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Insulation Calculator

Target R-value and insulation type in; depth, bags or board feet out.

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Your project

Insulation needed

Enter your numbers above.

Seal the attic floor before you blow in insulation

Gaps around light fixtures, plumbing stacks, wiring holes and the attic hatch leak warm air straight past the insulation. Sealing them with foam or caulk first often does as much for comfort and energy bills as the extra inches, and it is much harder to do once the insulation is down.

How much insulation you need

Insulation is rated by R-value, its resistance to heat flow, and R-values of layers simply add. So the calculator subtracts what you already have from your target and divides by the R-value per inch of the product: depth = (target R − existing R) ÷ R per inch.

The per-inch values are typical figures from the U.S. Department of Energy's Energy Saver guidance: about R-3.2 to 3.8 for loose-fill cellulose, R-2.2 to 2.7 for loose-fill fiberglass, R-2.9 to 4.3 for fiberglass batts, about R-3.6 for open-cell and R-6 to 7 for closed-cell spray foam. Your product's label is the final word, since it lists the settled thickness needed for each R-value.

For attics, ENERGY STAR recommends R-49 to R-60 across most of the country and R-30 to R-49 in the warmest zones. The cellulose bag count scales from a manufacturer's published coverage chart (roughly 47 bags of 25 lb per 1,000 sq ft to reach R-38), so treat it as a planning number and confirm with the chart on the bag you buy.

Where these numbers come from

The formulas and default values in this calculator are based on:

  • U.S. Department of Energy, Energy Saver insulation materials guide
  • ENERGY STAR recommended home insulation R-values by climate zone
  • National Fiber cellulose bag coverage chart

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.

Frequently asked questions

How do I know what R-value my attic has now?

Measure the depth of the existing insulation in a few places and multiply by the per-inch value for its type. Gray or tan fluffy fiber is usually cellulose; pink, yellow or white fiberglass is fiberglass. Old, compressed insulation performs below its original rating.

Can I put new insulation on top of old?

Usually yes, as long as the old insulation is dry and free of mold or pests. Lay new batts without a paper or foil facing when you add them over existing insulation, so you don't trap moisture between layers.

Why does blown insulation need to be deeper than the label R-value suggests?

Loose-fill settles over time. Manufacturers list both an installed and a settled thickness; blow in to the installed thickness so the attic still reaches your target R-value after it settles.

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