Which material is commonly categorized under flexible insulation?

The material commonly categorized under flexible insulation is cellulose. Cellulose insulation is made from recycled paper products and is designed to fill spaces in walls, attics, and other cavities where thermal insulation is needed. It is characterized by its ability to conform to various shapes, making it effective for various applications in construction. Cellulose is also known for its good thermal performance and is treated with fire retardants to enhance safety.

In contrast, the other materials listed do not fit the definition of flexible insulation. Plywood is a structural material used widely in construction for its strength and durability, but it does not provide insulating properties. Steel is a rigid metal known for its structural applications and heat conductivity rather than insulation. Concrete is a heavy and dense material typically used for structural purposes, and while it does provide some insulation, it is not categorized as flexible insulation. Thus, cellulose stands out as the correct choice for flexible insulation.