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Blown-in attic insulation: how deep and how many bags?

Find the settled and installed depth of loose-fill cellulose, fiberglass or mineral wool needed to reach a target R-value, with the number of bags and the cost.

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Bags needed

80

Settled depth to reach the target
13.6
Depth to blow, including settling
16.3
Weight of insulation
1,980
Settled volume
1,132
Material cost
$1,200

Insulating the attic floor

Heat rises, and an under-insulated attic is one of the largest sources of heat loss in a house. Blowing loose-fill insulation over the attic floor is one of the most cost-effective upgrades: it fills gaps around joists and wiring better than batts, and can be done in a day. The US Department of Energy recommends R-49 to R-60 for attics in most of the country.

The calculation

  • Thermal resistance needed = target R-value − existing R-value
  • Settled depth = resistance needed × thermal conductivity of the material (R-value per inch for each product)
  • Depth to blow = settled depth × (1 + settling allowance)
  • Weight = area × settled depth × settled density
  • Bags = weight ÷ bag weight, rounded up

Default properties: cellulose about R-3.6 per inch, 1.75 lb/ft³, 20% settling allowance; loose-fill fiberglass about R-2.6 per inch, 0.56 lb/ft³, 5%; loose-fill mineral wool about R-3.2 per inch, 1.75 lb/ft³, 10%. Each product’s bag label gives its own coverage chart, which you should follow for the final count.

Example

An attic of 1,000 sq ft with no insulation, brought to R-49 with cellulose in 25 lb bags at $15:

  • Settled depth: 13.6 inches
  • Depth to blow, including settling: 16.3 inches
  • Weight: about 1,980 lb
  • Bags: 80, for about $1,200 of material

If the attic already has R-19 of old batts, the cellulose only needs to add R-30, about 8.3 inches settled, and roughly 49 bags.

Before you start

  • Seal air leaks first: around the attic hatch, plumbing stacks, wiring holes, light fixtures and chimney chases. Air leaks can carry more heat than the insulation saves.
  • Keep insulation at least 3 inches away from chimneys, flues and recessed lights that are not IC-rated.
  • Install baffles at the eaves so insulation does not block soffit vents.
  • Staple depth rulers to the joists every few hundred square feet to check the depth.
  • Wear a mask, goggles and long sleeves, and keep walkways over joists.

Many home improvement stores lend a blowing machine free with the purchase of enough bags. Utility or state rebate programs may cover part of the cost; check what is available where you live.

Frequently asked questions

How much attic insulation do I need?

The US Department of Energy recommends R-30 to R-49 in warm climates and R-49 to R-60 in colder ones for an uninsulated attic, and topping up to R-49 or more in most of the country.

Why blow more than the target depth?

Loose-fill insulation settles over time, cellulose the most. Manufacturers state an installed thickness and a minimum settled thickness; blow to the installed depth so that the settled layer still reaches the R-value.

Cellulose or fiberglass?

Cellulose is denser, has a higher R-value per inch (about R-3.5) and blocks air movement better. Loose-fill fiberglass is lighter, does not absorb moisture and settles little, but needs more depth for the same R-value.

What should be done before blowing insulation?

Seal air leaks around lights, pipes and the attic hatch, keep insulation away from chimneys and non-IC rated recessed lights, install baffles at the eaves to keep soffit vents open, and fit depth markers.

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Results are estimates provided for information purposes. They do not replace professional advice.

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