How the Thickness Math Works — Four Verified Steps
Every “how many inches of cellulose for R-49?” question resolves through the same four-step derivation. There is exactly one correction factor between theory and the finished attic — the settling overage — and skipping it is the single most common DIY shortfall. Work the steps once with your own numbers and the full chart below becomes checkable at a glance.
Step 01 — Pick the settled R-value per inch for your material
InputSettled means the value after roughly one year of compaction — cellulose R-3.5, loose fiberglass R-2.2 (0.7 PCF) to R-2.5 blended, dense-pack cellulose R-3.7, closed-cell spray foam R-6.5. Using the bag-label number instead of the settled number is where derivations start drifting. The full Table 1 below lists all thirteen materials.
Step 02 — Divide the target R-value by R-per-inch
Key FormulaSettled thickness (inches) = Target R ÷ R-per-inch. Example: R-49 ÷ 3.5 = 14.0 inches of settled cellulose. The division is exact — no rounding until the end, or a 0.05-inch slip per step compounds across the chart.
Step 03 — Apply the ×1.15 settling overage for loose-fill only
Correction FactorInstall thickness = settled thickness × 1.15 for blown cellulose and fiberglass (12.5% and 7.5% settling, blended conservatively). 14.0 × 1.15 = 16.1 inches on the machine gauge. Dense-pack, batts, rigid foam, and spray foam skip this step — their settling is 0–2%, already inside the stated R-per-inch.
Step 04 — Verify on the deck with a ruler
VerifiedProbe at least ten spots after blowing and average the readings. If the average is below the settled target minus the material’s expected settling, more material goes in before the blower goes back. Pros staple depth rulers every ~300 sq ft so the check is visual instead of manual.
⚠️ Label vs settled: Bag coverage charts list “R-value at blown depth,” not settled depth. An AttiCat bag promising R-49 at 20.4” assumes fresh blow — measure after a year and you will find less. Always run the chart with settled R-per-inch.
❌ The #1 derivation error: Skipping Step 3. Blowing exactly 14.0” of cellulose “for R-49” settles to ~12.25” and R-43 — an 11-bag shortfall on 1200 sq ft. The ×1.15 is not optional for loose-fill.
💡 Skip the arithmetic: The estimator below runs Steps 2–3 instantly for your material and target. The full attic insulation calculator extends the same math to cost and bag count.
Thickness Estimator — Settled vs Blow Depth
Pick a material and a target R-value. The estimator runs Steps 2–3 of the derivation above and shows both the settled depth and the depth to blow.
Settled thickness (Step 2)
14.0"
49 ÷ 3.5 R-per-inch
Blow this depth (Step 3)
16.1"
settled × 1.15 settling overage
Values use the settled averages from Table 1. For bag counts and cost on your square footage, use the blown-in coverage chart.
Material Settled R-Value Per Inch Chart (°F·ft²·hr/BTU/in)
This is the master attic R-value chart: settled R-per-inch for every material that goes into an attic or its framing, with the settling rate that separates loose-fill from everything else. The Class column is the fire classification — A is non-combustible, C is what most cellulose and fiberglass achieve after fire-retardant treatment.DIY-Friendly refers to renting equipment, not skill.
| Material | Category | Settled R/inch | Settling % | DIY-Friendly | Class | Notes |
|---|---|---|---|---|---|---|
| Loose Cellulose | Blow-in | 3.2–3.8 (avg 3.50) | 12.5% | Yes | C | Most common attic retrofit; boric acid treated required |
| Dense-Pack Cellulose | Blow-in | 3.5–3.9 (avg 3.70) | 2.0% | No — high-pressure | C | Wall/rafter retrofit; 3.0–3.5 lb/cu ft |
| Loose Fiberglass 0.7 PCF | Blow-in | 2.2 | 7.5% | Yes | C | Budget pink fluffy; settles aggressively |
| Loose Fiberglass 2.0 PCF | Blow-in | 4.0 | 7.5% | Yes | C | High-density; needs 100 PSI minimum |
| Fiberglass Batt Standard | Batt | 3.40 | 0% | Yes | C | Open joist bays; new construction only |
| Fiberglass Batt High-Density | Batt | 4.00 | 0% | Yes | C | 2×6 cavities |
| Mineral/Rock Wool Loose | Blow-in | 3.20 | 2.0% | Partially | A | Non-combustible; coarser fiber |
| Mineral/Rock Wool Batt | Batt | 3.30 | 0% | Yes | A | Also provides soundproofing |
| EPS Rigid Foam | Rigid | 4.00 | 0% | Yes | C | Beadboard; cheapest rigid |
| XPS Rigid Foam | Rigid | 5.00 | 0% | Yes | C | Blue/green board; most moisture-resistant |
| Polyiso Rigid Foam | Rigid | 7.20 | 0% | Yes | B | Highest R/inch; pricier |
| PU Spray Foam Open-Cell | Spray | 3.5–4.0 (avg 3.70) | 0% | No | B | Class III vapor retarder |
| PU Spray Foam Closed-Cell | Spray | 5.5–6.5 (avg 6.50) | 0% | No | B | Class I vapor barrier + structural rigid |
Settling percentages are first-year values for attic loose-fill installed at label density. Dense-pack assumes 3.0–3.5 lb/cu ft wall-cavity density. Rigid foam and spray foam values assume no facers and standard formulations.
💡 How to use this chart: Find your material, read its settled R-per-inch, then jump to Table 2 for the pre-computed thickness. The homepage’s material reference panel carries the same R-per-inch values inside the main attic insulation calculator, so cost and bags update with the material switch.
Settled Thickness Needed to Reach R-30, R-38, R-49, and R-60
Every cell below is Step 2 of the derivation pre-computed: target R ÷ settled R-per-inch, rounded to 0.1 inch. The overage column is Step 3 — the multiplier between what you measure after a year and what you blow today.
| Target R-Value | Loose Cellulose @ R-3.5/in | Loose Fiberglass @ R-2.5/in | Dense-Pack Cellulose @ R-3.7/in | Closed-Cell Foam @ R-6.5/in | Installation Overage to Blow |
|---|---|---|---|---|---|
| R-30 | 8.6" | 12.0" | 8.1" | 4.6" | +15% loose-fill · 0% foam |
| R-38 | 10.9" | 15.2" | 10.3" | 5.8" | +15% loose-fill · 0% foam |
| R-49 | 14.0" | 19.6" | 13.2" | 7.5" | +15% loose-fill · 0% foam |
| R-60 | 17.1" | 24.0" | 16.2" | 9.2" | +15% loose-fill · 0% foam |
⚠️ “Install thickness” vs “settled thickness” — the #1 DIY mistake: Pro installers blow 115% of the settled target for loose-fill. A 14” settled R-49 cellulose target means blowing 16.1” initially (14.0 × 1.15). Do NOT skip this — the attic will settle 12.5% in year one, and if you blew only 14”, you land at R-43. The depth you see the day you finish is not the depth you keep.
Depth rulers make the settled-thickness target visible from the hatchPhoto: UnsplashDOE ENERGY STAR Recommended R-Values by Climate Zone
Thickness only matters relative to a target — and the target is set by where you live. The Department of Energy and ENERGY STAR publish recommended attic, wall, and floor R-values by IECC climate zone, reproduced here with the ventilation rule each zone expects. Find your zone on the DOE insulation map, then read across.
| Climate Zone | Representative Regions | Attic R | Wall R | Floor R | IRC Vent Rule | Key Notes |
|---|---|---|---|---|---|---|
| Zone 1 Hot-Humid | FL, south TX, HI | 30 | 13 | 13 | 1/150 | No vapor barrier on attic floor |
| Zone 2 Mixed-Humid | GA, NC | 30 | 13 | 13 | 1/150 | No vapor barrier |
| Zone 3 Mixed-Dry | CA valleys, AZ | 38 | 13 | 19 | 1/150 | Radiant barrier beneficial |
| Zone 4A Mixed-Humid | TN, VA, MO | 49 | 16 | 19 | 1/150 | Transition zone |
| Zone 4B Mixed-Dry | UT, CO, east OR | 49 | 16 | 19 | 1/150 | Balanced heating/cooling loads |
| Zone 5 Cold | OH, IL, PA | 60 | 20 | 25 | 1/300 + vapor barrier on warm side | Code minimum 2021 IECC |
| Zone 6 Cold | NY north, VT, ME | 60 | 20 | 25 | 1/300 + vapor barrier | Air sealing before insulating pays most |
| Zone 7 Very Cold | MN north, MT, ND | 60 | 21 | 30 | 1/300 + vapor barrier | Consider rockwool for fire safety |
💡 Zone targets, one line each: Zone 1–2 → R-30 · Zone 3 → R-38 · Zone 4 → R-49 · Zones 5–7 → R-60. The homepage’s climate-zone panel picks your DOE zone automatically and feeds the target into the calculator.
⚠️ Existing insulation stacks: If your attic already has R-19, you are not starting over — new material adds on top. Target minus current = what remains to blow, in the same R math. Removing old loose-fill is almost never worth the mess.
The 15% Settling Overage — Where the Number Comes From
The ×1.15 multiplier is not folklore; it is the blended math of two measured settling rates. Loose cellulose settles about 12.5% in its first year as the fibers pack and the boric-acit treatment distributes. Loose fiberglass settles about 7.5% — less, because glass fibers spring back, but its lower R-per-inch means the same R target needs so much more depth that the absolute loss is larger. Rockwool settles under 2%, dense-pack under 3%, spray foam and batts effectively 0%.
Run the correction on the two most searched targets. R-49 cellulose: 49 ÷ 3.5 = 14.0” settled → 14.0 × 1.15 = 16.1” blown. R-60 cellulose: 60 ÷ 3.5 = 17.1” settled → 19.7” blown. On fiberglass the effect is brutal: R-49 needs 19.6” settled and 22.5” blown — most 2×6 joist bays are buried entirely, and walking the deck afterward becomes impossible without walk boards.
If your attic was blown years ago and you suspect it short-settled, do not re-derive from memory: measure the actual depth and run it through our settling corrector calculator, which converts real depth into true R-value and the exact bag count to top up. The dense-pack vs loose-fill comparison explains when paying for density eliminates the problem entirely.
❌ Compounding error: Apply the overage once, at the end — never per step. 14.0 × 1.15 = 16.1 is correct. (14.0 × 1.05) × 1.05 × 1.05 = 16.2 drifts high, and every extra percent is real money in bags. One multiplication, then stop.
Continuous Insulation and the Joist Factor — Why R-49 Isn’t R-49 Everywhere
The chart’s R-per-inch values describe the insulation, not the assembly. Wood joists run at roughly R-1.2 per inch — a 2×6 joist is about R-7 sitting in a field of R-49. Because framing occupies roughly a quarter to a third of the attic deck once you count joists, double top plates, and partition tops, a nominal R-49 attic floor delivers an area-weighted R of about 38. That ~22% thermal bridging loss is invisible on the bag chart and very visible on the heating bill.
This is what the 2021 IECC’s continuous insulation mandate answers in walls: Zone 5 and colder exterior walls are required to add continuous foam over the framing so studs stop short-circuiting the cavity. Attic floors get the same treatment retroactively — rigid polyiso or XPS run above the joists (with walkable decking over it), or closed-cell spray foam applied at the roofline in a conditioned-attic conversion, removes the bridge by removing the wood from the thermal path.
Blown-in only
R-49 nominal ≈ R-38 effective. Joists stay in the thermal path. Cheapest, code-compliant, standard for vented attics.
Blown-in + rigid foam above joists
1” XPS adds R-5 continuously and covers the bridge. Deck becomes walkable. Adds height — check clearances at the hatch.
Foam at the roofline
Closed-cell on rafter undersides moves the boundary to the roof deck — no joist bridge, no vents needed. See our spray foam calculator for board-foot cost.
⚠️ Vent rule interaction: Deep loose-fill changes the ventilation math, too. Blowing to 16–20” buries the eave bays, so soffit baffles must be installed full-height before the machine runs — otherwise the 1/150 or 1/300 vent area the zone table assumes disappears under cellulose. Our baffle installation guide covers the fix.
How to Verify Installed Depth — the Ruler Method
Charts and receipts estimate; the deck measures. Verification takes ten minutes and settles every “did I get what I paid for?” question. Push a ruler or tape straight down to the drywall at ten or more spots — eaves, center, peak, and above any addition — and average. Blown fiberglass fluffs about 25% above its effective depth, so compress lightly and subtract; cellulose reads essentially true.
Convert the measured average with the same Step 2 formula in reverse: depth × R-per-inch = R-value. A 12” average in cellulose is R-42 — respectable, and R-49 is one more 2” layer away. Most renters’ machines cover 300–400 sq ft per hopper load, so mark your target depth on a joist with spray paint before you start: when the average ruler reading hits it, you stop buying bags.
Ten probe points, one average — the only number that mattersPhoto: UnsplashFrequently Asked Questions
What is the R-value of blown-in insulation per inch?▼
It depends on the material: blown-in cellulose delivers R-3.2–3.8 per inch settled (average 3.5), loose blown-in fiberglass delivers R-2.2 per inch at standard 0.7 PCF density or up to R-4.0 in high-density 2.0 PCF form, and loose mineral wool (rockwool) delivers about R-3.2 per inch. These are settled values — the numbers after the material has compacted for about a year. Bag labels sometimes show higher initial values that drop as the material settles.
How many inches of cellulose to achieve R-49?▼
Exactly 14.0 inches settled at the average R-3.5 per inch. Because loose-fill cellulose settles roughly 12.5% in the first year, pros blow 16.1 inches initially (14.0 × 1.15) so the deck reads R-49 after settling. If you blow only 14 inches, your finished attic settles out around R-42.5 — an 11-inch bag-count shortfall on a 1200 sq ft attic.
How many inches of cellulose to achieve R-60?▼
17.1 inches settled at R-3.5 per inch, which means blowing approximately 19.7 inches at installation time to account for the 15% settling overage. R-60 is the DOE recommendation for Zones 5–7. At that depth, fiberglass would take 24.0 inches settled (27.6 blown) — one reason cellulose dominates cold-climate attics where joists are only 2×6 or 2×8 deep.
What is the R-value of cellulose insulation?▼
Loose-fill attic cellulose: R-3.2–3.8 per inch settled. Dense-pack cellulose (compressed to 3.0–3.5 lb/cu ft in walls or rafter bays): R-3.5–3.9 per inch with under 3% settling. The difference comes from density — dense-pack is roughly six times denser than attic loose-fill, so it resists settling and stops air movement better per inch.
What is the R-value of blown-in fiberglass insulation?▼
Standard 0.7 PCF blown fiberglass is R-2.2 per inch settled — the lowest of the common attic materials — which is why reaching R-49 takes 19.6 inches settled versus 14.0 for cellulose. High-density 2.0 PCF blown fiberglass reaches R-4.0 per inch but requires a 100 PSI professional blower, so it is rarely a DIY option. Most big-box rental machines produce the 0.7 PCF product.
What is rockwool's R-value per inch?▼
Loose blown rockwool: about R-3.2 per inch settled with under 2% settling. Rockwool batts: R-3.0–3.3 per inch (R-15 in a 2×4 cavity, R-23 in a 2×6). Its advantage is not raw R-value — cellulose beats it slightly per inch — but a Class A non-combustible fire rating and moisture tolerance that neither cellulose nor fiberglass matches.
What R-value insulation do I need for my attic?▼
DOE and ENERGY STAR recommendations by zone: R-30 in Zones 1–2, R-38 in Zone 3, R-49 in Zone 4, and R-60 in Zones 5–7. The 2021 IECC code minimum for new construction in cold zones is R-60 for ceilings. If your existing depth is below target, you do not remove anything — you blow new material over the top, and the R-values add layer by layer.
What R-value insulation for exterior walls?▼
It is set by cavity depth, not choice: a 2×4 wall tops out at R-13 to R-15 with batts (R-13 code minimum in Zones 1–4, R-20 only with continuous exterior foam), while a 2×6 wall fits R-19 to R-21 batts (the R-20 code target in Zones 4–5). Dense-pack cellulose achieves R-13–15 in 2×4 walls and closed-cell spray foam R-22 in the same 3.5-inch cavity. You cannot reach wall targets by compressing bigger batts — compression destroys R-value.
How thick is R-38 blown insulation?▼
In cellulose: 10.9 inches settled (12.5 blown). In standard loose fiberglass: 15.2 inches settled (17.5 blown). In dense-pack cellulose: 10.3 inches. In closed-cell spray foam: just 5.8 inches. R-38 is the Zone 3 attic target and a common retrofit goal for 1970s–80s homes that originally shipped with R-19 or less.
What is R-60 insulation thickness in inches?▼
R-60 in loose cellulose is 17.1 inches settled — blow 19.7. In loose fiberglass it is 24.0 inches settled — blow 27.6, which will bury a 2×6 joist completely. In dense-pack it is 16.2 inches, and in closed-cell spray foam 9.2 inches. Check your truss or joist depth against these numbers before choosing a material: burying joists entirely is allowed, but it complicates future electrical work and attic storage.
Why does blown-in insulation settle, and how much R-value do I lose?▼
Loose-fill fibers are blown in with air trapped between them; gravity and vibration pack that air out over the first 6–12 months. Cellulose settles about 12.5%, loose fiberglass 7.5%, rockwool under 2%, and dense-pack or spray foam essentially 0%. R-value loss tracks the depth loss: a 12.5% thinner blanket delivers roughly 12.5% less R. Our settling corrector calculator computes the exact R you are living with and the bags needed to top up.
What is continuous insulation and does my attic need it?▼
Continuous insulation (ci) is a foam layer that covers framing, not just cavities — the 2021 IECC requires it over framing in Zone 5+ walls. Attic floors have the same hidden problem: wood joists conduct heat at roughly R-1.2 per inch, so R-49 between joists averages only about R-38 across the whole deck once ~22% of the surface (the joist factor) is accounted for. Adding rigid foam above the joists or spray foam at the roofline breaks those bridges; blown-in alone cannot.
How do I measure my attic insulation depth accurately?▼
Push a ruler or tape straight down to the drywall in at least ten spots spread across the deck — near the eaves, mid-attic, and the peak — and average the readings. Blown fiberglass fluffs 25% higher than its effective depth, so press lightly and subtract the fluff; cellulose reads true. Write the average on a joist with a marker, or install depth rulers (free at big-box insulation counters) every 300 sq ft so you can eyeball progress while blowing.