50 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
sufficient refrigeration to take care of the neat which finds its way 
through the insulation of the walls, floors, and ceilings of the cold- 
storage rooms. The greater the efficiency of the insulation the less 
heat will get through from without. There is a limit, however, to the 
amount of insulation that should be installed, which is the point 
where the interest on the money invested in insulation, the repairs 
and depreciation on same, balances the saving in operating expenses. 
There is no material known that will entirely prevent the passage 
of heat. However, there are some which offer a very high resistance, 
and are therefore termed nonconductors or insulators. The best 
heat insulators appear to be those that contain the greatest amount 
of entrapped air confined in the smallest possible air space. 
The function of cold-storage insulation, then, is to prevent the 
outside heat from passing through the walls, floors, and ceiling into 
the interior of the cold room. Therefore the problem is to minimize 
the passage of heat by interposing in the walls, floors, and ceiling a 
material or construction which will resist the transfer of heat from 
the outer to the inner side of the room. The materials most commonly 
used for this purpose are the different varieties of cork products, 
mineral wool, hair felt, rock wool, vegetable fiber, sawdust, mill 
shavings, etc., used in combination with wood, cement, masonry, and 
air spaces. At one time it was common practice in the construction 
of buildings for cold-storage purposes to provide a series of air spaces 
in the walls, some of which were as much as 12 inches wide, the 
supposition being that they were dead-air spaces. As a matter of 
fact they were not. As the air in contact with the warmer surface 
became heated it rose, while that in contact with the cooler surface 
fell, thus producing a circulation tending to equalize the temperature 
of the sides of the air space. Dead air, however, is a good nonconduc- 
tor, but unless the air spaces are properly proportioned, the above- 
mentioned air currents will be set up. Therefore, it is the present 
practice to fill in the spaces with some porous substance to break up the 
space into an indefinite number of small dead-air spaces which will 
effectually prevent circulation of the entrapped air. On the other 
hand, there is danger of packing the insulating material too closely, 
which will result in favoring the conduction of heat through the 
walls. 
Sawdust and mill shavings are mentioned in the above partial list 
of insulating materials, but they are not to be considered among 
the best. They can be had in any part of the country, and often 
without cost, and if kept dry are good insulators. It is a very difficult 
problem, however, to keep them dry, and when used, great care 
should be exercised in the construction and workmanship of the walls 
in order to keep out the moisture, 
