Blown-In Insulation Calculator

Blowing insulation into an attic? Enter the area, the R-value you have and the R-value you want. You get the bags to buy, with settling counted.

Your shopping list

44Bags to buy
11.81 inDepth to blow in
38 RR-value to add
  • Depth without settling allowance10.27 in
  • Settling allowance15 %

Bags come from the label coverage, so match the label to the R-value you add. Extra of 10% is included.

How to use the Blown-In Insulation Calculator

  1. Measure the attic floor and read your existing R-value.
  2. Pick cellulose or blown fiberglass and the R-value you want.
  3. Type the bag coverage from the label and read the bags to buy.

The math behind it

Depth to add = (target R – existing R) / R-value per inch
Installed depth = depth to add x (1 + settling %)
Bags = area / coverage per bag at the added R-value x (1 + extra % / 100), rounded up

A worked example

What we typed in

  • Attic floor area: 1,500 sq ft
  • R-value you already have: 11
  • Target R-value: R-49
  • Blown-in material: Blown cellulose (about R-3.7 per in)
  • Coverage of one bag at the R-value you add: 30 sq ft
  • Extra for mistakes and cuts: 10%

What we got

  • Bags to buy: 55
  • Depth to blow in: 11.81 in
  • R-value to add: 38 R
  • Depth without settling allowance: 10.27 in
  • Settling allowance: 15 %

For a related part of your project, use the Plywood Calculator.

Tips from the job site

  • The bag label has a chart with the number of bags per 1000 sq ft for each R-value. Use it as a check.
  • Mark the depth on rulers set into the insulation as you go.

Common questions

Does blown-in insulation settle?

Yes. Cellulose settles about 15% and blown fiberglass about 5%. The bag label shows settled values, so this is a safety margin.

Can I rent a blower?

Many stores lend one free when you buy a set number of bags. Ask the store.

See all Insulation Calculators.

Last checked: (worked example and form display).

Cellulose insulation calculator: use the bag coverage chart

Select Blown cellulose and enter the manufacturer’s square feet per bag for the R-value being added. For example, 1,200 square feet with 30 square feet per bag and 10% extra needs 44 bags: 1,200 ÷ 30 × 1.10 = 44. The 30 square feet is an example input, not a universal product coverage.

The depth shown uses an approximate R-value per inch and settling allowance. For installation, follow both the minimum bag count and the installed/settled thickness on the exact product label. See Greenfiber’s attic installation guide and coverage charts.