What You Should Know About Blow-In Cellulose Insulation

When you install insulation in an existing closed wall or in an attic, it likely will be a loose-fill product called blow-in cellulose insulation. With an attic, this type of insulation is just one option along with the other popular alternatives, fiberglass batts or blown-in fiberglass. But with enclosed walls, blowing in loose-fill cellulose insulation is still by far the most practical and cost-effective method.

What Is Blow-In Cellulose Insulation?

Cellulose insulation is a type of wood- or paper-based product. It is mechanically blown into or onto empty spaces in the structural part of a house to slow down the transmission of heat or cold.

What Is Loose-Fill Cellulose Insulation?

Cellulose insulation is thick, dense, and clumpy, with a consistency much like down feathers. The chief value of this shape and size is that the insulation can fit in enclosed areas (such as walls) and can conform around obstructions such as wires and ducts (found both in walls and in attics).

Cellulose insulation technically can come from any cellular plant source, such as corncobs or sisal. But commercial cellulose insulations are generally derived from wood, and more specifically from paper: recycled newspapers, cardboard, office paper, and other common waste paper products. For this reason, cellulose insulation is considered an eco-friendly home product.

How Cellulose Insulation Is Blown Into the Home

The most common type that homeowners will encounter is called loose-fill cellulose insulation. This is slightly different from another type of cellulose insulation, which is designed to be blown onto open walls, much like spray foam. In this second type, moisture introduced into the spray helps the cellulose stick to the wall. With loose-fill insulation, however, the cellulose is dry.

To fill finished walls, holes are drilled in the plaster or drywall to permit access of the blower nozzle. For attics, cellulose insulation is blown in parallel to the joists. It can be used by itself to fill in joist cavities that have no insulation or laid as a thick layer over the top of existing batts of fiberglass insulation.

The installation process for dry cellulose insulation looks like this:

  1. Densely packed bales of cellulose are fed into the hopper of an insulation blower powered by an electric motor. Rotating teeth or prongs at the bottom of the hopper fluff up the cellulose.
  2. The cellulose is blown into the attic or walls through long, flexible tubes that run from the blower to an application nozzle.
  3. The cellulose is allowed to fill the cavities or blanket existing insulation. No pressure is placed on the cellulose; it is allowed to settle over time.
  4. Walls are patched up and painted over.

Cellulose Insulation Advantages

There are a number of advantages to using cellulose insulation over other types:

  • Loose-fill cellulose insulation can settle around and conform to most of the obstructions found in walls and attics.
  • Loose-fill cellulose is relatively inexpensive, yet still has an R-value of about 3.5 per inch of thickness, compared to fiberglass’ R-value between 2.2 to 2.7 per inch.
  • When walls are already finished, injecting loose-fill cellulose insulation is one of the few ways of adding insulation. One alternative is to pull down the drywall and use fiberglass batts.
  • Cellulose insulation stands up reasonably well against insects and vermin because it is treated with borates.

Cellulose Insulation Drawbacks

There are also a few drawbacks to cellulose insulation:

  • While settling is one of blown-in cellulose insulation’s advantages, this can also be a problem, mostly with walls. Over time, the insulation can pack down and form pockets above the settled areas. These pockets become thermal bridges, which transmit heat or cold into the house. Settling in attics is less problematic for two reasons. First, attic spaces can be overfilled to account for settling. Second, when cellulose insulation in attics settles, no empty spaces are formed.
  • When cellulose soaks up moisture in enclosed areas, it can take a long time to dry out. Moisture dramatically cuts R-value and may lead to the formation of mold and mildew. Rigid or sprayed-in foam stands up better against moisture.

Is Cellulose Insulation Considered Green?

With cellulose, eco-friendliness is a debatable issue. On the one hand, it can be considered green because it uses up to 85 percent recycled materials. However, the remaining 15 percent, which includes the borate treatment, is 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 percent post-consumer recycled materials. So the green advantage of cellulose insulation may be less significant than it is sometimes portrayed.

Cellulose Insulation vs. Other Types

With closed walls, you have few other choices but to blow in insulation. Unless your home is going through some remodeling where the walls are being opened up, holes need to be bored into the walls and insulation injected. Here, the traditional favorite is blow-in cellulose insulation, although spray-in foam is becoming steadily more common.


Spray-in foam can have an R-value of up to 7 per inch of thickness, giving older homes a better R-value (or better insulation), which helps with heat and cold retention in various seasons.

With open walls, you can install fiberglass roll insulation, although spray-applied foam insulation is also possible.

For attics, the joists are often open and accessible and thus could be insulated with either blow-in cellulose or fiberglass batts. However, because of obstructions such as wires (and just because of its sheer ease), cellulose insulation is often blown into attics, as well.

Is Blown Cellulose Insulation a Fire Hazard?

Cellulose insulation’s source paper in its raw state is combustible. However, during manufacturing, cellulose insulation is treated with borates, which are Class I fire retardants. 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 resting on a bed of cellulose insulation held in your hand. Not only does the cellulose remain unaffected even while the penny begins to melt, but the insulating value of the cellulose is such that no heat is felt by the hand holding the experiment.

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