Cellulose R-Value per Inch: Loose-Fill vs Dense-Pack by Brand

Per-brand R-per-inch table, settling mechanics, how the FTC R-Value Rule governs coverage charts, and a depth-lookup for every DOE climate-zone target.

Cellulose R-Value per Inch by Brand and Method

The table below lists per-brand R-per-inch derived from manufacturer coverage charts — all on a settled-depth basis as required by the FTC R-Value Rule (16 CFR Part 460). R-per-inch represents the thermal resistance added for each inch of settled insulation thickness. Brands marked ⏳ have unverified primary TDS data.

Our loose-fill rating (~R-3.7 per inch (settled)) matches the commonly cited "3.7–3.8 R/in" range: values are derived from the manufacturer SANCTUARY Rev K 05/26 coverage chart on a FTC-compliant settled-depth basis, which is the legally defensible number on the bag.

How the R-per-Inch Math Works

External benchmark cross-check: Many sources (DOE insulation guides, generic online calculators) quote cellulose at approximately 3.4–3.8 R per inch. Our derived values for Greenfiber SANCTUARY (~R-3.7 per inch settled) are within that published range — reflecting the actual SANCTUARY Rev K 05/26 coverage chart at specific R-value tiers rather than a generic single-number claim. The slight variation (3.611 derived at R-13, 3.727 derived at R-60) reflects real compression behavior at deeper fills; higher depth rows show marginally higher R/in because density increases slightly under the column's own weight. Always verify against the actual coverage chart on your bag — that figure is the legally defensible spec.

Why Cellulose R-Value per Inch Varies by Brand and Method

Density Drives R per Inch

Cellulose R-per-inch is not a fixed constant — it varies with installed density. At typical loose-fill attic densities (1.5–2.0 lb per cubic foot), cellulose delivers 3.5–3.7 R/in. At dense-pack densities (3.5–4.2 lb per cubic foot, used in walls and cathedral ceilings), the same cellulose material delivers 3.5–4.0 R/in. Higher density packs fiber more tightly, reducing the air gaps that allow convective heat transfer through the insulation matrix.

FTC R-Value Rule — Why Coverage-Chart R Is Settled R

The FTC R-Value Rule (16 CFR Part 460) requires that every loose-fill insulation bag include a coverage chart using settled thickness — not fresh-blown thickness. Earlier unregulated charts used inflated fresh-blown depths that overstated final R-value. Today's coverage charts already encode settling — you install to the chart depth and hit the labeled R-value once settling completes within 48–72 hours. No additional buffer needed.

ASTM C739 vs ASTM C518 — The Test Method Distinction

Two ASTM standards govern cellulose thermal measurement: ASTM C739 (Standard Specification for Cellulosic Fiber Loose-Fill Thermal Insulation) sets manufacturing requirements and specifies maximum settled density at 1.6 lb/ft³ dry for loose-fill. ASTM C518 (Heat Flow Meter) is the steady-state thermal test used to derive R-per-inch for labeling — FTC requires C518 or C177 for all coverage chart R-values. Both references can appear on the same bag: C739 confirms the product quality specification; C518 (or C177) confirms the measured thermal resistance.

Greenfiber Cellulose: SANCTUARY vs SANCTUARY DEFENSE — R per Inch by R-Value Target

Greenfiber is the dominant cellulose brand at Home Depot and Lowe's. Two products span the current loose-fill product line — SANCTUARY (formerly INS515LD) and SANCTUARY DEFENSE (formerly INS541LD) — both standardized at 25 lb per bag with an identical loose-fill coverage chart per Greenfiber SANCTUARY Rev K 05/26. The DEFENSE branding emphasizes fire/insect resistance; thermal/coverage values are equivalent.

SANCTUARY and SANCTUARY DEFENSE from Greenfiber SANCTUARY Rev K 05/26 (@asOf 2026-06-02 (verified)). Knauf EcoFill WX (~3.7 R/in est.) omitted — TDS inaccessible. Verify from your bag's coverage chart. Use the Blown-In R-Value Calculator for estimated depth and bag counts (~38.6 bags/1,000 sq ft for SANCTUARY at R-30).

Dense-Pack Cellulose in Walls: R-Values by Stud Depth

At 3.5–4.0 PCF dense-pack density (~3.7 R/in), typical wall cavity R-values:

Dense-pack R-values assume 3.7 R/in — verify against your contractor's product TDS. Total wall R-value includes cavity R plus any exterior sheathing, WRB, or continuous insulation. DIY note: dense-pack is typically a pro-installed retrofit — it requires a high-pressure machine to hit 3.5–4.2 PCF, fabric netting on open framing, and an experienced operator to avoid blowouts. DIYers should stay with open-attic loose-fill unless they have specific dense-pack training and equipment.

Cellulose vs Blown Fiberglass — R per Inch

At R-49: Greenfiber SANCTUARY needs 13.3 in settled vs Owens Corning AttiCat's ~16.25 in — a ~3-inch advantage that matters near low eaves. At R-60: SANCTUARY at 16.1 in settled vs fiberglass at ~20.5 in. The trade-off: blown fiberglass does not settle (SANCTUARY cellulose settles ~7% per current Greenfiber chart) and does not absorb moisture; for dry attics in Zones 3–7 cellulose typically wins on cost-per-R-value.

Moisture and Humidity Effects on Cellulose R-Value

Cellulose is hygroscopic — it absorbs and releases moisture with ambient humidity. Short-term humidity swings (40–80% RH) have negligible impact on rated R-per-inch. Sustained elevated humidity or active wetting (roof leak, ice dam, chronic bath-fan vent into attic) can clump fibers and reduce loft, lowering effective R by 5–15% even after drying. Borate-treated cellulose dries to original performance in ~2–3 weeks of normal attic ventilation if the moisture source is corrected; ammonium-sulfate-treated cellulose can suffer corrosion of adjacent metal fasteners when wet. Numeric thresholds: joist moisture content above ~20% (per moisture-meter reading) indicates an active leak — investigate before adding insulation per ENERGY STAR Seal & Insulate; if more than ~10 sq ft of cellulose is visibly matted or has dark staining, plan for removal/replacement rather than drying in place. Avoid cellulose in attics with: ongoing roof leaks, bath/dryer/range-hood fans terminating in the attic, or chronic ice damming. Choose fiberglass until the moisture source is resolved. Settling: SANCTUARY chart implies ~7% per current Greenfiber chart (much less than the historical ASTM C-739 15–20% range).

Cellulose Insulation Cost Estimate

Cellulose bags retail at approximately $10–$16 per bag (2026 national average; verify before purchasing). Greenfiber SANCTUARY (25 lb) typically runs ~$14–$16; SANCTUARY DEFENSE (25 lb) ~$14–$18. At R-30 over 1,000 sq ft, SANCTUARY requires ~38.6 bags — approximately $540–$617. At R-49, ~70.8 bags — roughly $991–$1,133. Blower rental is free with 10+ bags at both Home Depot and Lowe's. Blown fiberglass (AttiCat) costs ~$18–$22/bag but requires fewer bags at R-30 (~29 vs ~38.6) — though at a higher per-bag price, the total cellulose cost-per-R-value advantage holds.

Cost estimates approximate as of May 2026 and vary by region, retailer, and time; verify before purchasing. DIY materials vs pro install: typical contractor turnkey runs $1.50–$3.00/sq ft at R-49 (materials + labor + machine + crew), so DIY materials-only estimates above are commonly 30–50% lower than a turnkey quote. Labor varies regionally — Northeast/West-Coast metros higher, interior markets lower.

How Many Inches of Cellulose for Your Climate Zone

The DOE and ENERGY STAR publish zone-based attic R-value targets aligned with the 2021 IECC (verify the edition adopted by your local building department). The table below uses Greenfiber SANCTUARY's settled R-per-inch (~3.7) — see the Greenfiber SANCTUARY / SANCTUARY DEFENSE TDS — to translate zone targets into installed depth. Always confirm with your bag's coverage chart.

DOE cost-effectiveness targets for existing homes; local jurisdictions may adopt different IECC editions. Confirm requirements with your local building department before permitted work.

Loose-Fill Cellulose Install Checklist

Follow these steps in order for a correct open-attic loose-fill install. Steps marked as code-dependent should be confirmed with your local building authority before starting permitted work.

1. Find your climate zone — Enter your ZIP at energystar.gov to get your DOE R-value target (see zone table above).
2. Read your bag's coverage chart — Find the sq ft per bag at your target R-value. Divide attic area by that number; round up to the nearest whole bag. This is your base bag count.
3. Air-seal all penetrations before blowing — Low-expansion spray foam or fire-rated caulk at top plates, plumbing stacks, electrical penetrations, and recessed light surrounds (use fire-rated boxes). Air sealing first recovers 15–30% effective R-value that convection through gaps would otherwise negate.
4. Install rafter baffles at every eave bay — Cardboard or rigid foam baffles extending 12+ inches above planned insulation depth to protect soffit vent airway. Required before blowing cellulose to prevent blocking vents.
5. Mark depth gauges every 4–6 feet — Place sticks or rulers at the coverage chart's minimum settled depth. Blow to slightly above that mark; SANCTUARY cellulose will settle ~7% per current Greenfiber chart in 48–72 hours to hit the target.
6. Blow in passes from eave to ridge — Start at the eaves and work back toward the access hatch so you don't compact previously blown areas. Reference the coverage chart on your bag (see step 2); install to the chart's minimum depth rather than estimating by eye.
7. Post-install: leave the attic insulation certificate visible at the access — IECC R303.1.1 / FTC 16 CFR Part 460 require an attic card or sticker showing installed R-value, settled thickness, coverage area, and product type. Inspectors look for it at every attic-access opening; for DIY jobs without a contractor card, write the same data on the inside of the hatch frame (R-value, settled depth, sq ft covered, brand + bag count, install date).

Common Cellulose Insulation Mistakes

These mistakes drive most underperformance reports in r/Insulation and contractor callbacks — each with a specific R-value shortfall or bag-count error.

• Depth-vs-R confusion — "10 inches is always R-38" is a common but wrong assumption. At SANCTUARY's ~3.7 R/in (settled), 10 in settled = R-37 — close to but not exactly R-38. Always look up depth at your target R from the coverage chart, or use the brand's R-per-inch with the inverse formula: inches = target R ÷ R-per-inch. Different brands yield different depths at the same R.
• Using dense-pack R/inch rates for loose-fill jobs — Dense-pack cellulose delivers 3.5–4.0 R/in at 3.5+ PCF density. If you apply that rate to an open-attic loose-fill project (which installs at 1.5–2.0 PCF), you will overestimate R by 15–25% and under-buy bags. Use the loose-fill coverage chart rate (3.5–3.7 R/in) for attic jobs, and the dense-pack rate only for wall/cathedral applications at confirmed ≥3.5 PCF installed density.
• Skipping air sealing before blowing — Insulation fills space but does not seal air leaks. Convective loops through unsealed top plates, recessed light penetrations, and plumbing stacks bypass the insulation layer and reduce effective R-value 15–30% in stack-pressure conditions. Air seal first — always — then insulate. This is the single most impactful pre-insulation step for energy performance.
• Mixing brands mid-job and assuming equal R/inch — Greenfiber SANCTUARY (25 lb) and SANCTUARY DEFENSE (25 lb) share the identical loose-fill coverage chart (~3.7 R/in settled across R-13..R-60). Mixing them with other (non-Greenfiber) brands across different attic zones can produce uneven R-value at the same installed depth. If you switch products mid-job, recalculate required depth separately for each product area.

Common Questions

What is the R-value per inch of cellulose insulation?

Loose-fill cellulose: ~3.7 R/in (settled) (SANCTUARY ~R-3.7 per inch (settled); SANCTUARY DEFENSE identical chart). Dense-pack at ≥3.5 PCF: 3.5–4.0 R/in. See the brand table above for the complete breakdown.

How many inches of cellulose do I need for R-38?

SANCTUARY: 10.4 inches settled. SANCTUARY DEFENSE: identical chart, also 10.4 inches settled. Always use the coverage chart on your specific bag per FTC 16 CFR Part 460 — it is the authoritative settled-depth number for your job.

Does cellulose insulation lose R-value over time?

R-per-inch does not change over time. Loose-fill cellulose settles ~7% per current Greenfiber chart in thickness within 48–72 hours, reducing total R proportionally — but the per-inch rate of the remaining material is unchanged. Dense-pack at ≥3.5 PCF does not settle appreciably.

What is the R-value of dense-pack cellulose vs loose-fill?

Dense-pack (3.5–4.2 lb/ft³): ~3.5–4.0 R/in. Loose-fill open attic (1.5–2.0 lb/ft³): 3.5–3.7 R/in. Higher density reduces air infiltration — the air-resistance component raises effective R/in. Dense-pack is used in walls, cathedral ceilings, and enclosed bays; not typical open-attic blowing.

How does cellulose R-value per inch compare to fiberglass?

Blown cellulose (3.5–3.7 R/in) vs blown fiberglass AttiCat (~2.79 R/in per DOE Insulation Guide). Cellulose reaches the same R in ~20–30% less depth — useful when eave headroom is constrained. Trade-off: fiberglass does not settle and resists moisture.

How do I add cellulose over existing fiberglass batts?

Blown cellulose is compatible over existing fiberglass batts. The calculation: existing R + cellulose top-up R = total target R. Example: 6-inch fiberglass batts at R-19 in a Zone 5 attic (DOE target: R-60). Cellulose needed: R-60 minus R-19 = R-41. At SANCTUARY's ~3.7 R/in (settled): 41 ÷ 3.7 = ~11.2 inches of settled cellulose to add, or approximately 52.6 bags per 1,000 sq ft (1,000 ÷ 19.0 sqft/bag at the R-38 coverage tier, the closest published row per SANCTUARY Rev K 05/26). Air-seal all penetrations before blowing — the existing batts provide some R but do not block air movement through gaps around their edges.

Estimates only — verify with your local building authority and a qualified contractor before construction. See our full disclaimer.