INSULATION CALCULATOR

Insulation Calculator — How Much Insulation Do I Need?

Calculate how much insulation you need for walls, attics, floors or crawl spaces. Choose from batt, blown-in, rigid foam or spray foam insulation and get a material quantity, bag or bundle count and estimated cost based on your target R-value.

Area to insulate

Insulation type

DOE recommended R-values by climate zone:

Waste allowance

Insulation needed
—sq ft

Enter area to calculate insulation needs.

Understanding R-Values

R-value measures a material's resistance to heat flow — the higher the R-value, the better the insulating performance. The US Department of Energy recommends different R-values by climate zone and location in the building:

  • Zone 1–2 (hot/humid, south FL, HI): Attic R-30, Wall R-13
  • Zone 3–4 (mixed, mid-Atlantic): Attic R-38, Wall R-13–19
  • Zone 5–6 (cold, upper Midwest, New England): Attic R-49, Wall R-20
  • Zone 7–8 (very cold, AK, northern MN): Attic R-49–60, Wall R-21

Insulation Type Comparison

  • Fiberglass batt: R-3.1 per inch. Most common, fits standard stud bays, easy DIY installation. Available in faced and unfaced.
  • Blown-in cellulose: R-3.5 per inch. Made from recycled paper. Ideal for attics and for retrofitting existing walls via small holes.
  • Blown-in fiberglass: R-2.2 per inch. Settles less than cellulose. Good for attics.
  • Rigid foam (EPS, XPS): R-3.8–5 per inch. Continuous insulation over studs, excellent for basement walls and exterior applications.
  • Spray foam (open-cell): R-3.5–3.7 per inch. Also serves as air seal. Ideal for sealing irregular cavities.
  • Spray foam (closed-cell): R-6–7 per inch. Highest R-value per inch, moisture barrier, structurally strengthens walls. Most expensive.

Where Insulation Matters Most

Heat moves through the path of least resistance. The areas that give the highest return on insulation investment are:

  • Attic floor: Up to 25% of heat loss in an under-insulated home — highest priority
  • Exterior walls: 15–20% of total heat loss
  • Basement and crawl space: 10–15% of heat loss — often overlooked
  • Windows and doors: Replace single-pane windows with double or triple-pane
  • Air sealing: Plugging air leaks with foam and caulk should be done before insulating for best results

Frequently Asked Questions

How many bags of blown-in insulation do I need for a 1,000 sq ft attic?

For R-49 with fiberglass blown-in (requires about 22 inches depth at R-2.2 per inch): approximately 30–35 bags depending on the product. Most bags specify sq ft coverage at target R-values on the packaging.

Can I add new insulation on top of old?

Yes, in most cases. In attics, you can add unfaced batts perpendicular to existing batts or blow in additional cellulose or fiberglass. Do not use faced batts as the top layer — moisture can get trapped between the two vapor retarders.

What is the difference between vapor barrier and vapor retarder?

A vapor barrier (Class I, 0.1 perms or less) is a sheet plastic like 6-mil poly used in crawl spaces and some walls. A vapor retarder (Class II, 0.1–1.0 perm) like kraft-faced batt is used on the warm-in-winter side of walls in cold climates.

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