Masonry Magazine March 1969 Page. 41
NCMA TEK 12
©1969, National Concrete Masonry Association.
An Information series from National Concrete Masonry Association
January, 1969
Estimating U-Factors for Concrete Masonry Construction
Excellent thermal insulation along with the ability to carry building loads is rather a unique feature of concrete masonry construction. Generally, materials which are good heat insulators cannot support loads, and most load-carrying materials are poor insulators and require added insulation to increase thermal resistance to an acceptable level. Load-bearing concrete masonry is the exception in that it possesses both high thermal resistance and high strength for load-bearing walls. In most instances, walls of concrete masonry have sufficient thermal insulation to meet the requirements imposed by climate and modern construction without the addition of "nonload-bearing" insulating material. And, when insulation must be added, the amount required is minimized due to the excellent inherent thermal characteristics of the concrete masonry wall.
Heat Transfer Symbols
84
Concrete Units
U = U-FACTOR
Overall heat transmission coefficient; the amount of heat, expressed in BTU transmitted in one hour through one square foot of a building section (wall, floor or ceiling) for each degree F of temperature difference between air on the warm side and air on the cold side of the building section.
R = 1/C = RESISTANCE
Overall resistance; the amount of resistance to heat flow between air on the warm side and air on the cold side of the building section.
k = CONDUCTIVITY
Thermal conductivity; the amount of heat, expressed in BTU transmitted in one hour through one square foot of a homogenous material one inch thick for each degree F of temperature difference between the two surfaces of the material.
C = CONDUCTANCE
Thermal conductance; the amount of heat expressed in BTU transmitted in one hour from surface to surface of one square foot of material or combination of materials for each degree temperature difference between the two surfaces. It should be noted that this value is not expressed in terms of per inch of thickness but from surface to surface.
f = FILM CONDUCTANCE
Film or surface conductance; the amount of heat, expressed in BTU transmitted in one hour from one square foot of a surface to the air surrounding the surface for each degree F temperature difference. The symbols fi and fo are used to designate the inside and outside surface conductances, respectively.
a = CONDUCTANCE OF AIR SPACE
Thermal conductance of an air space; the amount of heat, expressed in BTU transmitted in one hour across an air space of one square foot area for each degree F temperature difference.