Cellulose Insulation — R-Value, Types, and Installation

A cellulose insulation is a loose-fill thermal product made from recycled paper fiber treated with fire-retardant chemicals and blown or packed into walls, attics, and floors.

What It Is

Cellulose insulation consists primarily of shredded newsprint and cardboard treated with borate, ammonium sulfate, or ammonium phosphate to provide fire resistance, pest deterrence, and mold inhibition. It achieves an R-value of approximately R-3.2 to R-3.8 per inch, making it competitive with fiberglass batts at comparable installed depths.

Cellulose is installed by blowing through a 2-1/2 to 3-inch flexible hose connected to a machine that shreds, conditions, and fluffs the material before sending it into the cavity. For attic floors, the installer moves across the space filling to a target depth — typically 10 to 14 inches for R-38 in climate zones 4 through 8 as recommended by the IRC. For enclosed wall cavities, the dense-pack method injects material at 3.5 pounds per cubic foot through small 2-inch holes drilled in the sheathing or drywall.

Settled depth matters because cellulose compresses roughly 15 to 20 percent over the first year after installation. Installers compensate by blowing extra material so the settled depth still meets the target R-value. The installed depth and settled depth should both be documented on the insulation certificate posted in the attic or on file with the building department.

Types

Loose-fill cellulose is blown into open attic floors and is the most common residential application. Dense-pack cellulose is injected into closed wall cavities, rim joist bays, and cathedral ceiling rafter bays through small access holes. Stabilized cellulose is mixed with a small amount of water-activated adhesive so it clings to sloped or vertical surfaces without settling. Spray-applied cellulose, sometimes called wet-spray cellulose, is used in open wall cavities before drywall is installed and sets in place as the adhesive dries.

Where It Is Used

Cellulose insulation is used in attic floors, cathedral ceilings, sidewalls, basement rim joists, floor cavities over unconditioned crawlspaces, and knee walls in finished attics. It is one of the most common retrofit insulation materials because dense-pack installation requires only small 2-inch access holes rather than removing drywall or exterior sheathing. It is also popular in new construction as a cost-effective alternative to spray foam in wall cavities. Cellulose is particularly well-suited to older homes with irregular stud spacing and existing wiring, where batt insulation cannot conform to the cavity and spray foam may not be cost-justified.

How to Identify One

Cellulose appears as a gray, light brown, or bluish-gray fibrous material resembling shredded newspaper. In attics it looks like a loose fluffy blanket that conforms to the shape of the framing and obstructions. In walls it is confirmed by probing through a small hole or by viewing through an access opening. It compresses slightly under hand pressure, has a papery texture, and may carry a faint chemical odor from the borate treatment. It is distinctly different from fiberglass, which appears as pink, yellow, or white fluffy batts with a glassy sheen.

Replacement

Cellulose does not typically require full replacement unless it has been saturated with water from a roof leak, plumbing failure, or condensation. Wet cellulose clumps, settles dramatically, can develop mold, and may lose its fire-retardant effectiveness. Water-damaged cellulose must be removed by vacuum extraction and replaced with new material after the moisture source is repaired. Adding depth on top of settled but dry material is a common energy upgrade that does not require removing existing insulation — the new layer simply restores the original target depth.