Masonry Magazine June 1975 Page. 11
Technical Notes
on Brick Construction
Brick Institute of America 1750 Old Meadow Road, McLean, Virginia 22101
4
REVISED
Aug/Sep
1974
HEAT TRANSMISSION COEFFICIENTS
OF BRICK MASONRY WALLS
INTRODUCTION
In conjunction with the sudden recognition that the fossil fuels on our planet are, in fact, finite and non-renewable came the desire and need to use the available resources wisely. As this is written, many groups are in the process of drafting guidelines, standards and prescriptions aimed at the conservation of energy in new buildings.
Regardless of the results of these efforts, the true path to energy conservation lies in the knowledge of the true performance of materials, assemblies, systems and equipment, and therein the ability to accurately predict the true performance of the building as a whole.
This Technical Notes is aimed at providing information and methods of calculation for transmission coefficients and heat transfer values of brick masonry walls. These may be used in energy conservation studies and comparisons.
The 1972 edition of the ASHRAE Handbook of Fundamentals (American Society of Heating. Refrigerating and Air-Conditioning Engineers) states the following concerning heat transfer calculations:
"Currently used methods for estimating the heat transferred through walls, floors and roofs of buildings are largely based upon a steady-state or steady-periodic heat flow concept (Equivalent Temperature Difference Concept). The engineering application of these concepts is not complicated and has served well for many years in the process of design and selection of heating and cooling equipment for buildings. However, competitive practices of the building industry sometimes requires much more than the selection or design of a single heating or cooling system. Consulting engineers are requested to present a detailed comparison of alternative heating and cooling systems for a given building, including initial costs as well as short and long-term operating and maintenance costs. The degree of sophistication required for costs may make it necessary to calculate the heating and cooling load for estimating energy requirements in hourly increments for a year's time for given buildings at known geographic locations. Because of the large number of calculations that are involved, computer processing becomes necessary. The hour-by-hour heating and cooling load calculations, when based upon a steady heat flow or steady-periodic heat flow concept, do not account for the heat storage effects of the building structure, especially when one is interested in the net heat gain to the air-conditioned spaces."
Actual heat flow through a wall under normal weather conditions will involve diurnal cycles of solar radiation and air temperature, and is substantially different from that obtained from steady-state calculations.
Past research conducted by the Structural Clay Products Research Foundation, now a part of the Engineering and Research Division of the Brick Institute of America, indicates that the actual rate of heat transfer through typical building walls may be up to 20 percent less for masonry walls, up to 20 percent greater for wood frame walls and up to 60 percent greater for metal panel walls than the calculated rate based on published U values. This is due to the greater heat storage capacity of the masonry walls, which is sometimes referred to as capacity insulation or thermal mass. The National Bureau of Standards, in Building Science Series 45, states that the heat flows calculated by the steady-state method were 29 to 69 percent greater than those measured under dynamic conditions for masonry walls. Expressed another way, massive masonry walls perform 29 to 69 percent better than the steady-state U values indicate.
This Technical Notes shows how to determine the overall coefficient of heat transmission (U value) of various walls for use in steady-state or steady-periodic heat transfer calculations. Computer programs, such as those used by the National Bureau of Standards, cited in Building Science Series 45, "Dnyamic Thermal Performance of an Experimental Masonry Building", provide much closer values to the actual performance of walls than is possible using the steady-state and steady-periodic concepts of heat transfer.
Work is presently under way which hopefully will result in a simplified method of utilizing the dynamic concept in calculations without the aid of computers.