Blown-In Cellulose Insulation: Basics, Installation, and Pros and Cons

Loose Fill Cellulose Insulation In an Attic
Loose fill cellulose insulation can settle around the many obstructions found in attics and walls. slobo/Getty Images

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

What Is Loose Fill Blown Cellulose Insulation?

Cellulose insulation is a type of wood- or paper-based product. It insulates empty spaces in the structural part of a house to slow down the transmission of heat or cold. 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 generally, it is 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 type of cellulose is contrasted by a different type of cellulose insulation that is blown onto open walls, much like spray foam. Moisture introduced into the spray helps the cellulose stick to the wall. With loose fill insulation, though, the cellulose is dry.

  1. Densely packed bales of cellulose are fed into an insulation blower hopper. Teeth 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 the house.
  3. To fill walls, holes are drilled to permit access. For attics, cellulose insulation is blown parallel to the joists. It can be used on its own or blown on top of existing fiberglass batt insulation.
  4. The cellulose is allowed to fill the cavities. No pressure is placed on the cellulose; it is allowed to settle over time.
  5. Walls are patched up and painted over.

Cellulose Insulation Advantages

  • Loose-fill cellulose insulation can settle around and conform to most of the obstructions found in walls and attics.
  • 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.
  • When walls are left closed, 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 rolls. 
  • Cellulose insulation stands up reasonably well against insects and vermin because it is treated with borates.

Cellulose Insulation Problems

  • 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 of a problem for a couple of 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 best against moisture.

Is Cellulose Insulation Considered Green?

With cellulose, eco-friendliness is a debatable issue. On one hand, it is green because it uses up to 85 percent recycled materials. However, the remaining 15 percent, 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 percent post-consumer recycled materials.

Cellulose vs. Fiberglass and 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.

With open walls, you can install 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.

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 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 insulation, while holding all of this in your hand. This demonstrates that, even though the penny is affected, the cellulose will not catch fire. It also demonstrates the insulating properties of cellulose insulation, as the hand feels little of the heat produced by the blowtorch.