Attic Insulation Calculator

The DOE / ENERGY STAR recommendation depends on your IECC climate zone. For an uninsulated attic: Zone 1 → R-30; Zones 2–3 → R-49; Zone 4 → R-60; Zones 5–8 → R-60. If you already have 3–4 inches of existing insulation (verify with a ruler measurement to the joists), the target is lower per DOE: Zone 1 → R-25; Zones 2–3 → R-38; Zones 4–8 → R-49. Select your zone in the calculator above and the target R-value auto-fills. You can override it manually for project-specific or contractor-specified requirements — verify with your local building department for jurisdiction-specific code minimums.

Blown cellulose (Greenfiber SANCTUARY at R-3.7/inch settled per SANCTUARY Rev K 05/26; other cellulose brands at industry-conservative R-3.2/inch) is denser, resists air movement well, is made from ~80% recycled paper, and is typically the lower-cost option per R of performance — verify with your bag-label coverage chart. It settles ~20% per FTC 16 CFR 460 — the bag coverage rates in this calculator already account for settling. Blown fiberglass (R-2.5/inch) is lighter, easier to blow into tight spaces, and won't absorb moisture if there's a source of attic moisture. Requires roughly 40% more depth than cellulose to reach the same R-value — verify with your specific product's technical data sheet. The type comparison panel in the calculator runs both side-by-side for your attic.

Yes — in most cases you add the new layer directly on top of the existing insulation per DOE Energy Saver guidance. R-values are additive: R-11 existing + R-27 added = R-38 total. Important exceptions: if existing insulation is wet, moldy, or compressed, it must be removed before adding new. If your attic has vermiculite (a pebbly gray material common in pre-1980 attics), have it tested for asbestos before disturbing it — if positive, consult a licensed abatement contractor. Before adding new insulation, air-seal the attic floor — recessed lights, top plates, wire penetrations — so you're not trapping air leakage under the new layer.

The formula is: bags = ceil(sqft × added_depth_inches × coverage_constant). For Greenfiber SANCTUARY cellulose at R-3.7/inch settled (per SANCTUARY Rev K 05/26), reaching R-38 (~10.4 inch depth) requires approximately 53 bags per 1,000 sqft per the SANCTUARY coverage chart — verify with the coverage chart printed on your specific bag label. At R-49 (deeper), expect around 26 bags for a 500 sqft attic — verify with your bag-label coverage chart. The calculator uses coverage constants derived from the DOE R/inch value plus a 10% settling/waste buffer; always cross-check with the depth/bags table on your specific product.

Yes — and this step matters more than the R-value you reach. Per DOE Energy Saver guidance, air sealing alone can reduce heating and cooling costs 15–30%. Seal all penetrations first: recessed light rough-ins, top plates, wire and pipe holes, pull-down stair rough-ins, and any other gaps between conditioned and unconditioned space. Use low-expansion spray foam for gaps up to 1 inch; two-part foam kits for larger openings. Install rigid baffles at each rafter bay to maintain soffit ventilation before blowing — blocking soffit vents creates moisture problems and can void roof warranty.

Blown-in attic insulation is one of the more DIY-accessible home performance upgrades. Most homeowners complete a 500–1,000 sqft attic in a weekend. Requirements: safe attic access, walk-boards (never step on ceiling drywall), N95 respirator + goggles + skin coverage, and a blower rental (usually free with 10+ bag purchase at HD or Lowes). Fiberglass batts are even simpler — no blower needed, just cut and staple. Consult a licensed contractor if: the attic has vermiculite (possible asbestos), active moisture intrusion, structural access issues, or if air sealing requires electrical work around non-IC-rated recessed lights. See our full disclaimer.

R-value measures thermal resistance — how well a material resists heat flow per the FTC R-Value Rule (16 CFR 460). Higher R-value = better insulation. R-values are additive: if your attic has 3.5 inches of fiberglass batts (R-11) and you add 7.2 inches of blown cellulose (R-27), the total is R-38 — verify with your product's R-per-inch label. The formula: total R = existing_depth × R_per_inch[existing_type] + added_depth × R_per_inch[added_type]. Attics are the single largest source of heat loss in most homes — per ENERGY STAR Seal & Insulate, a well-insulated attic floor can reduce annual heating/cooling costs by 10–50% depending on climate and existing insulation level.

In most jurisdictions, adding blown-in insulation or batts to an existing attic floor is classified as ordinary maintenance and does NOT require a permit. However, some jurisdictions require a permit when bringing R-value up to current code minimum, or when work involves electrical modifications around recessed lights. Check with your local building department before starting. This calculator is a material quantity estimator — confirm all code and permit requirements with your local authority before construction.

Depth depends on your climate zone and target R-value per DOE Recommended R-Values. For Greenfiber SANCTUARY cellulose (R-3.7/inch settled per SANCTUARY Rev K 05/26), reaching R-49 requires roughly 13.3 inches of settled depth — verify with your bag-label coverage chart. Most zones 3–6 target R-49 to R-60, which translates to 13–16 inches of SANCTUARY cellulose (other cellulose brands at R-3.2/inch require ~15.3–18.8 in) — verify with your specific product's technical data sheet. The calculator above shows estimated depth based on your zone and insulation type.

R-38 is the DOE minimum for Zones 2–3 (warm climates like the Southeast and Southwest). For Zone 4 and colder, the DOE Recommended Levels target R-49 to R-60. If your existing attic has R-38 and you live in Zone 4 or above, adding insulation to reach R-49 typically pays back within 3–5 years in energy savings — verify with your local utility rebate program. Use the calculator above with your climate zone to see the DOE target for your zone.

Coverage depends on target R-value and insulation type — verify with your specific product's technical data sheet. A standard 25 lb bag of blown cellulose covers roughly 25–50 sq ft depending on the depth needed. At R-49 depth (~13 inches), one bag covers approximately 25 sq ft — verify with your bag-label coverage chart. Always cross-check with the coverage chart printed on your specific bag — manufacturers publish depth-vs-bags tables for their exact product per FTC 16 CFR 460.

For a 1,000 sq ft attic in Zone 4 targeting R-49 per DOE Recommended R-Values, plan for approximately 39 bags of blown cellulose, accounting for the standard 20% settling allowance per FTC R-Value Rule — verify with your bag-label coverage chart. For R-60 (zones 5–8), expect roughly 52 bags — verify with your bag-label coverage chart. The calculator above refines this for your exact zone, existing insulation depth, and insulation type.

Verify with each brand's current TDS — coverage and R/inch can vary by production batch. AttiCat (Owens Corning) is a fiberglass product that expands as it's blown, achieving roughly R-2.5/inch settled. At R-49, the Owens Corning AttiCat Product Data Sheet (Pub. No. 10011273-K, Dec 2025) shows 25 bags per 1,000 sq ft (39.9 sq ft/bag). It is widely available at Home Depot and sold in 27.5 lb bags — verify with current product packaging. Greenfiber SANCTUARY is a cellulose product rated at approximately R-3.7/inch settled — a denser material that resists air movement well and is made from ~80% recycled fiber. It covers 44.2 sq ft/bag at R-19 and 25.9 sq ft/bag at R-30 per the Greenfiber SANCTUARY Fact Sheet (Rev K, 05/26); on the same chart R-38 covers 19.0 sq ft/bag, R-49 covers 14.1 sq ft/bag, and R-60 covers 11.6 sq ft/bag — consult your bag-label coverage chart or Greenfiber SANCTUARY's technical data sheet. Knauf Jet Stream ULTRA is also fiberglass loose-fill — per the Knauf Jet Stream ULTRA Product Data Sheet (valid from 04/2025) — verify with this Knauf TDS — R-11 through R-60 coverage tables are published for both open-attic and cavity-wall applications. All three brands require the same basic installation approach: air-seal first, maintain soffit baffles, and hit both the required bag count AND the minimum depth.

Verify with the current AttiCat TDS — bag yield can vary by production batch. Per the Owens Corning AttiCat Product Data Sheet (Pub. No. 10011273-K, Dec 2025) — verify with this PDS — the published coverage rate at R-49 is 39.9 sq ft per bag, which equals 25 bags per 1,000 sq ft. To estimate your project: divide your net area to insulate (attic sq ft minus joists and obstructions) by 39.9, then round up to the next whole bag — verify with the AttiCat PDS coverage chart. Example: a 1,250 sq ft attic topping up to R-49 → 1,250 ÷ 39.9 = 31.3 → 32 bags — verify with the AttiCat coverage chart. If you already have existing insulation, first subtract its R-value from R-49 to find the additional R needed — verify with your bag-label R-per-inch — then use the calculator above to find the equivalent bag count — the tool handles the existing-depth offset automatically. Always cross-check with the coverage chart printed on your specific bag, since bag-yield can vary by production batch.

Bag yield (sq ft per bag) varies because the underlying materials have different densities and R-per-inch ratings — verify with each product's technical data sheet. Fiberglass (AttiCat, Knauf Jet Stream ULTRA) achieves about R-2.5 per settled inch, so you need more depth — and more material — to reach R-49 than cellulose does. Cellulose (Greenfiber SANCTUARY) delivers roughly R-3.7 per settled inch, so each bag covers more sq ft at a given R-target because less depth is needed. A second factor is settling: cellulose settles ~20% after installation per FTC 16 CFR 460, which is already factored into the bag-coverage tables printed on each bag label. Manufacturer fill machine calibration also plays a role — each brand's blower velocity and nozzle design affects how the product expands and settles in your attic — verify with the installation guide for your specific blower. The practical takeaway: always compare brands at the same R-value target, not by sq ft/bag in isolation, and consult your bag-label coverage chart. The Knauf Jet Stream ULTRA Product Data Sheet (valid from 04/2025) — verify with this Knauf TDS — publishes the full R-11 through R-60 coverage tables.

Bags needed = ⌈ attic area (sq ft) ÷ coverage constant (sq ft per bag at target R-value) ⌉ — verify with your product's bag-label coverage chart. The coverage constant varies by product: blown cellulose (Greenfiber SANCTUARY example) covers 14.1 sq ft per bag at R-49; blown fiberglass (AttiCat) covers about 39.9 sq ft per bag at R-49 per the Owens Corning AttiCat Product Data Sheet. For a 1,000 sq ft attic targeting R-49 from bare joists: ~71 bags of SANCTUARY cellulose or ~25 bags of AttiCat — verify with the AttiCat coverage chart. Enter your attic area, climate zone, and existing depth in the calculator above for a precise count — the tool adjusts for both bare-joist and partial-layer starting points.