There are at least two kinds of mineral wool made in this country. The more common kind is made by converting the slag of blast furnaces (the best being from slag that does not contain iron), mixed with certain rocks while in a melted condition, to a fibrous state.
Its appearance is much like that of wool, being soft and fibrous, but in no other respect are the materials alike. Mineral wool made from slag appears in a variety of colors, principally white, but often yellow or gray, and occasionally quite dark. The color, however, is said to be no indication of the quality, as all of the peculiar properties of the material are present in equal proportions in any of the shades. The other kind of mineral wool is known as rock wool, and is made from granite rock raised to 3,000 degrees of temperature. It is claimed to be absolutely free from sulphur and the only odorless wool manufactured; it has been approved by the U. S. War Department. Its color is white and its general appearance is the same as that made from slag. The peculiar nature of both kinds is that of a mass of very fine, pliant, but inelastic, vitreous fibres interlacing each other in every direction and forming an innumerable number of minute air cells. Its great value in the insulation and protection of buildings lies in the number of air cells which it contains, combined with its resistance to heat or fire. In common slag wool 92 per cent. of the volume consists of air held in minute cells, while in the best grade the proportion of air reaches as high as 96 per cent.' This confined air makes it one of the best, if not the best, of the non-conductors of heat, and to a less degree of sound. Aside from these qualities it is very durable, contains nothing that can decay or become musty and is almost a sure protection against rats and vermin. Being itself incombustible it greatly retards the burning of wooden floors or partitions which have the spaces filled with it. The greatest value of this material is as an insulator of heat, but it is also a valuable non-conductor of sound. In the opinion of the author, however, it can be considered only as a "muffler" of the sound waves, as he can think of no practical way in which it can be used so as to-entirely separate the floor and ceiling, as it would be crushed by laying the floor cleats upon it. As a muffler or filling between the beams, however, there is probably nothing that is superior. As previously stated, the author considers that the most complete insulation from sound (without separate beams) would be obtained by floating the flooring on Cabot's Quilt or a very thick felt, with the spaces between the floor cleats filled with mineral wool.
Mineral wool, when used alone as floor deafening, may be laid on boards cut in between the joists, or on top of sheathing lath when that is used. The thickness of the wool should be at least 2 inches. Mineral wool is particularly desirable for filling the spaces between the studding of outside walls and partitions and between the rafters of the roof. It may be used to great advantage in partitions around bathrooms or water closets, and around water pipes when placed in partitions. In outside walls and attic roofs, as a protection from the heat of summer or the cold of winter, it is of the greatest value. By lathing the under side of the rafters with sheathing lath, and filling on top with 2 or 3 inches of mineral or rock wool, the comfort of the room below will be greatly increased. Flat roofs over inhabited rooms may be covered with rough boards and 1 ¾-inch cleats nailed on top, as in Fig. 178, and the spaces filled with wool, and the roof sheathing then nailed to the cleats. This would not only greatly increase the comfort of the rooms, but greatly retard the progress of fire from the outside. (When insulating against heat, nails driven through the insulating material do no harm.) When using mineral wool in floors it should be packed in closely, but not jammed so as to break the fibres, which are naturally very brittle. In partitions it is packed between the lathing, so as to fill the space completely, the wool being put in after the lathing has reached a height of 2 or 3 feet, then more laths put on, the space filled, and so on to the top; it should not be dropped from any considerable height, for the breaking up of the fires destroys the insulating qualities of the material. In fact the tendency of mineral wool to settle and consolidate is the only drawback, except cost, to its use for insulation. The wool behind the lathing will not prevent the plaster from keying.
Mineral wool is sold by the pound, and in estimating the quantity of wool required, 1 pound per square foot of filling, 1 inch thick, should be allowed for ordinary wool and \ pound for selected wool. The price of the ordinary wool is about $1.25 per hundred pounds, and of selected wool $2.