How many bags of insulation do I need?

Blown-in insulation is sold in bags labelled with coverage at a given R-value, and the label is the whole calculation: a bag of loose-fill fibreglass covers about 57 sq ft at R-19 and about 29 sq ft at R-38, because reaching a higher R-value means blowing a thicker layer. Enter the area and the target R-value and this returns the bags and the depth you are aiming for.

Area to insulate (sq ft)

Attic floor area, excluding joists you are not covering.

Target R-value R-13 (about 3.5 in)R-19 (about 5 in)R-30 (about 9 in)R-38 (about 11 in)

Extra allowance (%)

Covers joist bays, obstructions and settling.

Bags needed

19bags

Coverage per bag 57.0 sq ftInstalled depth 5.0 in

Typical fibreglass coverage per bag at the listed R-values. Cellulose, mineral wool and manufacturer-specific products differ; use the coverage printed on the bag for the final order.

The depth matters as much as the bag count, because the only way to verify the job afterwards is to measure how deep the insulation sits. Mark the target depth on the joists before you start — a piece of tape at 11 inches is a better instruction to anyone helping than any number of bags.

Attic insulation is the highest-return work most houses need, and the usual target in a cold climate is R-49 to R-60 in the attic floor. Two things get in the way: obstructions such as chimneys, flues and recessed lights, which need a dam and a clearance, and baffles at the eaves, which must stay open so the roof can vent. Neither is counted here, and both reduce the effective area you can cover.

How the calculation works

Bags = ceil(area x (1 + allowance / 100) / coverage per bag), where coverage per bag is the manufacturer's published figure at the chosen R-value. Installed depth is the depth the manufacturer specifies for that R-value.

The R-value is a property of the installed thickness, not of the bag: the same bag of loose fill gives R-19 at 5 in and R-30 at 9 in, which is why coverage falls as the target rises.

• Coverage figures used are typical published values for loose-fill fibreglass: 88.5 sq ft at R-13, 57 at R-19, 37.9 at R-30, 29.3 at R-38 per bag.
• The area is flat and free of obstructions; deduct for skylights, chimney chases and anything that must stay clear.
• Blowing is done to the marked depth and raked level, which is what makes the coverage figure achievable.
• Cellulose has different coverage per bag — roughly 40 sq ft per bag at R-19 — so use its own label if you choose cellulose.

Sources: Typical loose-fill fibreglass coverage per bag: 88.5 sq ft at R-13, 57 at R-19, 37.9 at R-30, 29.3 at R-38; US DOE recommended attic R-values by climate zone

Worked examples

Questions

How many bags of insulation do I need for 1,000 sq ft?

18 at R-19, 27 at R-30 and 35 at R-38 with the coverage figures used here. Always confirm against the bag you buy.

How deep should attic insulation be?

About 5 in for R-19, 9 in for R-30, 11 in for R-38 and 14-16 in for R-49 to R-60. Depth is the only reliable check after installation.

What R-value should an attic have?

R-49 to R-60 for most cold-climate attics and R-38 to R-49 in milder zones. The US DOE publishes a zone map and it is worth checking your zone rather than guessing.

Can I blow insulation over existing insulation?

Usually yes, if the existing material is dry and not compressed. Do not cover knob-and-tube wiring, and keep a 3 in clearance around recessed light fixtures unless they are rated for contact.

Does cellulose use the same bag count?

No. Cellulose is denser and covers less per bag — roughly 40 sq ft at R-19 rather than 57 — so the label on the bag is the figure that matters.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.