Blown-In Insulation Calculator
Work out how much blown-in insulation your attic needs - enter the area, what is already up there and your climate zone to get the depth to add, the volume, the bag count and what a professional install should cost.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
existing R = existing depth (in) × R per inch of the existing material
R to add = target R − existing R · depth to add (in) = R to add ÷ R per inch of the new material
volume (cu ft) = area × depth to add ÷ 12
bags = ⌈volume ÷ bag yield⌉ · coverage per bag = bag yield ÷ (depth ÷ 12)
Assumptions & limits
- Target R-values are the code minimums in IRC 2021 Table N1102.1.3: R-30 for climate zones 0–1, R-49 for zones 2–3 and R-60 for zones 4–8.
- R per inch uses the midpoint of the published ranges: loose-fill cellulose 3.5 (range 3.2–3.8) and loose-fill fiberglass 2.5 (2.2–2.7). Manufacturers state the figure for their own product. Batts (3.1–3.4 per inch) can be selected as the EXISTING material, where only R per inch matters, but are not offered as the new material: they are sold by the bundle at a stated square-foot coverage rather than by installed volume, so a volume-based bag model prices them wrong.
- Bag yield is an INPUT, not a constant, because it is product-specific. The defaults are 18 cu ft installed for a 25 lb cellulose bag (settled density ≈ 1.4 lb/cu ft, published range 1.3–1.6) and 50 cu ft for a 30 lb loose-fill fiberglass bag (≈ 0.6 lb/cu ft). Bag PRICE follows the material selector for the same reason — $13 for a 25 lb cellulose bag, $45 for a 30 lb loose-fill fiberglass bag. Read the chart on the bag you are buying and enter the real number — the FTC R-Value Rule, 16 CFR 460, requires that chart to be printed.
- The tool reports coverage per bag at your target R alongside the bag count, so it can be reconciled directly against that chart.
- Installed prices are $1.00–$2.50 per sq ft at R-49 depending on material, scaled by the R-value actually being added. They assume an accessible attic with room to work.
- Excluded: air sealing, baffles, attic-hatch insulation, knee walls, removal of contaminated insulation, and any electrical remediation such as boxing out old recessed cans. Air sealing in particular is separate work and usually the higher-value half of the job.
- The calculation is for a vented attic floor. Conditioned attics, cathedral ceilings and roof-deck spray foam are different assemblies and are not modelled here.
Worked example
A 40 × 25 ft attic with 4 in of existing cellulose, target R-60 (climate zone 4–8), new cellulose at 18 cu ft per bag and $13 per bag
- Existing R = 4 × 3.5 = R-14 → R to add = 60 − 14 = R-46
- Depth to add = 46 ÷ 3.5 = 13.1 in (17.1 in total)
- Volume = 1,000 × 13.1 ÷ 12 = 1,095 cu ft → bags = ⌈1,095 ÷ 18⌉ = 61
- Coverage per bag at this R = 18 ÷ (13.1 ÷ 12) = 16.4 sq ft
Result:About 13.1 in to add — 61 bags of cellulose, roughly $714 to $912 DIY, or $1,130 to $2,350 blown in by a contractor.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on September 1, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
Start from what is already up there
Almost nobody insulates a bare attic. There is usually something between the joists already, and it counts — which means the useful question is not "how much insulation do I need" but "how much more". Measure the existing depth with a ruler pushed to the drywall, identify what it is, and the arithmetic falls out.
inches to add = (target R − existing depth × R per inch) ÷ R per inch of the new material
Four inches of old cellulose is R-14. Against a zone 5 target of R-60 that leaves R-46 to find, which is 13.1 more inches of cellulose. Skip the subtraction and you buy forty per cent more material than the attic needs.
What the code actually asks for
| Climate zone | Ceiling R-value | Cellulose depth | Loose fiberglass depth |
|---|---|---|---|
| 0–1 (Gulf coast, south Florida, Hawaii) | R-30 | 8.6 in | 12.0 in |
| 2–3 (the South and lower Southwest) | R-49 | 14.0 in | 19.6 in |
| 4–8 (most of the country northward) | R-60 | 17.1 in | 24.0 in |
Those R-values are IRC 2021 Table N1102.1.3, not a rule of thumb — they are the minimum a new ceiling has to meet. Two things follow. Most existing houses are well below them, because the requirement has climbed steadily over the decades and nobody retrofits on a code cycle. And the depth column is worth reading before you pick a material: the same R-60 is 17 inches of cellulose or 24 inches of loose-fill fiberglass, which is the difference between clearing the joists comfortably and running out of headroom under the roof deck at the eaves.
R per inch, and why the bag chart is the authority
| Material | R per inch | Settled density | Where it wins |
|---|---|---|---|
| Loose-fill cellulose | 3.2–3.8 | ~1.4 lb/cu ft | Shallow attics, awkward cavities, best R per inch of the loose fills |
| Loose-fill fiberglass | 2.2–2.7 | ~0.6 lb/cu ft | Light weight on old ceilings, does not settle much |
| Fiberglass batts | 3.1–3.4 | n/a | Clean, regular joist bays — sold by coverage, not by volume, so not a blown option |
| Open-cell spray foam | 3.5–3.9 | n/a | Conditioned attics and roof decks, air seals as it goes |
| Closed-cell spray foam | 6.0–7.0 | n/a | Tight spaces where depth is the constraint |
The calculator uses the midpoints and lets you change the bag yield, because that is the number that genuinely varies between products. Every bag of loose fill carries a coverage chart — minimum thickness, maximum coverage and minimum weight per square foot at each R-value — and it is there because the Federal Trade Commission's R-Value Rule requires it. That chart, not a website, is the authority for the product in your cart. The tool prints coverage per bag at your target R next to the bag count so you can hold the two side by side.
Air seal first — it is the higher-value half
Insulation slows heat moving through material. It does nothing about air moving past it, and a typical attic leaks a great deal of warm air: recessed light cans, plumbing vent stacks, the attic hatch, wiring penetrations, top plates and the chimney chase. Blowing fresh insulation over those leaks buries them — the leak continues, and now it is under sixteen inches of cellulose where nobody will ever find it.
So the order of operations is: seal the penetrations, box out any recessed cans that are not rated for insulation contact, fit baffles at the eaves so the fill cannot drift into the soffit vents, insulate the hatch, and only then blow. It is also why a contractor quote that includes air sealing is not simply the expensive one — it is a different scope, and usually the better buy.
DIY or hire it out
Blowing loose fill is genuinely DIY-able. Most home centres lend the blower free with a minimum purchase, the machine is not difficult, and the material is cheap: the worked example on this page is about $714 to $912 in cellulose against $1,130 to $2,350 to have the same job blown in. What you are buying with the contractor is a truck-mounted blower that does in three hours what a rental unit does in eight, plus — on a good crew — the air sealing.
Two jobs to hand over regardless: an attic with knee walls or a finished room in it, and any attic where the existing insulation is contaminated by rodents or moisture. Both are removal jobs before they are insulation jobs.
Common mistakes
- Ignoring what is already there. The most expensive mistake and the easiest to avoid — it is one ruler measurement.
- Adding faced insulation on top. The kraft or foil facing is a vapour retarder. A second one partway through the assembly traps moisture between the layers. Anything added on top must be unfaced.
- Burying the soffit vents. Loose fill drifts into the eaves and blocks the intake air that keeps the roof deck dry. Baffles are a few dollars each and are not optional.
- Insulating over unrated recessed lights. Older cans that are not marked IC (insulation contact) need a clearance or a cover. This one is a fire issue, not a performance one.
- Chasing depth past the point it pays. Going R-19 to R-38 saves far more than R-49 to R-60. Once you are near code, spare budget belongs in air sealing.
Related planning
Insulation and cooling load are two halves of the same problem: the BTU calculator sizes an air conditioner to a room, and the mini-split sizing calculator does the same for a ductless unit — both get easier to satisfy once the attic is right. If the ceiling below is coming down as part of the work, the drywall calculator sizes sheets, compound and tape, and the paint calculator handles the finish. The BTU converter is useful for reading equipment specs while you compare systems.
Figures are planning estimates based on IRC 2021 minimums, published R-per-inch ranges and 2026 US prices. Bag coverage is product-specific — read the chart on the package, which the FTC R-Value Rule requires, and enter that figure before you buy.
Frequently asked questions
How do I calculate how much blown insulation I need?
Three steps, and only the third is product-specific. First, how much R-value you still need: subtract what is up there (existing depth times the R per inch of that material) from your target. Second, the depth that buys it: R to add divided by the R per inch of the new material - 3.5 for cellulose, 2.5 for loose-fill fiberglass. Third, the bags: area times depth divided by twelve gives cubic feet, and the bag chart on the package converts that to bags. That chart is not marketing - the FTC R-Value Rule requires it.
How many inches of blown-in insulation do I need for R30?
It depends on the material, and by more than people expect. Cellulose runs about R-3.5 per inch, so R-30 is roughly 8.6 inches. Loose-fill fiberglass runs about R-2.5 per inch, so the same R-30 takes about 12 inches - forty per cent more depth for the same performance. That matters in a shallow attic where the depth has to clear the joists and stay under the roof deck, and it is why the two materials are not interchangeable by the inch.
How much does a 25 lb bag of blown insulation cover?
The bag itself does not have one coverage - it has a chart, because coverage depends on the R-value you are blowing to. A 25 lb bag of cellulose yields roughly 18 cubic feet installed, which covers about 25 sq ft at R-30, about 15 sq ft at R-49 and about 12.5 sq ft at R-60. Loose-fill fiberglass bags cover far more area per bag because the material is much lighter per cubic foot. Always read the chart on the bag you are actually buying and enter that figure above.
How much does it cost to insulate a 2,000 sq ft attic?
Doing it yourself, topping a 2,000 sq ft attic from R-14 up to R-60 takes about 122 bags of cellulose - roughly $1,600 in material, plus the blower, which most home centres lend free with a minimum purchase. Having it blown in professionally runs about $2,300 to $4,700 for the same job. The gap is smaller than it looks: a crew brings a truck-mounted blower, does it in a few hours, and will usually air-seal the penetrations first - which is the part that actually decides whether the insulation performs.
How much insulation should you put in your attic?
The building code sets a floor by climate zone. IRC 2021 Table N1102.1.3 asks for a ceiling R-value of R-30 in zones 0 and 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. In cellulose that is roughly 9, 14 and 17 inches. Going above the code figure has real but diminishing returns - the jump from R-19 to R-38 saves far more than the jump from R-49 to R-60 - so put spare budget into air sealing before extra depth.
Should I air seal before adding insulation?
Yes, and it is the single highest-value thing on this page. Insulation slows heat conduction; it does nothing about air moving through gaps. Recessed lights, plumbing stacks, the attic hatch, top plates and chimney chases all leak warm air straight into the attic, and burying those leaks under fresh insulation makes them harder to reach without making them stop. Seal first, then blow. It is also why professional quotes that include air sealing are not simply dearer than ones that do not.
Can I put blown-in insulation over existing batts?
Yes - loose fill over old batts is normal and effective, as long as the batts are dry and the existing R-value is counted, which the calculator does. Two conditions matter. The added material must be unfaced: a second vapour barrier partway through the assembly traps moisture between the layers. And the eaves need baffles so the loose fill cannot drift into the soffit vents and block the airflow that keeps the roof deck dry.