Adding Insulation to Existing Walls

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How to determine which type of insulating material to use for your wall insulation retrofit fit project.

Home improvement shows, media, and government agencies have long cited the benefit of adding additional insulation to attics, ceilings, crawlspaces, and basements since they are relatively easy to access and the project can be accomplished by willing home owners. However walls are also responsible for heat loss and gain in a typical home. Due to the large surface area, walls can be more difficult to insulate, but wall insulation can also save homeowners a great deal of money in utility bills.

While many home centers and rental centers now carry insulation blowers for do it yourselfers primarily for insulating attics, but adding insulation to an existing wall usually requires professional installation due to the wall construction, electrical wiring, and the presence of existing insulation. This insulation process is often referred to as “drill and fill.”


Basic Retrofit Insulation Process

1. Technicians drill several holes in the exterior or interior wall surfaces about 12" from the ceiling, another hole about 3 feet from the floor. Holes must be drilled in each stud cavity.


2. The technician then uses an insulation blower, to install cellulose or loose fill fiberglass insulation into your walls.


3. The holes in the walls are plugged, either with plastic plugs or with drywall compound.

Comparison of Blown In Insulation Materials

The two most popular materials for drill and fill applications are cellulose and loose-fill fiberglass. Each material has its advantages and your insulation contractor should go over them with you before beginning the project. Key characteristics you should consider include thermal resistance, air resistance, ease of installation, cost, settling, indoor air quality, and recycled content.

Loose-fill Fiberglass


Loose-fill Cellulose

Thermal Resistance

If your greatest concern is thermal resistance, commonly called R-value, then fiberglass can provide a higher R-value with less material than cellulose. Fiberglass can also provide up to 30 percent better air resistance than cellulose when installed correctly.


The tools and techniques are similar regardless of which material is used, but the materials affect the installation process.

Cellulose fibers are produced by grinding up newspapers and other paper products combined with fire retardant chemicals. Cellulose fibers are more course and irregular compared to fiberglass, making fiberglass easier to install. The smaller fibers of fiberglass means that the blowers can use lower air pressure which reduces the chance of equipment failure and hose clogging. Fiberglass insulation can also be blown in into smaller holes, such as the mortar joints between brick veneer walls.

Density and Cost

Installation costs will vary depending on the amount of material used. For dense packing applications where loose-fill insulation is blown under high pressure to an enclosed cavity the recommended dense-pack density for cellulose is 3.7 lbs per cubic foot while it is 2.2 lbs. per cubic foot. 


Another important factor to consider for drill and fill insulation is settling. Cellulose is more prone to settling due to the weight of the material, the irregular size, and the potential for moisture absorption.  Loose-fill fiberglass insulation is less likely to settle inside a wall cavity when installed at densities of 1.5 lbs. per cubic foot or higher. Fiberglass fibers are more flexible and tend to spring back into their original shape after the air pressure is removed after installation.

Recycled Content

If you are concerned about recycled content you have several factors to consider. Cellulose is about 80 percent recycled newspaper and 20 percent fire retardant materials. Fiberglass insulation is the largest secondary market for recycled glass containers, or about 2.2 billion pounds of recycled post-consumer glass.

Indoor Air Quality

Since the average person spends around 90 percent of their time indoors, indoor air quality is an important consideration for building materials.  Homeowners should look for products that improve indoor air quality by reducing formaldehyde levels in the home. Cellulose typically has little to no negative effect on IAQ since it has no volatile organic compounds (VOC) but the dust from the cellulose and the newspaper ink can aggravate allergies in some individuals. 

Drill and fill insulation retrofits can boost energy savings in the summer and winter and new products and techniques are providing homeowners with more options to save money.


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