A common method of collecting fibrous material, particularly fibrous mineral material, involves distributing the fibrous material onto a collecting surface to form an insulation pack. The fibers can be collected as a pack on an endless belt positioned beneath the fiber distributing apparatus, and the pack can be built up continuously on the moving belt. Such fiber collection apparatus can be combined with an exhaust fan positioned to draw gases through the belt and thereby force the fibrous material toward the belt to form the pack. Apparatus for forming packs, such as insulation packs as thick as 3 inches or more, can use a series of fiber distributing means. The exhaust fans associated with such apparatus for producing relatively thick insulation packs can have an extremely strong suction effect on the fibrous material, since the manufacture of thicker insulation packs generally requires stronger suction fans.
A common problem with such apparatus is that the fan pulls the initially deposited fibers onto the belt so that the bottom layer of the insulation pack is denser than the upper portions. It is believed that the greater the final thickness of the insulation pack, the greater is the density variation from the bottom to the top of the pack. The fact that the bottom portion of the insulation pack is of greater density than the upper portion of the pack is undesirable in that the greater density portion does not compensate for the increased material usage by providing an equivalent increased thermal insulative value.