What is the least expensive insulation for a ceiling?

The least expensive insulation option for a ceiling is typically fiberglass batts or rolls, which are widely available and easy to install. Another economical choice is cellulose insulation, made from recycled paper products, which can be blown into ceiling spaces. Both options provide decent thermal resistance at a lower cost compared to foam board or spray foam insulation. However, it's important to consider local building codes and the specific climate when selecting insulation.