Insulation Calculator

Find out how deep your insulation needs to be to reach a target R-value — and how many bags to buy from the chart on the bag.

sq ft
R

Rough estimate: inches × 2.5 (loose fiberglass) or × 3.5 (cellulose).

sq ft

From the chart on the bag; 0 skips the bag count.

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R-values per inch are typical figures; the product label is the final word.

Your results

Insulation depth to add19.6 in
R-value to addR-49
Material R-value per inchR-2.5
Volume of insulation1,633 cu ft

Insulation depth by R-value

Jump straight to a worked answer:

R-13 thicknessR-19 thicknessR-30 thicknessR-38 thicknessR-49 thicknessR-60 thickness
Insulation Calculator diagram

How the insulation calculator works

R-value to add = Target R − existing R Inches to add = R-value to add ÷ R-value per inch of your material Bags = Area ÷ coverage per bag at that R-value (from the bag chart)

What R-value does an attic need?

ENERGY STAR publishes retrofit recommendations by climate zone. For an attic with little or no insulation, the targets are R-30 in zone 1 (the warmest parts of the country), R-49 in zones 2 and 3, and R-60 in zones 4 through 8. If the attic already has 3–4 inches, the recommended levels are R-25, R-38 and R-49 respectively.

Climate zoneUninsulated atticAttic with 3–4 in existing
1R-30R-25
2–3R-49R-38
4–8R-60R-49

R-value per inch by material

Different materials insulate differently per inch, so the same target R needs different depths:

MaterialTypical R per inchInches for R-49
Blown fiberglassR-2.519.6 in
Blown celluloseR-3.514 in
Fiberglass battsR-3.215.3 in
Mineral wool battsR-3.314.8 in
Open-cell spray foamR-3.613.6 in
Closed-cell spray foamR-6.57.5 in

Tips before you add insulation

  • Air-seal first: gaps around light fixtures, pipes and the attic hatch leak more heat than thin insulation does.
  • Keep insulation away from recessed lights not rated for contact and maintain clearance around chimneys and flues.
  • Install baffles so soffit vents stay open.
  • Blown-in insulation settles; bag charts list both installed and settled thickness — use the settled figure.

Sources

Frequently asked questions

How many inches is R-49 insulation?

About 19.6 inches of blown fiberglass (R-2.5 per inch), 14 inches of blown cellulose (R-3.5 per inch) or roughly 15.3 inches of fiberglass batts.

How many inches is R-30?

About 12 inches of blown fiberglass, 8.6 inches of cellulose or 9.4 inches of fiberglass batts.

Can I put new insulation on top of old?

Usually yes, as long as the old insulation is dry and free of mold. Unfaced batts or loose fill can go over existing material; never put a vapor retarder between layers.

Related guides

Last reviewed November 1, 2026. Results are estimates — confirm with your supplier.

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