Energy Efficiency Comparison Study of Concrete Block Structures

Abstract

The Natural Resources Research Institute (NRRI), with a grant from the Minnesota Department of Administration, has evaluated the energy saving potential of an insulated concrete block product manufactured by Sparfil International, Inc. The blocks used in the study contained taconite tailings (Sparfil-Tac) in place of sand. A comparative evaluation of two 400 ft2 test buildings, identical in all respects except for wall construction (conventional 12-inch concrete block versus 12-inch Sparfil-Tac insulated block), shows the Sparfil-Tac product to have superior thennal insulating properties. Both buildings were heated electrically. The Sparfil-Tac test building consumed 44.6% less electricity than its conventional concrete block counterpart over a six-month test period (December 26, 1989, through June 27, 1990). This energy consumption differential, while significant, was less than expected. A 65 .4 % reduction in energy consumption should have been realized, based on heat loss calculations for all test building components. A guarded hot box test (ASTM C-236) perfonned by Twin City Testing Corporation showed the Sparfil-Tac product to have a thennal resistance (R-value) of 10.02 rather than the expected 24.5. This lower R-value is supported by the heat loss calculations which show the Sparfil-Tac test building to have an R-value of 8.45. To determine if the lower R-value was due to faulty block fabrication or some other factor, an additional guarded hot box test was performed on Sparfil's regular (non-taconite tailings) block. The second test showed these blocks to have an R-value of 10.80 rather than the R-value of 24.5 claimed in Sparfil product brochures. The R-value discrepancy is most likely due to the improper application of heat flow concepts by Sparfil International when calculating thermal resistances for their block products. This is particularly true for blocks whose hollow cores are filled with expanded polystyrene (EPS) inserts. The two guarded hot box tests show that the EPS inserts do not increase the insulating value of the blocks to the extent claimed. The economics of each building type were compared by using the difference in energy consumption between the two buildings. Based on quoted power and construction costs, a payback period ranging from 3.6 to 10.1 years was calculated for the 400 ft2 Sparfil-Tac product; had the block performed as expected, the payback period would have, instead, ranged from 2.5 to 7 years. The variable payback period reflects the range of construction costs used in the economic analysis. Economies of scale show the Sparfil-Tac product is more costcompetitive in larger building applications. Additional guarded hot box thermal testing of regular Sparfil blocks is recommended given the difference between the actual (test-based) and claimed (calculation-based) R-values. Calculated (in effect, idealized) values should only be relied on when they are supported by experimental results. Based on this additional testing, further economic analyses could then be performed. Based on the results of the study described in this report, Sparfil-Tac blocks are superior to regular concrete blocks in terms of R-value achieved per square foot of construction cost ($/ft2 + R-value). When construction does not require concrete blocks, other construction techniques will yield a considerably smaller ratio ($/ft2 + R-value), thus more insulating for the construction dollars spent.