LayoutWise

Insulation Calculator

Enter the area and the R-value you have and want. You get the depth and bags of blown-in insulation for an attic, or the batts for walls and floors.

Your space

1. Area & type
2. R-value

Bags

48

25 lb cellulose

Add

R-30

R-19 → R-49

Depth

8.6"

settled, on top

Volume

714 cu ft

of insulation

Cross-section

Attic floor
+8.6" blown cellulose over R-19

Shopping list

  • ✓Blown cellulose: 48 bags (25 lb) to add R-30 — 8.6" settled depth
  • ✓Blowing machine (often free to borrow with a bag purchase), attic rulers every 300 sq ft
  • ✓Baffles at the eaves to keep soffit vents open, and foam or caulk to air-seal first

ENERGY STAR recommends R-49 to R-60 in most U.S. attics; check the climate-zone map for yours. Bag counts use typical settled densities — the coverage chart on the bag is the final word.

How much attic insulation do I need?

R-values add up. Subtract what's already there from your target, then divide by the material's R-value per inch to get the depth to add. ENERGY STAR recommends R-49 to R-60 in most U.S. attics (R-30 to R-49 in the warmest zones).

Example: a 1,000 sq ft attic with about R-19 (6–7 inches of old fiberglass), topped up to R-49 — that's R-30 more. Blown cellulose at about R-3.5 per inch needs 8.6 inches; blown fiberglass at about R-2.5 per inch needs 12 inches.

Measure what you have

Push a ruler down into the insulation in a few spots. Old fiberglass batts run about R-3 per inch, loose fiberglass R-2.5, cellulose R-3.5. Air-seal gaps around lights, pipes and the attic hatch before adding more — it often saves more energy than the insulation itself.

Batts for walls and floors

FramingCommon batt
2x4 wallsR-13 or R-15
2x6 wallsR-19 or R-21
Floors over crawlspacesR-19 to R-30
Attics, cathedral ceilingsR-30 to R-38

Don't squash a thicker batt into a thinner cavity — an R-19 batt squeezed into a 2x4 wall only performs at about R-13.

Sources