Cellulose Insulation Basics: Blowing-In, Fire, Soundproofing, and More

Loose Fill Cellulose Insulation In an Attic
Loose Fill Cellulose Insulation In an Attic. Getty/slobo

When you install blown-in attic or wall insulation, chances are pretty good that it will be a product called cellulose insulation.

What is it made of?  Why install loose-fill instead of fiberglass batts?  Most importantly, does cellulose insulation present a fire hazard?

Cellulose Insulation Essentially Is Paper

While cellulose insulation has been used for many years, it really was not until the 1970s that it came into widespread use in the U.S. and other developed countries.

Strictly speaking, cellulose insulation can come from any cellular plant source (corncobs, sisal, sawdust, etc.).  But generally it is made from recycled newspapers, cardboard, office paper, and other waste paper products.

What Is Loose Fill Cellulose?

There are several different types of cellulose insulation, but the most common type that homeowners will encounter is called loose fill. 

Using an insulation blower, pellets of cellulose are blown into attics or walls (with holes drilled to permit access) and allowed to fill the cavities. 

No pressure is placed on the cellulose; it is allowed to settle over time.

Why Insulate With Paper Products When Fire Is a Hazard?

The source paper in its raw state is combustible.  However, during manufacturing, cellulose insulation is treated with borates, which are a Class I fire retardant.

Class I refers to ordinary combustibles such as wood and paper, as opposed to Class II combustibles such as flammable liquids, grease, gasoline, oil, etc.

As a demonstration of cellulose insulation's fire retarding capacity, it is possible to use a blowtorch to warp a penny on a bed of cellulose, while holding all of this on your hand.  This demonstrates two things:

  • Even though the penny is affected, the cellulose will redden, scorch, and turn black, but it will not actively catch fire.
  • Your hand, only a couple of inches below, is insulated from the heat.

R-Value Rating of Cellulose for Energy Efficiency

Loose fill cellulose has an R-value of about 3.5 per inch of thickness, compared to fiberglass' R-value between R3 to R4 per inch.

Fiberglass rolls or batts more completely fills wall cavities, if only because the walls must be opened up in order to install the batts.  So, fiberglass is considered to have an overall better R-value than cellulose insulation when it comes to real-world performance.

Loose-Fill Cellulose vs. Moisture, Insects, and Vermin

No flexible insulation performs well against moisture--whether loose-fill cellulose or fiberglass batt or roll.  Rigid or sprayed-in foam stands up best against moisture.

Fiberglass is better against moisture than loose fill cellulose because it is less prone to water absorption.  However, due to its denseness, it can retain water for a long time--long enough for mold and mildew to form.  

Cellulose more completely soaks up moisture and takes a long time—if ever—to dry out. If you have owned a down coat or sleeping bag, you know the warnings against getting this insulating material wet: moisture dramatically cuts R-value.

Regarding insects and vermin, cellulose insulation does well because of the treatment with borates.

How Green and Eco-Friendly Is it?

With cellulose, eco-friendliness is a debatable issue.

On one hand, it is green because it uses up to 85% recycled materials.  However, the remaining 15%, which is made up of the boric treatment, is the part that can be considered less-than-green because it is a chemical treatment.

Fiberglass insulation may use recycled materials, as well.  Owens-Corning, one of the biggest names in fiberglass insulation production, reports that it uses between 53% and 73% post-consumer recycled materials.

Cellulose vs. Fiberglass:  Which Type to Use?

With closed walls, you often have no other choice. Unless you are going through some remodeling where the walls are being opened up, you will need to blow in insulation.

With open walls, you will be installing fiberglass roll insulation.

 Spray-in foam is becoming a favorite method of wall insulation in homes, as well.  

For attics, the joists are often open and accessible.  However, because of obstructions like wires (and just because of its sheer ease), cellulose insulation is often blown into attics, as well.