102. No material is known which will totally obstruct the passage of heat by conduction. There is, however, a great difference in the conductivity of materials, and those which conduct heat very slowly are called by the general name of non-conductors.

In a general way, the efficiency of non-conducting materials is proportional to their lightness per cubic foot.

Thus, heat passes through a solid plate of glass with comparative freedom, but a layer of "mineral wool" or "slag wool" (which is merely glass blown into fine threads) of the same weight per square foot of surface, exhibits very much less conducting power.

The materials used as non-conductors vary greatly in durability, and this fact should be carefully considered when estimating the saving that may be accomplished by using them.

All non-conducting coverings should be well protected against displacement, or the entrance of either air or moisture. A covering of painted canvas, well secured by copper wire, is usually sufficient for pipes which are not exposed to the weather, but in all exposed places great care should be taken to make the outer casings perfectly waterproof.

Table 16. Non-Conducting Coverings

The differences in the efficiencies of the various non-conductors now on the market are much less than usually stated in advertising literature.