Skip to content

Insulation Calculator

How many inches of insulation to add to your attic to reach the recommended R-value, counting what's already there.

Attic
sq ft
ENERGY STAR's recommended total for an attic, based on the 2021 energy code. Find your zone on the ENERGY STAR or DOE climate zone map.
What's there now
What's there now
Measure in several places with a ruler and use the average.
What you'll add
What you'll add
Adding
Insulation to add 13.5 inches deep From R10 now to R60: add R50, about 13.5 in.

R-value

R-value to add
50
Existing R-value (approx.)
10
Target R-value
60

Depth and volume

Total depth when done
17.5 in
Volume to add
1,126 cu ft

How much attic insulation do I need?

ENERGY STAR recommends a total of about R60 for attics in most of the US (zones 4–8), R49 in warmer zones 2–3 and R30 in zone 1. Estimate your existing R-value (depth × about 2.5 per inch for loose fiberglass, 3.7 for cellulose), subtract it from the target, and divide by the new material's R per inch. With 4 in of loose fiberglass (R10) you need to add R50: about 13.5 in of blown cellulose.

Worked example: topping up a 1,000 sq ft attic

  1. What's there: 4 in of loose fiberglass × 2.5 = R10.
  2. Target: R60 for climate zones 4–8.
  3. To add: R60 − R10 = R50.
  4. Depth: blown cellulose gives about R3.7 per inch: 50 ÷ 3.7 = 13.5 in, for a total of 17.5 in.
  5. Volume: 1,000 sq ft × 13.5 in ÷ 12 = 1,126 cu ft. The bag label tells you how many bags cover your area at your target R-value.

Inches needed for an uninsulated attic

Climate zone (target)Blown celluloseBlown fiberglassFiberglass batts
Zone 1 (R30)8.1 in12 in9.4 in
Zones 2–3 (R49)13.2 in19.6 in15.3 in
Zones 4–8 (R60)16.2 in24 in18.8 in

Using DOE's R per inch: cellulose 3.7, loose fiberglass 2.5, fiberglass batts 3.2. Products vary; the bag or batt label gives the exact R-value and depth.

Estimating your existing R-value

The Department of Energy's rule of thumb for insulation already in an attic:

  • Loose fiberglass (light, fluffy, yellow, pink or white): depth × 2.5
  • Rock wool (dense, gray or off-white, sometimes with black specks): depth × 2.8
  • Cellulose (small gray pieces, like shredded newspaper): depth × 3.7
  • Fiberglass batts (rolls or blankets): depth × 3.2

If you see shiny, pebble-like granules, that may be vermiculite, which can contain asbestos. Don't disturb it, and get it tested before working in the attic.

Before you insulate

  • Air seal first. Seal gaps around light fixtures, pipes, wires, chimneys and the attic hatch; insulation doesn't stop air leaks.
  • Keep vents clear. Use baffles at the eaves so insulation doesn't block soffit vents.
  • Mind the clearances around chimneys, flues and recessed lights that aren't rated for contact with insulation.
  • Read the bag chart. Blown insulation bags list the coverage at each R-value and the settled depth to aim for.

Frequently asked questions

How many inches of insulation do I need in my attic?
For R60 in most of the US: about 16 in of blown cellulose or 24 in of blown fiberglass in an empty attic. In warmer zones (R49): about 13 in of cellulose or 20 in of fiberglass.
What R-value should attic insulation be?
ENERGY STAR recommends R60 for uninsulated attics in zones 4–8, R49 in zones 2–3 and R30 in zone 1, based on the 2021 International Energy Conservation Code.
What is the R-value of 6 inches of fiberglass?
About R15 for loose fiberglass (6 × 2.5) or about R19 for fiberglass batts (6 × 3.2), using DOE's rule of thumb.
Is cellulose or fiberglass better for an attic?
Both work. Cellulose gives more R-value per inch (about 3.7 vs 2.5 for loose fiberglass), so you need less depth; fiberglass is lighter and doesn't absorb water. Follow the product's instructions either way.
Can I put new insulation on top of old?
Usually yes, as long as the old insulation is dry and not moldy, and it isn't vermiculite. Don't put faced batts on top of existing insulation; use unfaced batts or blown insulation.

Guides

Related calculators

Drywall CalculatorDrywall sheets, screws, tape and joint compound for a room, with cost.Roofing CalculatorRoofing squares, shingle bundles, nails, underlayment and drip edge.Square Footage CalculatorSquare feet of any room or area, in sq ft, sq yd, m² and acres.Paint CalculatorGallons of paint for a room's walls and ceiling, with doors, windows and coats.

Sources and method

Last checked October 12, 2026. How we build and test every calculator: methodology.