Masonry Magazine December 1986 Page. 22
As a result of the analysis of the large data base of thermal mass results, the graph in Figure 3(3) was generated to illustrate reasonable expected load reductions for heating and cooling by the switch to mass wall construction. In mild climates, where the largest portion of the nation's building activity is taking place, thermal mass can provide significant insulation cost reductions.
Researchers from LBL performed work for the DoE "Affordable Housing Guidelines Project" (AHG) under congressional mandate to expand the initial data base which covered only wood frame construction. Researchers(4) quantified the effects of heat capacity, overall wall U-value and insulation position in a parametric study using a typical 1540 square foot, 3 bedroom-2 bath, slab on grade home.
Earlier results by NBS showed that slab on grade homes had more conservative thermal mass responses than crawl space, or basement equipped homes, when changing from frame walls to insulated masonry walls. The study was specifically oriented around "typical" home construction and did not intentionally utilize passive solar effects tending to boost apparent thermal mass responses.
LBL results show energy is saved using thermally massive walls in homes for all twelve climates studied (Table 1). The results range from rather low annual savings in cloudy, cool San Francisco to large benefits in Atlanta, Medford, Oregon and Phoenix. Larger net savings occur using mass walls with an R-5 insulation level than for mass walls with an R-20 insulation level compared to equally insulated wood frame walls. This is because in the less well insulated case, more gross energy is "on the table" to be saved by more energy conserving design.
According to LBL, the Phoenix, Arizona climate type has the greatest potential for mass benefits. Figure 4 illustrates the tradeoff of U-values with exterior and interior insulation position for the Phoenix climate, between frame walls and masonry walls having heat capacity (HC) greater than 6 Btu/sq. ft. Similar plots could be generated for other cities from the LBL data.
Using a draft copy of LBL's newly developed PEAR 2.0 computer program(5) that reads out the huge data base of DOE 2.1 computer runs of the AHG project, it can be demonstrated for Phoenix that an R-5 exterior insulated concrete block wall of heat capacity (HC) of 7 Btu/square foot (typical of 8 inch, partially grouted walls) consumes similar heating and cooling energy (45.68 million Btu/yr) as an identical prototype house with R-11 frame walls (45.42 million Btu/yr). The same level of R-11 insulation on the masonry wall results in a net decrease of 3.6 million Btu/yr. This illustrates the important fact that seemingly small differences in annual energy use between equally insulated frame and masonry buildings may result in large differences in insulation levels when tradeoffs are recognized.
In a colder climate like Chicago, PEAR 2.0 predicts smaller differences between frame and masonry wall insulation requirements. In a Chicago home using an R-38 ceiling, double glazed windows and an R-19 frame wall, 112.04 million Btu/yr is used for heating and cooling. An otherwise similar masonry walled home using R-14 exterior insulation and HC-7 uses 112.08 million Btu. This means that a masonry wall insulated with two inches of polyisocyanurate exterior insulation with lath and stucco should provide equal or better thermal performance than an R-19 frame wall in this climate. An advanced preinsulated block wall might only require one additional inch of exterior insulation to perform equally well. The thermal mass data support
Figure 1. Wall Mass Heating Load Reductions Compared to M-Factor Corrections
tion on the masonry wall results in a net decrease of 3.6 million Btu/yr. This illustrates the important fact that seemingly small differences in annual energy use between equally insulated frame and masonry buildings may result in large differences in insulation levels when tradeoffs are recognized.
In a colder climate like Chicago, PEAR 2.0 predicts smaller differences between frame and masonry wall insulation requirements. In a Chicago home using an R-38 ceiling, double glazed windows and an R-19 frame wall, 112.04 million Btu/yr is used for heating and cooling. An otherwise similar masonry walled home using R-14 exterior insulation and HC-7 uses 112.08 million Btu. This means that a masonry wall insulated with two inches of polyisocyanurate exterior insulation with lath and stucco should provide equal or better thermal performance than an R-19 frame wall in this climate. An advanced preinsulated block wall might only require one additional inch of exterior insulation to perform equally well. The thermal mass data support
Figure 2. Wall Mass Cooling Load Reductions
LEGEND
For Figures I and 2.
LA = Los Angeles, CA
TFL = Tampa, FL
LC = Lake Charles, LA
DFW = Dallas/Ft. Worth, TX
ATL = Atlanta, GA
ALB = Albuquerque, NM
NYC = New York City, NY
DC = Washington, DC
BST = Boston, MA
CHI = Chicago, IL
CLV = Cleveland, OH
MAD = Madison, WI
PHO = Phoenix, AZ
DEN = Denver, CO
KC = Kansas City, MO
SEA = Seattle/Tacoma, WA
KNX = Knoxville, TN
ORL = Orlando, FL