How much attic insulation do I need?
Short answer: inches to add = (ENERGY STAR target R − current R) ÷ R per inch of the new material. In zone 5 the target is R-60. If you have 3.5 inches of fiberglass batts (about R-9.5), you need about R-50.5 more. That is roughly 13.7 inches of blown cellulose.
1. Your attic
Find your zone on the ENERGY STAR map linked below the calculator. Measure insulation depth with a ruler in a few spots and use a typical reading.
R per inch defaults: fiberglass batts 2.7 (midpoint of the 2.2 to 3.2 range), blown fiberglass 2.2, blown cellulose 3.7, mineral wool 3.1 (from a 2005 Montana State University Extension table; values vary by product). If your bag or batt label lists a different value, enter it. Enter 0 inches if the attic has none.
2. What you need
- You have now
- R-9.5
- ENERGY STAR target
- R-60
- R-value to add
- R-50.6
- Depth to add
- 13.7 in
3.5 in × 2.7 per inch
Total for Zone 4C, 5 or 6. With 3 to 4 in already there, add R-49.
Target minus what you have
blown (loose-fill) cellulose, about 1,139 cu ft over 1,000 sq ft
Your current depth is in ENERGY STAR's 3 to 4 inch column. Its table says to add R-49 in this zone. This calculator shows R-50.6 because it uses your measured depth.
Seal air leaks before you add insulation. Step only on joists or truss chords, and keep insulation 3 inches from recessed lights that are not IC-rated.
How this is calculated
- Current R = depth in inches × R per inch of the existing material.
- Target = ENERGY STAR's recommended level for an uninsulated attic in your zone (R-60).
- R to add = target − current R (never below zero).
- Inches to add = R to add ÷ R per inch of the new material (3.7).
- Volume = attic area × inches to add ÷ 12, in cubic feet. Total depth after adding is about 17.2 in.
- Loose-fill R-value depends on settled density. We do not estimate bags. Use the coverage chart on the bag for your target R-value.
ENERGY STAR attic targets by climate zone
These are ENERGY STAR's recommended levels for retrofitting existing wood-framed homes. They are based on the 2021 International Energy Conservation Code. Find your zone on theENERGY STAR climate zone map.
| Zone | Attic, uninsulated | Attic, 3 to 4 in already there (add) |
|---|---|---|
| 1 | R-30 | R-25 |
| 2 | R-49 | R-38 |
| 3 | R-49 | R-38 |
| 4A and 4B | R-60 | R-49 |
| 4C, 5 and 6 | R-60 | R-49 |
| 7 and 8 | R-60 | R-49 |
R-value per inch of common attic insulation
| Material | R per inch | Inches for R-60 from scratch |
|---|---|---|
| Fiberglass batts or rolls | 2.2 to 3.2 (we use 2.7) | about 22 |
| Blown fiberglass | 2.2 | about 27 |
| Blown cellulose | 3.7 | about 16 |
| Mineral (rock) wool | 3.1 | about 19 |
Values come from a 2005 Montana State University Extension table, the most recent public per-inch list we found. R per inch varies by product, so check the label of the one you buy. Loose-fill R-value depends on how densely it is installed. The bag's coverage chart lists the depth and bag count for each R-value. Use it to buy material.
Seal air leaks first
ENERGY STAR's DIY guide has you air-seal the attic before you check and add insulation. Insulation slows heat flow, but it does not stop air from leaking through gaps. See how to find and seal air leaks.
Insulation and ice dams
In cold climates, heat escaping into the attic melts roof snow, and it refreezes at the roof edge as an ice dam. Proper insulation and air sealing keep the attic cold in winter. A history of ice dams can point to serious air leaks. Readhow to prevent ice dams.
Safety in the attic
- Walk on joists or truss chords, never on exposed drywall or insulation.
- Wear a particulate respirator or double-strap dust mask, gloves, a hat and disposable coveralls.
- Keep insulation at least 3 inches from recessed "can" lights unless they are rated IC (Insulated Ceiling).
- Codes usually require 1 inch of clearance from metal flues and 2 inches from masonry chimneys. Check your local code.
- Seal around flues and chimneys with high-temperature caulk, not spray foam.
Common questions
How do I know if I need more attic insulation?
ENERGY STAR says to add more if your insulation is level with or below the tops of the floor joists. If you can easily see the joists, you likely need more.
Can I put new insulation on top of old insulation?
ENERGY STAR's table includes a column for attics that already have 3 to 4 inches, with an amount to add on top. That assumes the existing layer stays in place.
Why does ENERGY STAR list two numbers for my zone?
The first is the total for an attic with no insulation. The second is how much to add if 3 to 4 inches is already there. The calculator uses your measured depth instead of the 3 to 4 inch assumption.
How many bags of insulation do I need?
It depends on the product. Divide your attic area by the coverage per bag shown on that product's chart for your target R-value. We do not estimate bags because coverage differs by brand.