How to use
- Measure the attic floor area receiving insulation and find your climate zone through the ENERGY STAR guidance.
- Select the attic-condition column. Review the displayed table value before entering any R-value credit.
- Use the material's thermal value for a rough depth estimate, and enter bag coverage for the intended installed R-value when the label provides it.
How the estimate is calculated
The selected zone and attic-condition column give the table target. This tool subtracts the R-value credit you enter, with a floor of zero. Estimated added depth is the remaining R-value divided by R per inch, and volume is attic area × depth ÷ 12. Bag count is separately rounded up from attic area divided by label coverage; it is not inferred from the depth estimate.
Example: a 1,000 sq ft uninsulated attic in zone 4C, 5, or 6 uses R60. With zero R-value credit and 3.2 R/in, estimated added depth is 18.75 in and volume is 1,562.5 cu ft. No bag count is given without coverage from a product label.
ENERGY STAR attic table values
| Climate zone | Uninsulated attic | Attic with 3–4 in |
|---|---|---|
| 1 | R30 | R25 |
| 2 | R49 | R38 |
| 3 | R49 | R38 |
| 4A–4B | R60 | R49 |
| 4C, 5, 6 | R60 | R49 |
| 7–8 | R60 | R49 |
Source: ENERGY STAR Recommended Home Insulation R-Values, reviewed for this estimate, based on 2021 IECC Table R402.1.3. This calculator uses the attic columns of that table. The condition column and the separate credit field are distinct inputs: existing depth is not automatically converted into an additional subtraction. Leave the credit at zero when the chosen column already describes the addition you intend to make.
Why the bag chart beats R-per-inch math
Installed thickness and coverage depend on the product and how it is placed. A generic 3.2 R/in value is a commonly used planning figure, not a guaranteed material rating. A Louisiana DNR table summary lists loose-fill cellulose around 2.8–3.8 R/in and high-density fiberglass batts around 3.2–3.5 R/in; the original was not fetched. Choose the actual product data rather than treating those ranges as a specification.
For a zone 3 attic using the existing-layer column, the model target is R38. At 2.5 R/in it estimates 15.2 in. If the label covers 40 sq ft per bag at the intended R-value, an 800 sq ft area needs 20 bags. That label-based count takes precedence over a generic volume conversion. Metric entry expresses the thermal rate as US R-value per centimeter; the R numbers remain US R-values, not RSI.
Common mistakes
- Using coverage printed for a lower R-value than the planned installation.
- Subtracting the old layer again after selecting a column that already accounts for it.
- Confusing US R-value with metric RSI after changing length units.
- Applying this attic depth directly to a wall cavity or another assembly.
Sources
ENERGY STAR insulation R-values supplies the fetched attic table and zone guidance. Louisiana DNR insulation table is search-summary planning context only. The attempted energy.gov material pages returned 404 when reviewed and are not cited as verified product data.
Related calculators
- BTU Calculator — room air conditioner starting capacity
- Square Footage Calculator — combine attic floor sections
Frequently asked questions
Where do I find my climate zone?
Use the climate-zone guidance linked from the ENERGY STAR insulation page. Confirm the correct zone and attic condition before choosing a table column.
Is R per inch a guaranteed product value?
The default is a planning figure. Actual product thickness and coverage at the intended R-value should come from its label.
Why is there a separate R-value credit field?
It makes any subtraction explicit. The tool subtracts that credit from the selected column, so enter zero when the column already represents the addition you plan.
What if my credited R-value exceeds the target?
Estimated additional R-value, depth, and volume become zero. A supplied bag coverage is still shown as a separate area-based packaging calculation, not an instruction to add material.
Does metric mode convert R-value to RSI?
R numbers remain in US R-value units. Only area, depth, volume, and the thermal value per centimeter change their length basis.