Insulation · Project planning
Attic Insulation Calculator
Estimate a retrofit thermal deficit from known existing R or measured depth and an entered product R per inch. Choose quantity coverage for the additional thickness.
By Alish Gautam, Software Developer · Updated · Calculation methodology
Start with the editable example below. Dimensions retain their physical size when units change. Region sets currency labels without converting currencies; confirm entered quotes after changing region. Thermal inputs labeled US R retain that unit in every region.
Project takeoff
Whole packages round up. Allowance is separate from purchased surplus. Prices omit labor, delivery, tax and other items unless explicitly entered.
- Gross measured area
- 120 ft²
- Opening / exclusion deductions
- 0 ft²
- Known existing US R
- 19
- Target US R
- 49
- Additional US R needed
- 30
- Estimated additional thickness
- 8.571 in
- Net requirement
- 120 ft²
- Allowance
- 12 ft²
- Planned requirement
- 132 ft²
- Unrounded package requirement
- 3.3
- Packs / units to buy
- 4
- Purchased coverage
- 160 ft²
- Surplus beyond planned requirement
- 28 ft²
Supplier-ready material list
- Packs / units to buy
- 4
Check before ordering
- Pack coverage must match the additional thickness, not the total target resistance.
Geometry / layout breakdown
| Surface | Material | Gross (ft²) | Net (ft²) |
|---|---|---|---|
| Room / surface 1 | floor | 120 | 120 |
Depth-derived R is an estimate. Loose-fill products require their coverage chart at the additional thermal resistance; this linear thickness estimate does not generate a bag curve.
How it works
R deficit = max(0, target R − existing R). Additional thickness = deficit / product R per inch. Zero deficit produces zero new packages.
Assumptions and limitations
Depth-derived R is an estimate. Loose-fill products require their coverage chart at the additional thermal resistance; this linear thickness estimate does not generate a bag curve.
Whole packages round upward after the chosen allowance. Physical geometry and purchase reserve are separate. Manufacturer and project specifications override editable examples; the selected region does not establish a local code requirement.
ENERGY STAR attic reference
The published retrofit cases below are a reference. Select a custom target appropriate to your project; these cases do not establish locally adopted-code compliance.
| Zone | Uninsulated attic US R | Existing 3–4 in case US R |
|---|---|---|
| 1 | 30 | 25 |
| 2 | 49 | 38 |
| 3 | 49 | 38 |
| 4A/4B | 60 | 49 |
| 4C/5/6 | 60 | 49 |
| 7/8 | 60 | 49 |
Sources and boundaries
ENERGY STAR: Recommended Home Insulation R-Values
Open the source · Web guidance; edition not stated · Reference checked 2026-10-04
Scope: US residential retrofit recommendations
Used for: Displayed climate-zone attic reference table.
Does not establish: Local adopted-code compliance; Manufacturer bag coverage.
The page states its guidance is based on 2021 IECC Table R402.1.3; preserve separate bare-attic and existing-insulation columns.
ENERGY STAR: Well-Insulated and Sealed Attic
Open the source · Web guidance; edition not stated · Reference checked 2026-10-04
Scope: Attic air sealing and insulation planning
Used for: Measure existing insulation before top-up; Air-sealing context.
Does not establish: Exact product R per inch; Universal installed depth.
Measured depth can estimate existing performance; product and condition remain important.