34 



CIRCULAR 7 4 0, U. S. DEPARTMENT OF AGRICULTURE 



be equivalent to a 1 inch thickness of shavings. The structural 

 materials in a wall act as insulation, but usually some are not of much 

 value in retarding the heat flow. A 12-inch concrete wall, for example, 

 adds about as much insulation as f 10 inch of shavings. Two thick- 

 nesses of -Jf -inch fir boards are almost equivalent to 1 inch of shavings, 

 provided the cracks are closed tight. 



Table 6. — Heat-insulation values of various materials in dry condition 



Material 



Density 



Heat 



conduc- 

 tivity i 



Thick- 

 ness 



equiv- 

 alent to 

 12 inches 



of mill 

 shavings 2 



Material 



Density 



Heat 

 conduc- 

 tivity i 



Thick- 

 ness 



equiv- 

 alent to 



12 inches 

 of mill 



shavings 2 



Planer shavings — 

 Corkboard . 



Pounds 



per cubic 



foot 



8.7 



8.3 



5.0 



13.2 



B. t. u. 



per hour 



0.40 



.27 



.26 



.31 



Inches 

 12.0 

 8.0 



7.5 



9.25 



Pumice gravel 3 



Fir (across grain) . . 

 Concrete 



Pounds 



per cubic 



foot 



18.8 



26.0 



B.t. u. 



per hour 



0.61 



.76 



12.0 



4.9 



1.2 



Inches 

 18.0 

 23.0 



Redwood-bark 



360.0 



fiber 



Cinder concrete 



Cinders . . 



97.0 



60.0 



147.0 



Fiber insulation 



36.0 















1 For each degree (F.) of difference through 1 square foot of material, 1 inch thick. 



2 Based on data published in the Refrigerating Data Book, issued by the American Society of Refrigerat- 

 ing Engineers (3) , except the data for redwood-bark fiber, which are from the Guide of the American Society 

 of Heating and Ventilating Engineers {2). 



3 Grain ^2 to Me inch in diameter. 



The quantity of heat passing through a wall with 12 inches of dry 

 shavings depends upon the temperature difference between the two 

 sides. When the temperature is 65° F. outside and 32° inside, each 

 1,000 square feet of such a wall may be expected to permit the passage 

 of about 26.000 B. t. u. per day. That is, 11,000 square feet of such a 

 wall will permit the loss of about 1 ton of refrigeration. Approxi- 

 mately the same quantity would be passed by equal areas of the vari- 

 ous materials shown in table 6 if they were of the thickness shown. 

 For walls twice as thick, the heat flow would be only half as great; 

 for a wall only one-third as thick, three times as much heat would pass 

 through. 



In fill insulation, such as shavings, sawdust, and redwood-bark 

 fiber, the resistance is influenced by the density of packing. In verti- 

 cal walls, especially, such material must be packed tight ; otherwise 

 settling will occur and leave spaces unfilled after the wall is closed 

 up. In these comparisons of various materials it is assumed that all 

 are dry. 



Moisture in any of these materials reduces their effectiveness and 

 will cause some to rot. All should be installed so as not to accumu- 

 late moisture. Moisture condenses on surfaces cooler than the air but 

 not on those that are warmer. The insulation material in a wall or 

 roof is usually colder than the outside air. It is important therefore 

 for the insulation to be protected against the outside air by a barrier 

 against water vapor, such as coatings of asphalt or vaporproof paper 

 on the outside of the wall. A barrier is not necessary on the inside, 

 since a wall seldom picks up moisture from the inner, or cold, side. 

 In fact, any moisture that may be present in the insulating material 

 tends to leave the wall and condense on the cooling: coils inside the 



