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RED Calc Free Help Loose-Fill Insulation
Loose-Fill Insulation Tool User Guide

Loose-Fill Insulation
Red Calc Tool User Guide

What this tool can do for you

This tool will help determine the information you need for insulating an attic with loose-fill insulation. The insulation can be cellulose, fiberglass, rock wool, or any other loose-fill insulation. Four results can be calculated, including the all-important number of bags of insulation.

Installation of cellulose in attic

Calculated values

  • Settled R-value or settled depth.
  • Bags of insulation required.
  • Installed depth of insulation.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal ranges (for inputs). Read more.
  • This tool is for added insulation only; it does not account for existing insulation.
  • Wood framing is assumed for the ceiling/floor structure.

Inputs and field measurements

  • Insulation bag weight - from insulation manufacturer.
  • Settled density - from insulation manufacturer.
  • Settled R-value per thickness - from insulation manufacturer.
  • Net area - area to be covered with loose-fill insulation, from field measurements.
  • Exposed joist height - height of ceiling/floor joist exposed above existing insulation, from field measurements.
  • Settled R-value - insulating value you want to add; or
  • Settled depth - your desired total settled depth of insulation.
  • Settling percentage - from insulation manufacturer.


Loose-fill insulation is a common solution for insulating an attic. It will cover the surfaces well without voids or gaps, is inexpensive to install, and is readily available. All three of the most common loose-fill insulations are considered "environmentally positive" because they are manufactured from recycled waste.

Cellulose is the favored type of loose-fill insulation used for the DOE-funded low-income weatherization programs. This fire-retardant insulation is made from recycled newspaper and boxes and does a good job of slowing airflow, even at loose-fill densities. Additional advantages include the ability to store small amounts of water and, because it is usually manufactured locally, it is not shipped long distances. Cellulose has the disadvantages of braking down when it gets saturated with water and it is relatively heavy. Because of its weight, it is not recommended for use in mobile home ceilings or floors. Loose-fill cellulose will settle from 10 to 20 percent over time due to gravity and vibration. The settled density range for loose-fill cellulose is 1.5 to 2.0 lb/ft3 (36 to 24 kg/m3) and its settled R-value per inch is from 3.2 to 3.8 (0.222 to 0.2636 RSI per cm).

Fiberglass is lighter than cellulose and will not be permanently damaged if it is saturated with water. It may be used in the ceilings and floors of mobile homes because its light weight. Currently, about 20 to 40 percent of the fiberglass sold as insulation is recycled. This is a significantly lower percentage than cellulose, but the fiberglass industry continues to make gains. Disadvantages of fiberglass include an inability to slow airflow as well as cellulose and rock wool and a higher cost than the other two common insulations. Loose-fill fiberglass will settle at a rate of from 2 to 4 percent over time. The settled density range for loose-fill fiberglass is 0.5 to 1.0 lb/ft3 (10 to 14 kg/m3) and its settled R-value per inch is from 2.2 to 2.7 (0.1526 to 0.1873 RSI per cm).

Rock wool is similar to fiberglass in that it is spun from molten material, but from blast-furnace slag rather than glass. This process uses products that would otherwise be discarded. Of the three loose-fill insulations discussed here, it is the least common. Rock wool will settle at a rate similar to fiberglass, from 2 to 4 percent over time. The settled density for loose-fill rock wool is 1.7 lb/ft3 (27 kg/m3) and its settled R-value per inch is from 3.0 to 3.3 (0.208 to 0.229 RSI per cm).

Table of recommended insulation specifications
Source: Department of Energy. 'Loose-Fill Insulations' (May 1995)

Best practices

Loose-fill insulations should always be installed at the manufacturer's recommended densities. In the Loose-Fill Insulation tool, include this density as the "Settled density" input.

Do not "fluff" the insulation. "Fluffing" occurs when loose-fill insulation is installed below the manufacturer's minimum density. If fluffing occurs, the project requires fewer bags of insulation, which allows air to flow through the insulation more easily and leads to excessive settling. Fiberglass insulation is easier to fluff than cellulose or rock wool.

Install the insulation evenly with no voids. Ensure that all code requirements for setbacks and damming are followed.


  • Carpenter, Stephen C., “Characterization of Framing Factors for Wood–Framed Low–Rise Residential Buildings” ASHRAE Transactions (Feb. 2003): v 109, Pt 1.
    Comment: Source of framing factor percentages used in this tool.
  • Department of Energy. "Loose-Fill Insulations" (May 1995).
    Comment: Good overview of loose-fill insulation installation.

Related RED Calc Free tools

  • Parallel-Path Equivalent R-Value: Determine the overall equivalent R-value of an area made up of smaller areas with different R-values.
  • Surface Heat Transfer: Determine the heat transfer through a surface area with a given R-value and temperature difference across the surface.
  • Dense-Pack Insulation: Determine the installed density or required bags of insulation when dense packing enclosed cavities.

Related external resources

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