Cellulose insulation is manufactured from shredded recycled paper and other organic waste. It is treated with borax for flammability and smouldering resistance; this also makes it unattractive to vermin, and resistant to insects, fungus and dry rot. Unlike mineral fibre and glass fibre insulation it does not cause skin irritation during installation. Recycled cellulose has a low embodied energy compared to mineral and glass fibre insulation, and when removed from a building it may be recycled again or disposed of safely without creating toxic waste. (Treatment with the inorganic salt, borax, ensures that cellulose insulation conforms to BS 5803 Part 4: 1985 – Fire Test Class 1 and Smoulder Test Class B2.)

Cellulose insulation may be used directly from bags for internal floors and also lofts where the required eaves ventilation gap must be maintained. For other cavities, including sloping roof voids, the material is dry injected under pressure, completely filling all spaces to prevent air circulation. In breathing walls, cellulose insulation is filled inside a breathing membrane,
which allows the passage of water vapour through to the outer leaf of the construction. Cellulose may be damp-sprayed in between wall studs before the wall is closed. Cellulose is a hygroscopic material, which under conditions of high humidity absorbs water vapour and then releases it again under dry conditions. Cellulose is an effective absorber of airborne sound.

Loose-fill cellulose insulation (LFCI) is detailed in the standards BS EN 15101 Parts 1 and 2: 2013. Classification with respect to settlement, resistance to mould and fire (BS EN 13501–1: 2007, BS EN 13823: 2010 and BS EN ISO 11925–2: 2010) is required. (The thermal conductivity of cellulose insulation is 0.035 W/m K in horizontal applications and 0.038–0.040
W/m K in walls.)