Insulation Calculator
Attic bags by the FTC coverage chart — R target credited for what is already up there, rounded UP to the next charted row, never interpolated.
Insulation Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Cellulose chart only — fiberglass bags need their own label numbers.
- Flat attic floors; kneewalls and vaults are separate jobs.
- No dense-pack wall pricing.
In short: A 1,200-ft² attic to R-38: 1,200 ÷ 19.0 ft² per bag = 63.16 → 64 bags of blown cellulose, 10.9 in settled depth, $832.00 at $13 a bag. The chart is the law — bags are charted at R-19/30/38/49/60, never in between.
Formula
net R = target − existing credit
bags = ⌈area ÷ chart ft²/bag at next charted R ≥ net⌉
cost = bags × $/bag
charts are settled-basis — never interpolated between rows
[('19.0', 'ft² per bag at R-38, cellulose chart'), ('10.9 in', 'settled depth at R-38'), ('3.5 / 2.5', 'R per inch: cellulose / blown fiberglass'), ('R-60', 'the loose-fill chart ceiling')]
Worked Example
- Measure attic floor area; probe existing depth to the joists.
- Credit existing R: inches × 3.5 (cellulose) or 2.5 (blown fiberglass).
- Subtract the credit from the zone target.
- Round UP to the next charted R; divide area by bag coverage.
- Price the bags; rent the blower free.
1,200 ft², no existing, R-38 target: chart row R-38 (19.0 ft²/bag, 10.9 in) → 63.16 → 64 bags = $832.00 at $13. Same attic to R-49: 1,200 ÷ 14.1 → 86 bags = $1,118.00.
Strengths & Limits Of This Model
Where this engine is strong
- FTC chart rows verbatim — checkable on the bag
- Existing-insulation credit without double-paying
- Round-up-to-charted-R honesty, never interpolation
Where it stops
- One product family charted
- No air-sealing or ventilation quantities
Practical Use Cases
Attic retrofits
Bags before the blower rental.
Zone upgrades
DOE target by climate band.
Top-ups
Credit the insulation already down.
Budget checks
$12-15 bag reality vs quotes.
Label verification
Chart numbers to check the bag.
Methodology & Editorial Standards
The FTC bag chart is the single authority: rows at R-19/30/38/49/60 with Greenfiber-class coverage (44.2 / 25.9 / 19.0 / 14.1 / 11.6 ft² per bag) and settled depths (5.4 / 8.6 / 10.9 / 14.0 / 17.1 in). The net target (entered R minus existing credit) rounds UP to the next charted row — never interpolated. Bags ceil whole; cost is bags × free-entry price. Targets outside 19-60 and negative credits are refused; net ≤ 0 returns an honest zero with air-sealing advice.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Insulation Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many bags of blown insulation do I need?
Area ÷ the bag’s charted coverage at your target R, rounded up. A 1,200-ft² attic to R-38 cellulose: 1,200 ÷ 19.0 = 64 bags.
What R-value should my attic have?
DOE bands: R-30 to R-49 in zones 1-3, R-38 to R-60 in zones 4-5, R-49 to R-60 in zones 6-7. R-49 is the common retrofit target where bands meet.
Why round R up instead of interpolating?
Bags are charted only at R-19/30/38/49/60 — the FTC chart has no R-42 row. Interpolating invents coverage no manufacturer printed; rounding up to the next charted row is how the chart is meant to be read.
Does cellulose settling change the bag count?
No — the FTC charts are on a settled basis and the 10-14% settling is already inside the coverage number. Blow to the initial thickness printed beside it.
Cellulose or blown fiberglass?
Cellulose: R-3.5-3.7 per inch settled, fewer inches, more bags (R-49 = 14.1 ft²/bag). Fiberglass: ~2.5-3.0 per inch, negligible settling, far more coverage per bag (AttiCat ~39.9 ft²/bag at R-49). Same R, different trucks.
How do I credit existing insulation?
Estimate its R: inches × 3.5 for settled cellulose, × 2.5 for blown fiberglass, × 3.2 for batts. Subtract from the target; if nothing remains, add nothing.
What does blown insulation cost?
Cellulose bags run $12-15 at the big boxes — a 1,200-ft² R-38 job is about $832 in bags, and the blower rents free with a minimum bag purchase.
Should I air-seal before blowing?
Yes — DOE guidance is to air-seal ceiling penetrations first, then blow. Loose-fill slows heat flow; it does not stop the stack effect pulling air through them.