What is Cellulose Insulation?

by Erik North on May 10, 2011

what is cellulose insulation


What is cellulose insulation? Is cellulose insulation really the oldest form of insulation? What kinds of cellulose insulation are used in modern homes and buildings?


Cellulose is the cell structure of any plant fiber. Cellulose insulation is insulation made from some type of plant fibers. Pretty straightforward, no? For something so simple, cellulose has a long varied history with as many drawbacks as the obvious advantages.

Historically the kinds of plant fibers pressed into service as insulation have included straw, hay bales, newspapers, sawdust or cotton. Today cellulose insulation has been standardized as the gray fluffy stuff pictured above.

Cellulose is typically used as wall cavity or attic cavity insulation. There are several different approaches to application (dry vs. wet, loose vs. dense) which helps increase heat retention and usually dampens noise levels as well.

Modern cellulose insulation is made from either ground up paper (usually recycled) or denim (also usually recycled). It is heavily treated (around 15% by volume) with boric acid, borax or ammonium sulfate. These chemicals are not hazardous for humans but are very effective flame retardants and help reduce issues with pests.


Cellulose has a very long history of helping homes better retain heat. This has included straw, hay bales, newspapers, sawdust or cotton…really anything at hand with a reasonable chance of keeping the building warmer was stuffed into the walls. This was also not so coincidentally the great era of mice nests.

Modern cellulose use (meaning ground paper with added fire retardants) grew in the second half of the 20th century, massively accelerating during the 70s oil shocks. It gained popularity for its low cost and facility of use for retrofits.

However bad publicity spread for several reasons (poor quality product related to lack of regulation, boric acid shortages, possibly apocryphal stories about insulation based fires). Fiberglass nimbly stepped into cellulose’s place as the residential standard for insulation in the US. Over the last decade, cellulose use has risen though it occupies a much smaller market share than fiberglass.

Current Uses

Cellulose is used extensively in modern construction and weatherization (OK, you probably knew that). Cellulose can be blown as loose fill insulation in attic flat cavities, dense packed into walls and floors during energy retrofits or wet sprayed for new construction.

Loose fill cellulose insulation is primarily used in attic flat cavities. During early 70s retrofits, loose fill cellulose was used for filling empty wall cavities. Cellulose blower machine limitations (and frankly limitations in understanding wall cavity air movement) constrained our retrofit options. These loose fill retrofits compress and settle, leaving wide gaps in the wall cavities.

Dense pack cellulose is the preferred modern technique for adding cellulose as retrofit insulation. Dense packing wall cavities adds thermal insulation, radically reduces trans-shell air flows and provides some level of sound proofing.

Wet spray cellulose has water added during application. It air seals the wall cavity, provides added structural strength with the same thermal and sound retardant properties as dense packing. Wet spray cellulose is almost always applied during new construction prior to adding finished drywall.

Advantages and Disadvantages of Cellulose Insulation

Cellulose occupies a particular low end niche in the spectrum of insulations. The thermal resistance of cellulose is comparable to fiberglass but unlike fiberglass, cellulose impedes air flow (and air transported heat loss).

When blown into the stud cavities, cellulose gets everywhere (trust me…everywhere). It flows around wires, pipes and electrical fixtures, eliminating air pockets and restricting air transported heat loss.

Cellulose is very inexpensive, being made from shredded paper and low cost chemicals. Those chemicals (the boric acid, borax or aluminum sulfate mentioned earlier) provide superb resistance to mold, pests and fire.

Most of the volume (approx 80%-85%) in cellulose is recycled newspaper. Cellulose has more recycled material than any other commercially available insulation. Finally, cellulose doesn’t use any greenhouse gases as propellants like spray foam formulations.

Enough with the advantages…disadvantages:

Like I mentioned above, when retrofitting densepacked cellulose into wall cavities it will get everywhere. Densepacked cellulose will spill into the house through any wall penetrations. Expect to be cleaning the dust and cellulose particles for a good while. Homes with furnace duct systems can expect some of this dust to be recirculated through the house.

Cellulose weighs several times that of fiberglass or rock wool. This usually isn’t an issue unless insulating at the attic slope (applied directly to the roof). One would need to account for the added insulation weight in calculating roof weight bearing loads.

Installation of loose fill, dense pack and wet spray cellulose also require experience, specialized equipment and training. Fiberglass can be installed by any weekend warrior. The installers should be experienced with each cellulose form.

Ultimately cellulose a great (and very green) tool for energy auditors and weatherization retrofits.

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