What type of insulation is required for make-up air ducts installed within 18 inches of a Type I commercial kitchen hood?

Noncombustible insulation is required for make-up air ducts installed within 18 inches of a Type I commercial kitchen hood due to safety and fire prevention regulations. Type I hoods are designed to exhaust smoke and greasy air from cooking operations, which can involve high temperatures. The proximity of the ducts to the hood necessitates the use of materials that will not ignite or contribute to the spread of fire in case of a flame or high heat scenario.

Noncombustible insulation can withstand high temperatures and meets building codes that are critical for commercial kitchen environments. This insulation helps ensure that any heat generated does not compromise the integrity of the ductwork or build up in surrounding areas, maintaining both operational efficiency and safety standards.

The other options suggest insulation types that do not align with fire safety requirements. Combustible insulation could pose a significant fire hazard in this context, while thermal insulation is a broad category that may not address the potential for combustion. Flexible insulation might refer to materials that are easier to work with but do not inherently provide the noncombustible properties required in environments with significant heat exposure like those near a Type I hood. Thus, noncombustible insulation is the appropriate choice for safety in these scenarios.