How much insulation do I need? R-value, depth and bag count

"How much insulation do I need?" is really two questions. First: how much R-value does your climate zone call for? Second: how many bags, batts or board feet does it take to reach that R-value over your area? Get the first one right and the second is simple arithmetic. Here is the method, worked for a 1,000 sq ft attic.

Step 1 — Find your target R-value

The International Energy Conservation Code (IECC) sets minimum R-values by climate zone (1 is the hottest, 8 the coldest). The 2021 edition — the one most recent code updates adopt — asks for:

• Attic / ceiling: R-30 in zone 1, R-49 in zones 2–3, R-60 in zones 4–8
• Wood-frame wall cavity: R-13 in zones 1–2, R-20 in zones 3–6, R-21 in zones 7–8 (zone 5 and colder add R-5 continuous insulation on the outside)
• Floor over an unheated space: R-13 in zones 1–2, R-19 in zones 3–4, R-30 in zones 5–6, R-38 in zones 7–8

Many areas still enforce the 2018 edition, where the attic figure is R-38 in zones 2–3 and R-49 in zones 4–8. Your local building department has the final word, but for an upgrade, aiming for the newer number is rarely wasted money.

Step 2 — Turn R-value into depth

depth (inches) = target R ÷ R per inch

Each material has its own R per inch. Typical planning figures:

• Blown cellulose: R-3.5 per inch
• Blown fiberglass: R-2.5 per inch
• Fiberglass batts: about R-3.1 per inch (high-density batts about R-4.3)
• Mineral wool batts: about R-4.2 per inch
• Closed-cell spray foam: about R-6.5 per inch; open-cell about R-3.7

For an attic at R-60: cellulose needs 60 ÷ 3.5 = 17.1 inches settled; fiberglass needs 60 ÷ 2.5 = 24 inches. Cellulose settles, so it is blown about 11% deeper than the finished depth — roughly 19 inches on the day. Loose fiberglass does not settle appreciably.

Step 3 — Turn depth into bags

A bag of blown insulation holds a fixed amount of fibre, so the deeper you go, the fewer square feet each bag covers. That is why the chart printed on the bag is the real authority. Averaged across published manufacturer charts, a 25 lb bag of cellulose fills about 200 square-foot-inches, and a 30 lb bag of loose fiberglass about 784.

sq ft per bag = sq-ft-inches per bag ÷ depth, then bags = area ÷ sq ft per bag, rounded up.

For a 1,000 sq ft attic:

• R-60, cellulose: 200 ÷ 17.1 = 11.7 sq ft per bag → 86 bags
• R-60, blown fiberglass: 784 ÷ 24 = 32.7 sq ft per bag → 31 bags
• R-49, cellulose: 14 inches, 14.3 sq ft per bag → 70 bags
• R-49, blown fiberglass: 19.6 inches, 40 sq ft per bag → 25 bags

The fiberglass bag count is lower because each bag is much fluffier, not because it is cheaper — compare cost per square foot at the same R-value, not cost per bag.

Topping up an existing attic

Most attics already have something. Measure the depth, multiply by the material's R per inch, and only buy the gap. An attic with about R-19 of old fiberglass that you are bringing to R-49 needs R-30 more: with cellulose that is 30 ÷ 3.5 = 8.6 inches, 23.3 sq ft per bag, or 43 bags for 1,000 sq ft — well under the 70 bags a bare attic needs.

Walls: batts by the package

For walls, the stud size and spacing pick the product. A 2 × 4 wall at 16" on center takes 15"-wide R-13 batts; a common package covers about 97 sq ft. Measure the wall area, subtract the doors and windows, and divide: 950 sq ft of net wall ÷ 97 = 9.8 → 10 packages. Package coverage varies by brand, so check the label before you buy.

Common mistakes

• Counting bags from the wrong chart. Bag coverage differs by product and even by machine setting. Use the chart on the bag you are buying.
• Forgetting settling. Cellulose blown to exactly the target depth ends up short once it settles.
• Compressing batts. A batt squashed into a thinner cavity loses R-value.
• Blocking the vents. Keep soffit baffles in place so the attic can still breathe.

Our app: Smart Insulation Calculator

Smart Insulation Calculator is a free Android app built around exactly this chain: target R-value, material, depth, area, bags. Pick your climate zone and code year (IECC 2009 to 2021) and it fills in the code minimum for attics, walls, floors, crawl space and basement walls, rim joists and slab edges. It then works out the depth, the settled depth, and the number of bags, batt packages, foam kits or rigid boards — showing every step, with every default editable because the real answer is on the bag. The figures in this article are the app's own defaults.

It is available on Google Play for Android only, and the calculators also run in any browser at smartinsulationcalculator.com. Boarding the walls afterwards? See how much drywall you need.

Not sure which R-value to aim for in each part of the house? See what R-value insulation you need by climate zone. Closing the walls up afterwards? How much drywall do I need counts the sheets, mud and tape.

Blowing insulation into existing walls instead of fitting batts? Blown-in insulation for walls gives the dense-pack pounds per square foot for 2x4 and 2x6 walls and a whole-house bag count.