REFEIGERAnON 53 



Brick is a much better conductor of heat and therefore a poorer 

 insulator. In accordance with results obtained in Germany/ 

 the value of n for a brick wall lo inches thick is 8.5 B. T. U. 

 If an air space is added to the wall, this figure is reduced to 

 6.1 B. T. U. If the wall is absolutely air-tight, the thickness 

 of the air space is immaterial. 



According to the same report, sandstone and limestone are 

 still higher conductors of heat than brick. In a 12-inch wall 

 built from sandstone, the value was found to be 11. o B. T. U.; 

 and in a wall built from limestone of the same thickness, n 

 proved to be equal to 13 B. T. U. 



For a single-sashed window, the factor n may be taken as 

 12; for a double-sashed window, as 7 (Box). 



D. Thickness of Insulation. — The heat-conducting powers 

 of any insulation are approximately inversely proportional to 

 its thickness, other conditions being equal. A lo-inch brick 

 wall will transmit 8.50 B. T. U., whereas a 40-inch wall under 

 the same conditions will transmit only 3.85 B. T. U. 



2. Temperature to he Maintained. — A decrease in the tem- 

 perature to be maintained requires a corresponding increase in 

 insulating power. This is illustrated in the following table: 



Temperature from — 10 to 5° F., use 6 in. cork 



S to 20° " " 5 " " 



20 to 32° " " 4 " " 



32 to 45° " " ,S " " 



" " 45 and above " 2 " " 



3. Climatic Conditions. — These will naturally affect the dif- 

 ference in temperature on the two sides of a wall and will con- 

 sequently affect the transmission of heat. They should be 

 given due attention when constructing a cold-storage room 

 close to an engine room or a boiler room, or when constructing 

 it with an outside wall face, or when locating it on a south 

 rather than a north exposure. 



4. Other Factors. — The materials used for insulation should 

 be free from rot, mold, and offensive odors. In both nature of 



' Siebel's Compend. of Mechanical Refrigeration and Engineering, 191 1, p. 382. 



