Blow-In Insulation: The Basics

Closeup of blown-in insulation

The Spruce / Jason Donnelly

When you think about adding insulation to your home, the image that usually comes to mind is long strips of fiberglass insulation in faced batts or unfaced rolls. Batts of faced fiberglass insulation is the standard material used to insulate wall and joist cavities during new construction, and improving the insulation in attics often involves laying a "blanket" of unfaced fiberglass rolls across the attic floor.

But fiberglass batts and rolls have limitations for retrofit applications, and homeowners are wise to consider blow-in insulation as an alternative for improving the home's insulation R-value. R-value is the industry standard for measuring the resistance value of specific insulating materials; the higher the R-value, the better the material insulates.


The term blow-in insulation (or loose-fill insulation) refers to the process of filling stud or joist cavities or covering attic floors, with any loose material that has a good insulating R-value. While there are a variety of materials that can be used, including styrofoam pellets or loose fiberglass fibers, the most common material used for blow-in applications is cellulose material.

Made from recycled newspaper, cardboard, and other wood-based materials, blow-in cellulose is treated with boric acid and other substances that make it flame resistant and mold-resistant. It is then packaged in tightly packed bales or bags. Installation involves the use of a mechanical blower/hopper that churns up the cellulose material with paddles to loosen it and mix it with air, then blows the fluffy insulation through hoses to wherever it is required. These are very expensive machines but they are available for rent, and some home improvement centers will loan out the machines for free to homeowners buying large quantities of insulation. More often, though, this installation is performed by a contractor specializing in the process.

Blow-in insulation is widely considered the best means of adding insulation in existing construction, both for attic spaces and in wall cavities.


Adding insulation in an attic can be done by hauling unfaced rolls of fiberglass insulation into an attic and unrolling them to form an uninterrupted blanket across the attic floor, but this can be an exhausting process, especially if you only have a small access hatch. Blow-in insulation offers a much faster way to significantly improve your attic insulation. In a matter of an hour or two, an installation specialist can blow a thick blanket of loose insulation across the floor of the attic using a single hose run up through the attic hatch. For wall cavities, blow-in insulation is the only practical way to improve the R-value of stud cavities, short of removing entire wall surfaces to install faced fiberglass batts.

Blow-in cellulose insulation is a favorite among homeowners who prefer green products, since the material is made entirely from recycled paper and wood products, with few synthetic processes or chemicals involved.

How It Is Done for Attics

When attics are insulated with blow-in insulation, the installation crew typically consists of two individuals. One worker moves around the attic with the blower hose, spraying fluffed-up insulation across the floor and into cavities; the insulation easily settles around any attic obstructions. The other worker operates the blower unit from an interior room or outside the house, feeding bags or bales of cellulose into the hopper and controlling the air mixture to keep the hoses blowing freely. Together, the workers will apply a layer of insulation to a thickness that achieves the desired R-value. Where a fiberglass blanket already exists in the attic, the additional blow-in insulation is usually spread right on top of the fiberglass.

How It Is Done for Walls

With existing, closed-up walls, blow-in insulation is the highly preferred way to go—unless you want to rip off exterior siding or interior drywall to access wall cavities. A major remodeling project offers a good time to insulate with fiberglass batts, but otherwise, blow-in insulation offers a far easier method of improving the R-value of walls.

The technicians typically drill two holes in each wall cavity—one about 12 inches from the ceiling and the other about 3 feet from the floor. Usually, this is done on the exterior face of the walls, though it can also be done from inside the home through drywall or plaster. Using an insulation blower, the technicians force cellulose or another loose-fill insulation material into each wall cavity. The holes in the wall cavity are then filled with plastic plugs, which can be painted to match the exterior walls. One disadvantage of this process is that these plugs are sometimes hard to conceal, especially on stucco or brick walls.

Two component expanding closed cell foam insulation is sometimes the best option for walls in older homes with 2x4 exterior walls. Holes are drilled in a similar way as cellulose for installation. It can act as a vapor barrier and have an R value of up to 7 per inch. It does not settle like cellulose or fiberglass.

What Is an Insulation Blower?

An insulation blower is a machine that forces insulation into wall cavities, attics, and other spaces. It consists of a hopper to hold the loose insulation, an engine, a blower, and a hose.

Blow-in insulation is not a perfect solution for walls. Especially in older homes, wall cavities may have obstructions, such as electrical conduit, fire blocks, and plaster "keys" that prevent insulation from filling the cavities. Good technicians will have techniques for getting around these obstructions. And no matter how well it is installed, all blow-in insulation will settle somewhat over time, which reduces the insulation value.

Comparison With Fiberglass Batts

Blow-in insulation is quite comparable in cost and R-value when compared to fiberglass batts or blankets.

  • DIY-friendly. Blow-in insulation is difficult for DIYers, especially for wall cavities. Attic insulation can be a DIY project with a rented or borrowed blower but is still a challenge. By contrast, installing fiberglass batts or rolls, while labor-intensive, is not hard for DIYers.
  • Material cost. The insulation itself costs about the same for comparable R-values. Blow-in cellulose costs about $30 for each 19-pound bag, which will cover about 40 square feet at minimal R-value. Expect to pay $600 to $1200 for materials for insulating an attic floor or the walls on a small, 1,000 square foot house.
  • Installation cost. Because it is usually done by a professional crew, blow-in insulation is usually somewhat more costly. Expect to pay $40 to $70 per hour for professional installation. An attic usually takes four to five hours, total, including set-up and break-down.
  • R-value. Blow-in cellulose offers an R-value of about 3.2 to 3.8 per inch. Fiberglass batts offer about 3.7 per inch.


When blown-in attic cellulose gets wet, it takes a long time to dry out—if it ever completely dries. A roof leak or ice dam, for example, can saturate cellulose insulation in an attic or wall cavity. While the insulation is treated to hinder mold, the additives are not foolproof. After cellulose gets moldy, it is a long, arduous project to scoop it out into plastic contractor's bags and haul it down, bag by bag.

And although cellulose blown-in insulation is relatively fireproof, it may be subject to smoldering when subjected to high heat or fire. This can be a concern in attics, especially around recessed light fixture canisters. Before blowing in attic insulation, it is wise to look for recommendations from manufacturers of recessed light fixtures on how to deal with heat issues.