installed), the thickness of new insulation required, and how much material to buy in the unit it is actually sold in — batts, foam boards, cellulose bags or spray-foam kits. Choose from fiberglass, rockwool, cellulose, EPS, XPS, polyiso, and open- or closed-cell spray foam, switch between imperial and metric, add a price per unit, and read off your total material and cost.

Beyond the estimate, the learning guide explains the R-value system, what each material is best at, how many board feet a foam kit really covers, and where condensation hides. Everything runs locally in your browser; nothing is uploaded.

• Target by assembly: One-touch presets for attic, wall, floor and basement edge set sensible R-value targets based on US climate-zone practice, or enter your own.
• Existing insulation: Enter what is already installed; the calculator only dimensions the additional thickness needed to reach the target.
• Seven material families: Fiberglass batt, rockwool batt, cellulose loose-fill, EPS, XPS, polyiso foam board, and open- or closed-cell spray foam, each with its true R-value per inch.
• Quantity in the unit you buy: Packs of batts, 4×8 ft foam boards, 18 kg cellulose bags, and 600-board-foot spray foam kits, each with its real coverage math.
• Layer logic: Foam board and batt jobs automatically round up to whole layers, so the stacking recommendation matches what you actually install.
• Imperial or metric: Area in sq ft or sq m, thickness in inches, centimetres and millimetres, and R-value with RSI conversion on a dedicated tab.
• Cost estimate: Enter a price per pack, board, bag or kit and get a material budget instantly, updated as you change the inputs.
• Full report: A material report table with plain-text, CSV and JSON export for your invoice, budget sheet or building-permit file.
• Live working: A maths line shows the exact addition R ÷ R-per-inch ÷ coverage steps behind every number on screen.
• Learning guide: R-per-inch reference table, climate-zone targets, condensation and vapour control notes, tips and formulas built into the tool.

1. Tell the tool where the insulation goes. Click Attic, Wall, Floor or Basement for a sensible R-value target, or type your own on the R-value line.
2. Enter your surface area. Choose sq ft or sq metres and type the wall, floor or ceiling area you plan to insulate.
3. Add existing insulation. If you are topping up an attic, enter its current R-value; for a new build leave it at zero.
4. Pick the material. Fiberglass or rockwool for deep spaces, cellulose for blow-in attics, foam board for sheathing and basements, spray foam for thin or airtight cases.
5. Choose the product size. The batt-pack R or foam-board thickness appears when relevant so the layer math matches what is on the shelf.
6. Add the price (optional). Put your cost per pack, board, bag or kit in the cost field to build a material budget.
7. Read the live estimate. Added R-value and required thickness appear first, with the quantity and cost grid beneath.
8. Review the working. The maths line shows target minus existing R, divided by R-per-inch, and then converted to packs or sheets.
9. Export the report. Use text, CSV or JSON to take the quantity and budget into your purchase order or quote.
10. Check the guide. The learning tab explains R-value, condensation, vapour barriers and the right material for each spot.

Example 1 — Attic top-up. A 1,000 sq ft attic already has R-11 and your local table wants R-49. You need R-38 more. Rockwool at R-4 per inch needs 9.5 inches; using two layers of R-19 packs means about 131 packs to cover the area twice.

Example 2 — Foam board wall. A basement wall is 600 sq ft and needs R-13. XPS at R-5 per inch needs 2.6 inches, so two layers of 1.5-inch board. That is 2 layers × 19 sheets of 4×8 board = 38 sheets to buy.

Example 3 — Spray foam ceiling. A 400 sq ft ceiling wants R-24 of closed-cell foam (R-6.5 per inch → 3.7 inches, so one ~4-inch pack of kits). Board feet needed: 400 × 3.7 ≈ 1,480 BF, or about 3 sixty-foot kits.

Example 4 — Cellulose calculate. A 900 sq ft attic needs 7 inches of cellulose (R-3.7/in → R-25.9). That is 525 cu ft of void ≈ 14.9 m³ ≈ 447 kg, so roughly 25 bags of 18 kg blow-in cellulose.

Example 5 — Checking a contractor quote. A contractor quotes for R-38 in a 2,000 sq ft attic using fiberglass (R-3.2/in → 11.9 inches, so 3 layers of R-13). The calculator gives ~3 layers × 196 packs ≈ 588 packs, a quick cross-check before you sign.

• Pay only for the R you need: The tool stops overspending by targeting your climate zone instead of guessing at extra thick layers.
• Buy the right unit: Packs, boards, bags and kits are converted automatically so you never translate a volume into a shopping-basket quantity.
• Budget before you order: Add local prices and get a material total that survives a per-product price check.
• Topping up or new build: Existing-R input makes upgrades as easy to size as fresh installs.
• Imperial and metric honest: R or RSI, sq ft or sq m, inches or millimetres — every output converts clearly.
• Explainable math: Each figure follows from the previous one in the working line, so nothing is a black box.
• File-friendly exports: Plain text, CSV and JSON reports drop into purchase orders, budgets and permit submissions.
• Private by design: No uploads, no accounts, no logging — enter any real numbers with confidence.