Masonry Magazine August 1977 Page. 8
FEA STUDY/MASONRY WALLS
continued from page 7
Three-Way Savings
By more accurately accounting for the thermal benefits provided by masonry walls, building owners and developers can better size their heating and cooling systems needs. Three primary and tangible effects are realized by the mass of masonry. They are: 1) Masonry slows the amount and rate of transfer of heat through the building envelope; 2) masonry reduces "peak load" requirements in a building, which reduces cost (particularly when peak charges are part of the rate structure) and conserves fuel, and 3) masonry, because it is hand-fitted on site, reduces the chance of air infiltration the direct exchange of heat due to cracks around windows, doors, and between panelized components of system-type structures or cracks developed by unplanned shrinkage, creep or other secondary movements.
The combined result of these three energy bonuses is a more efficient building envelope (new, existing, renovated or retrofitted buildings), one which can enable the installation of smaller, less costly heating, ventilating, air-conditioning (HVAC) equipment.
Envelopes, however, are only one component of thermal performance. The interaction of envelope, HVAC system and the distribution system must be considered to accurately evaluate total performance.
Masonry walls reduce heat transfer and distribute it more evenly over a longer time scale in both heating and cooling operations, thus conserving energy associated with the distribution of mechanically treated air. When dynamic calculations are considered, masonry will usually reduce the size of HVAC equipment by reducing peak load demand as compared to steady-state equipment sizing calculations.
Heat Transfer-Difficult to Determine
Until recently, architects and engineers have had a simple formula for determining the amount of heat being transferred through any building envelope. They measure a wall, look up the corresponding "U" value for the material on a pre-figured calculation sheet, multiply by the expected temperature differential, and establish the approximate amount of heat that escapes through the wall.
This procedure would be correct if the materials in question were in a vacuum and didn't join each other. But heat transfer is much more complicated than this. Temperature changes, sharp corners, sudden changes in thickness and the joints of high and low conductivity materials cause additional heat losses.
In order to determine more accurately heat loss and transfer, a new factor for planning and measuring was developed. Called the "M" factor, it is already making simple steady-state "U" value equations obsolete. The "M" factor graph or curve serves as a simple correction factor for dynamic analysis. It has been presented to leading code and industry organizations and government agencies.
Only two numbers are required to use the "M" factor graph: The number of degree days in the locale (obtainable from the U.S. Weather Bureau), and the weight per square foot of the wall. (See Figure 2.)
The graph shows that in all cases, the masonry walls perform better than lighter weight walls with the same "U" value rating. The heavier the wall, the greater the apparent savings.
Before the "M" factor was available to accurately determine heat transfer and loss, the building envelope was a much misunderstood element. It has, at different times, been blamed for both wasting and saving energy.
Most often, it has been misused as a source of energy conservation. In reality, the "skin" of a building is only one of the sources for wise energy conservation practices.
Government and industry estimates currently trace between two and 15 percent of all energy consumed in buildings directly to the walls or envelope. The other amounts are attributed to lighting and its distribution, HVAC equipment, hot and chilled water service systems and other internal building activities and machinery.
While these estimates often are accurate, they do not relate the direct effect of the walls of a building on overall energy performance. In other words, the positive effects of the envelope on mass during constantly fluctuating temperature conditions and air infiltration are not considered. Occupants remain more comfortable in masonry structures because fluctuating external temperatures are not immediately felt.
Historical Advantages
Historically, mankind has "known" that masonry buildings were thermally stable, which made them warmer in winter and cooler in summer than buildings with thinner, lighter-weight walls made of other materials. Examples of the superior thermal performance of masonry range from sun-baked Egyptian buildings and tombs to Gothic cathedrals and most buildings erected until the early twentieth century.
The advent of newer wall materials, from glass and steel to plastic panels, coupled with the now obsolete theory of infinite energy resources, reduced the apparent need for masonry walls to matters of aesthetics, fire protection and low maintenance. Though these desirable benefits of masonry are still valid today, the elimination of masonry as a viable energy conservation material can be short-sighted and costly.
Mass Does Improve Insulation
In designing new or remodeled structures and in planning energy-related retrofitting, building owners and designers can greatly benefit by understanding and using the new "M" factor technology. Insulation added to any opaque wall may be beneficial to the energy efficiency of that wall. But there is a point at which additional insulation is not cost-effective.
However, in dynamic temperature conditions, the material around that insulation can greatly enhance its performance. In other words, masonry can optimize the effects of insulation through its mass properties. Because masonry walls slow the transition of heat into and out of continued on page 31
8 MASONRY/NOV/DEC.