Look up at the ceiling in a sports arena, a warehouse, or a large retail store and you might see a textured, slightly fuzzy coating on the exposed deck above. That is spray cellulose — one of the most cost-effective acoustic treatments for large-volume spaces. Made from recycled newspaper and treated with fire retardants, it turns bare concrete and steel into sound-absorbing surfaces at a fraction of the cost of suspended ceiling tiles.
TLDR
Spray cellulose is a fibrous acoustic insulation made from recycled newsprint and other cellulose fibres, mixed with adhesive and fire retardant, and spray-applied directly onto ceilings, walls, or structural decks. It absorbs sound through the porous absorber mechanism — air particle oscillations within the fibre matrix are converted to heat by viscous friction. At 25 mm thickness, spray cellulose achieves NRC 0.65 to 0.75. At 50 mm, it reaches NRC 0.85 to 1.00. The product is available in various textures and can be painted or tinted. Brands like ICC K-13, Sonaspray FC, and Thermofiber are widely used in gymnasiums, warehouses, airports, restaurants, and exposed-structure lofts. Spray cellulose is valued for its sustainability (75% or more recycled content), cost-effectiveness on large surface areas, and ability to conform to irregular surfaces like corrugated metal decks and exposed structural steel.
Real-World Analogy
Imagine spraying wet papier-mache onto a ceiling and letting it dry. The result is a thick, fibrous layer full of tiny air pockets between the paper fibres. When sound hits this surface, it enters the pockets and gets absorbed through friction — just like sound getting lost in a thick blanket. Spray cellulose is essentially engineered papier-mache: the fibres are precisely sized, the adhesive holds them together permanently, and the fire retardant makes it safe for commercial use.
Technical Definition
Spray cellulose consists of cellulose fibres (typically 75 to 85% recycled newsprint) bound with a water-based adhesive and treated with borate or ammonium sulphate fire retardants. The material is mixed with water in a blending machine and applied through a spray nozzle at pressures of 30 to 80 psi. As the material dries (typically 24 to 48 hours), it forms a rigid, porous matrix bonded directly to the substrate.
The absorption mechanism follows the classical porous absorber model. Sound waves entering the material encounter a network of randomly oriented fibres. Air particles oscillating within the fibre network experience viscous drag, converting acoustic kinetic energy to thermal energy. The efficiency of this conversion depends on:
- Thickness: The primary variable. Every additional 10 mm of thickness extends the low-frequency absorption range downward. At 25 mm, absorption is effective above 500 Hz. At 75 mm, it extends to 125 Hz.
- Density: Typical installed density is 24 to 64 kg/m³. Higher density improves low-frequency absorption but increases material cost and drying time.
- Flow resistivity: Per ISO 9053, the airflow resistance through the fibre matrix determines how efficiently sound energy is absorbed. Optimal flow resistivity for spray cellulose is 10,000 to 30,000 Pa·s/m².
Performance Data (per ISO 354:2003)
| Thickness | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz | NRC |
|---|---|---|---|---|---|---|---|
| 25 mm | 0.16 | 0.44 | 0.79 | 0.90 | 0.88 | 0.82 | 0.75 |
| 50 mm | 0.47 | 0.82 | 1.00 | 1.00 | 0.97 | 0.91 | 0.95 |
| 75 mm | 0.73 | 1.00 | 1.00 | 1.00 | 0.99 | 0.94 | 1.00 |
Fire Performance
When treated with borate compounds, spray cellulose achieves Class A fire rating per ASTM E84 (flame spread index less than 25, smoke development index less than 50). This is critical for code compliance in commercial buildings.
Why It Matters for Design
Spray cellulose fills a niche that other treatments cannot easily reach. In large-volume spaces — gymnasiums, warehouses, airports, convention centres — the ceiling area can be enormous (thousands of square metres), and the budget per square metre must be low. Suspended tile ceilings in these spaces are expensive due to the grid structure, and the ceiling geometry is often irregular (exposed trusses, corrugated metal deck, pipes, and ductwork).
Spray cellulose conforms to any surface shape. It coats corrugated deck profiles, wraps around pipes, and fills the gaps between structural members. This is impossible with rigid tiles or panels. The result is continuous acoustic treatment on surfaces that would otherwise be untreatable.
In loft-style restaurants and offices with exposed ceilings, spray cellulose provides absorption without concealing the industrial aesthetic. It can be tinted to match the deck colour or sprayed in contrasting colours for a design feature.
The sustainability story is strong: spray cellulose has one of the lowest embodied carbon values of any acoustic treatment (recycled paper content reduces virgin material demand), it requires no suspended grid or mounting hardware, and at end of life it is biodegradable.
How AcousPlan Uses This
AcousPlan includes spray cellulose products in its material database with thickness-dependent absorption coefficients. When you apply spray cellulose to a ceiling in the room builder, you select the application thickness and the platform calculates frequency-dependent RT60 using the correct absorption data. The material comparison tool lets you evaluate spray cellulose against suspended tiles or acoustic plaster for the same ceiling area.
Related Concepts
- What is Acoustic Plaster? — Similar spray-applied, different material
- What Are Acoustic Ceiling Tiles? — The grid-based alternative
- What is NRC? — Absorption rating system
- What is RT60? — The metric spray cellulose improves
Calculate Now
Compare spray cellulose at different thicknesses against other ceiling treatments. AcousPlan shows the RT60 and STI impact for your specific room geometry.