Attic Insulation Calculator
By Stephen Morrison, Editor Last reviewed
Pick your climate zone and tell us what's in the attic now. You get the R-value to add, from ENERGY STAR's recommendations, and how many bags of blown-in insulation or packages of batts that takes, read from your product's own label.
Quick answer: in climate zone 5, ENERGY STAR recommends R-60 for an uninsulated attic, or adding R-49 if you already have 3–4 in. With Owens Corning AttiCat, its chart gives 19 bags per 1,000 sq ft for R-38; use your bag's chart for the R-value you need.
Recommended attic R-values by climate zone
| Zone | Uninsulated attic | Add to 3–4 in already there | Floor over unheated space |
|---|---|---|---|
| Zone 1 | R-30 | R-25 | R-13 |
| Zone 2 | R-49 | R-38 | R-13 |
| Zone 3 | R-49 | R-38 | R-19 |
| Zone 4A or 4B | R-60 | R-49 | R-19 |
| Zone 4C | R-60 | R-49 | R-30 |
| Zone 5 | R-60 | R-49 | R-30 |
| Zone 6 | R-60 | R-49 | R-30 |
| Zone 7 | R-60 | R-49 | R-38 |
| Zone 8 | R-60 | R-49 | R-38 |
Source: ENERGY STAR, based on the 2021 International Energy Conservation Code. Local code may require more for new work.
How the calculator works
Old insulation by depth: depth × 2.5 per inch (low end, so we don't overcount it)
Bags: area ÷ 1,000 × bags per 1,000 sq ft (from the label)
Batts: whole layers of your batt's R-value; packages = area × layers × (1 + extra%) ÷ coverage
We read coverage from the product label instead of a formula, because loose fill's coverage depends on the product and how it's blown. The label's chart is what the manufacturer stands behind.
R-value per inch, by material
| Material | R per inch | Notes |
|---|---|---|
| Fiberglass batt or board | 2.5–4 | Not an air barrier; vapor control depends on the facing. |
| Mineral wool rigid board | 3–4 | PNNL lists the rigid board; batts weren’t in the table. |
| Expanded polystyrene (EPS) board | 3.8–4.4 | Typically a Class III vapor retarder. |
| Extruded polystyrene (XPS) board | 5 | Typically a Class II vapor retarder. |
| Polyisocyanurate (polyiso) board | 6–6.5 | PNNL: about 6; DOE’s guide: 6.5 for 1 in. Can lose some R-value as it ages, mostly in the first two years (DOE). |
| Closed-cell spray foam | 6–6.5 | PNNL: about 6 (high-density); DOE’s guide: 6.5 for 1 in. |
| Open-cell spray foam | 3.6 | DOE’s guide lists R-12.6 for 3.5 in (about 3.6 per inch). DOE says not to use it below ground level. |
Sources: PNNL's Building America Solution Center and DOE's Guide to Home Insulation. DOE's live Energy Saver insulation pages returned “not found” when we checked on Oct 30, 2026, so we also cite an archived copy.
Example: blown-in depth by R-value
Owens Corning's AttiCat chart gives these minimum thicknesses. Settling, it says, “is negligible, with no impact on R-value.”
| R-value | Minimum thickness |
|---|---|
| R-30 | 10.75 in |
| R-38 | 13.5 in |
| R-49 | 17 in |
| R-60 | 20.5 in |
Do these first
- Air seal the attic floor: gaps around pipes, wires, flues and the attic hatch. ENERGY STAR's savings estimate is for sealing and insulating together.
- Fix roof leaks and moisture before covering anything. Planning a new roof? Do it first: see the roofing calculator.
- Follow the product's instructions for clearances, protective gear and ventilation.
Doing it yourself or hiring? Run the numbers with the DIY vs hire a pro calculator, and put the work in your project plan. Terms like R-value and vapor retarder are in the glossary.
Common questions
How much insulation do I need in my attic?
It depends on your climate zone. ENERGY STAR, using the 2021 International Energy Conservation Code, recommends R-30 for an uninsulated attic in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8. If you already have 3–4 in, it recommends adding R-25 (zone 1), R-38 (zones 2–3) or R-49 (zones 4–8).
How many bags of blown-in insulation do I need?
Read it off the bag. Every loose-fill product has a coverage chart listing bags per 1,000 sq ft for each R-value. For example, Owens Corning's AttiCat chart gives 19 bags per 1,000 sq ft for R-38, so a 1,000 sq ft attic takes 19 bags. Enter your product's figure and your attic area above.
How deep should blown-in insulation be?
At least the minimum thickness on the bag chart for your R-value. For AttiCat, that is 10.75 in for R-30, 13.5 in for R-38, 17 in for R-49 and 20.5 in for R-60. Other products differ, so use your own bag’s chart.
Should I air seal before adding insulation?
Yes. ENERGY STAR estimates homeowners save an average of 15% on heating and cooling costs (11% of total energy costs) by air sealing and adding insulation. It also notes that, combined with attic insulation, air sealing can help reduce ice dams.
What is my climate zone?
The US is divided into zones 1 (hottest) to 8 (coldest) by county. ENERGY STAR’s insulation page has the map; your local building department can also tell you.
Sources
- ENERGY STAR, Recommended home insulation R-values (based on the 2021 IECC, Table R402.1.3)
- ENERGY STAR, Methodology for estimated savings from sealing and insulating
- ENERGY STAR, Attic air sealing project
- PNNL Building America Solution Center, Typical R-values and vapor retarder classes of common insulation materials
- U.S. Department of Energy, Guide to Home Insulation (PDF)
- U.S. Department of Energy, Types of insulation (archived copy; the live page returned 404 on 2026-10-30)
- Owens Corning, AttiCat expanding blown-in insulation coverage chart (PDF)
Formulas and figures last checked against these sources on. Spotted an error? Tell us and we'll fix it.
More calculators
Roofing Calculator
Roof area, squares and shingle bundles from house size and pitch, with starter, ridge cap, underlayment and drip edge.
Concrete & masonryConcrete Calculator
Cubic yards and 40–80 lb bag counts for slabs, footings and post holes.
Walls & paintPaint Calculator
Gallons and quarts for a room, minus doors and windows, with the ceiling optional.
Walls & paintDrywall Calculator
Sheets for walls and ceiling, plus joint compound and tape, from your room size.