Asbestos Millboard

Asbestos millboard is a dense, compressed board manufactured primarily from chrysotile (white asbestos) fibres, often with small amounts of amosite. The asbestos content typically ranges from 35 % to 70 %, making it one of the higher-concentration asbestos products. Millboard was produced by forming wet asbestos-fibre slurry on a cylinder mould, similar to paper-making, and then pressing and drying the resulting sheets.

Millboard was widely used from the 1920s through the mid-1980s as a heat-resistant barrier in numerous settings. Common applications include heat shields behind boilers, fireplaces, and wood-burning stoves; linings for electrical switchgear, fuse boxes, and distribution boards; gaskets and spacers in high-temperature industrial equipment; and fire barriers within building structures.

The material has a characteristic cardboard-like appearance but is significantly denser. It is usually grey, brown, or off-white and is typically 1-6 mm thick. Millboard is classified as a mixed-friability material: when in good condition, it is relatively stable and does not readily release fibres. However, when aged, water-damaged, or mechanically disturbed through cutting, tearing, or abrasion, it can generate significant quantities of airborne asbestos dust.

The risk level for millboard is moderate (Risk Level 3) compared to sprayed coatings or lagging, but it should never be underestimated. Cutting or sanding millboard produces very high fibre concentrations. In the UK, work on millboard may qualify for non-licensed work or licensed work depending on the scope and condition. In the US, OSHA classifies its disturbance as Class II asbestos work.

Millboard is frequently overlooked during building surveys because it can be hidden behind modern fittings. A thorough Type 2 management survey or refurbishment and demolition survey is necessary to locate concealed millboard in older properties.