Porous wood fibre boards can be divided into two main groups according to their weight; light boards of 110-170 kg/m3 and heavy boards of 240350 kg/m3- The former are produced in thicknesses from 20 to 200 mm, the heavy boards from 6 to 160 mm, sometimes as thinner sections laminated together. Compressive strength varies between 20 and 200 kN/m2 for the light boards, and between 100 and 300 kN/m2 for the heavy variety. Thus there is a broad range of possible applications.
The light boards have thermal insulation as a main function. The lightest types are quite elastic and can be pressed between joists or studs; otherwise the boards are mounted continuously on the outside of the construction so as to form a complete thermal layer without cold bridges.
The heavier boards have an equally important function as exterior cladding forming the stiffening (bracing) of the buildings. With water-repelling agents added they are often used as wind-proofing and as sub-roofing. Other types can be rendered on the outside surface. Yet another function of the denser types is as soundproofing under floor coverings.
The boards are produced by both dry and wet processes (see page 280). In the dry process, polyurethane glue (4%) or melted polyolefine fibres (10%) are used; in the wet process the natural lignin in the material forms the glue. Some recycled paper can be added in both cases.
Fire retardants such as ammonium sulphates or ammonium phosphates (about 3%) are commonly used. Aluminium sulphate speeds the action of the lignin in the wet process. By adding laccase even more lignin becomes available.
In the wet process, boards of over 30 mm thick must be laminated, and here polyvinyl acetate adhesive (PVAC) or waterglass adhesive is added. For exposed or humid conditions such as for exterior sheathing, water repellents are added, including bitumen, lateks, paraffin wax, colophony and waterglass, normally in tiny amounts, except for up to 20% in the case of bitumen.
Boards may be delivered with tongue and groove to ensure wind-proofing and no cold bridges. They are easy to cut and work with, and are fixed with staples or screws. As acoustic layer they are laid loose.
Energy consumption is considerable in the wet process, but additional glues are not needed; whereas the dry process normally demands high quantities of problematic glues. These include polyurethane adhesive and phenol-resorcinol-formaldehyde PRF. Bitumen may be used with both types and, being a fossil oil product, reduces the environmental advantages considerably. Stapled boards will be difficult to demount without breakage; re-use otherwise is possible, as well as energy recycling under condition that the synthetic glues and bitumen components can be taken care of.
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