What Is Cellulose Insulation?

Cellulose insulation is one of the most efficient and eco-friendly insulation products. The main ingredient of is recycled paper products, mostly made from trees. Some manufacturers also make cellulose insulation from cardboard, cotton, straw, sawdust, hemp, and corncobs.

Cellulose Insulation

How Cellulose is Made

To make cellulose insulation, manufacturers feed recycled paper products into a hammermill which produces small chips. They treat the resulting cotton-like product with a fire retardant, such as boric acid, which also repels insects. 

Cellulose contains the highest percentage of recycled material of any insulation. It also has less embodied energy–the total energy required to produce a product–than most other insulation products.

Types of Cellulose

There are four main types of cellulose insulation. The product is the same–but they use different additives to achieve the desired results for different areas of the house.

Loose Fill Dry Cellulose Insulation

Loose-fill cellulose is common for walls and attics. It has an R-value of up to R-3.8 per inch, regardless of location. Contractors use it in most attics with flat ceilings or ceilings with a pitch of less than 3/12. Any more slope and the cellulose slumps down –leaving little or no insulation towards the ceiling peak.

Stabilized cellulose is available for vaulted ceilings between 3/12 pitch and 5/12 roof pitch. Any slope over 5/12 requires blanket and/or batt insulation.

Loose-fill cellulose is applicable as wall insulation in new construction and renovations. New construction installations use netting to hold the product in place during installation and before drywalling.

Contractors can also use loose fill when the drywall is already installed by removing drywall plugs at the top of each stud cavity and blowing in cellulose. It’s possible for the cellulose to settle–leaving uninsulated spots.

For renovation applications, professionally installed dense-pack cellulose will solve the slumping problem. Dense pack is installed by applying pressure to each cavity as the cellulose is added. 

Wet Spray Cellulose

Wet spray cellulose is ideal for open-stud new construction since it eliminates settling problems. Wet cellulose–sometimes with an added adhesive–will stick to the exterior sheathing and studs and fill all holes and cavities. It doesn’t need netting to hold it in place. Once the cellulose is dry–approximately 24 hours–contractors can cover it with drywall.

Wet spray cellulose is often used on ceilings for insulation and soundproofing. It works the same way as it works on walls – it won’t fall off wet or dry, and you can cover it in 24 hours.

Stabilized Cellulose Insulation

Stabilized cellulose is an attic loose-fill insulation. You can use it in attics with sloped roofs by adding a small amount of water to activate an adhesive, preventing it from sliding. Stabilized cellulose is approved for use on slopes of up to 5/12 (41.66%).

Stabilized cellulose also reduces product settling–meaning you need less insulation. Less insulation also reduces the weight on the ceiling.

Low Dust Cellulose Insulation

Cellulose is a dusty product during installation – a problem for people who may be sensitive to newspaper dust or dust in general. Low dust cellulose contains a small amount of oil or dust dampener to reduce dust during installation. Once settled, cellulose does not produce any more dust.

Advantages of Cellulose Insulation

The R-value of cellulose is R-3.6 – R-3.8., which is better than fiberglass insulation–blown in and batts, rock wool insulation, and most other lower-cost insulations. It’s not as good as most rigid board insulation.

R-Value and Thermal Performance

Having an R-value of R-3.8 is only the beginning of the cellulose advantage. Good thermal performance requires a well-sealed building envelope. Sealing includes such things as air infiltration, airflows, and thermal bridging.

Cellulose is good at filling around wall and ceiling protrusions –reducing air pockets, and producing a more complete insulation blanket. Dense-pack cellulose helps prevent air infiltration and limits heat loss through convection.

One study by the University of Colorado had cellulose-insulated houses losing 26.4% less heat energy than a comparable fiberglass-insulated house. Cellulose also reduces the amount of energy required for heating by 20% – 30%.

Sound Attenuation

Cellulose provides two soundproofing qualities–mass and damping, which reduces noise vibrations passing through drywall and along cavities. Cellulose is approximately three times denser than fiberglass and provides better sound insulation.

Fire Retardant

Cellulose has the highest–class 1–fire safety rating because of its boric acid treatment. Some manufacturers are adding ammonium sulfate to the retardant blend for even better fire safety.


Cellulose costs approximately $1.20 – $2.80 per square foot–supplied and installed–depending on type and location. Loose fill in the attic is at the low end of the scale, and wet spray is at the upper end. Fiberglass wall batts and fiberglass blown-in are slightly less expensive. Rock wool insulation–batts and blown-in are more expensive.

Dense-pack cellulose insulation costs up to $4.20 per square foot. The higher cost is due to the amount of product and the techniques needed to accomplish the job. 

Disadvantages of Cellulose Insulation

As advantageous as cellulose insulation is, it has a few issues. Take these into consideration when choosing an insulation product.


Blowing cellulose into attics is a simple operation. Many building supply outlets not only sell bagged loose-fill cellulose, but rent machines for the job, making loose-fill cellulose easy to DIY.

Installing dense-pack wall insulation and applying wet spray insulation are not DIY abilities. Crawling around in constricted attic spaces is not an attractive prospect for many people. Hiring professional installation companies increases cellulose insulation costs, and it’s sometimes difficult to find professional installers.


Improperly installed loose-fill cellulose can settle inside a wall cavity, leaving some areas uninsulated. Most cold spots are at the top of wall cavities, but they can occur wherever there is an obstruction, such as wires, plumbing, braces, or electrical boxes. Most cold spots will occur below obstructions because they prevent insulation from filling the gaps.


Cellulose weighs approximately three times more than fiberglass for the same R-value. It is not terribly heavy but could cause problems when blown into attics with thin or weak drywall.