STEADY STATE THERMAL PERFORMANCE OF CONCRETE MASONRY UNIT WALL SYSTEMS

STEADY STATE THERMAL PERFORMANCE OF CONCRETE MASONRY WALLS

A steady state thermal analysis has been performed on popular masonry walls systems and their details. A finite difference heat conduction code developed at the Oak Ridge National Laboratory (ORNL) was used for thermal modeling [Childs 1993]. The accuracy of the predicting of clear wall R-values was validated by using 19 published test results for masonry, wood-framed, and metal-stud walls (maximal discrepancy between test resulted and simulated R-values was below 6%). Considering that the precision of the guarded hot box method is reported to be approximately 8% [ASTM 1989], the ability of the used computer model to reproduce the experimental data was found as satisfactory.

Figure 1 illustrates a collection of 12-in. (30 cm) thick masonry wall units that were analyzed for this paper. In the clear wall thermal analysis, the following six Concrete Masonry Units (CMUs) were studied: solid block, two-core hollow block, cut-web block, multicore block, solid block with interlocking insulation insert, and solid block with serpentine insulation insert. The thermal resistance for each unit was estimated for five different values of concrete thermal resistivity: 0.19 (1.32) 0.28 (1.94), 0.40 (2.77), 0.59 (4.09), and 0.86 h•ft²•EF/BTU per in. (5.96 mK/W). These values approximately correspond, respectively to the following densities of concrete: 120 (1,920), 100 (1,600), 80 (1,280), 60 (980), and 40 lb/ft³ (640 kg/m³) [ASHRAE 1993 ].

Typically, CMUs are produced in the U.S. with the normal density concrete - 140-120 lb/ft³ (2,240-1,920 kg/m³). Using normal density concrete, hollow blocks can be manufactured in compressive strengths, ranging from 1500 to 4000 psi (10 to 27.5 Mpa) based on net area [Drysdale et. al. 1994].

In addition to this traditional production, a great variety of several shapes of CMUs made of lightweight concretes are available in several countries. Lower thermal conductivity of these concretes improves the thermal performance of such units. Unfortunately, lower compressive strength reduces the load that can be carried by walls made of CMUs manufactured of lightweight concretes. Compressive strengths of lightweight concretes are sometimes 10 times lower than for normal density concretes - ranging from 290 to 1500 psi (2 to 10 Mpa) [Roszak 1989, RILEM 1993, Mielczarek 1989]. So, lightweight concretes are mostly used to produce solid CMUs, or left-in-place forms where necessary strength is provided by poured structural concrete.

Most popular lightweight concretes are listed below:

Expanded Shale, Clay, and Slate (ESCS) concretes of densities 80-100 lb/ft³ (1,600-1,280 kg/m³) are also used in the U.S. for CMUs production. For thermal calculations for CMUs made of these concretes, Expanded Shale Clay and Slate Institute recommends the following range of thermal resistivities: 0.40-0.27 h•ft²•EF/BTU per in. (2.79-1.89 mK/W) [ESCSI 1992].

In Europe, Lightweight Expanded Clay Aggregate (LECA ) concrete - 28-40 lb/ft³ (450-640 kg/m³) is widely used for CMUs production. Thermal resistivity of the LECA concrete is reported as between 1.07- 0.9 h•ft²•EF/BTU per in. (7.70-6.29 mK/W) [LECA 1991].

Also, mostly in Europe, CMUs are made of wood concrete - 28-40 lb/ft³ (500-1000 kg/m³) is widely used for CMUs production. Thermal resistivity of the wood concrete is reported as between 0.90-0.41 h•ft²•EF/BTU per in. (9.09-2.86 mK/W) [Mielczarek 1989, Nanazasvili 1983, Wyszynski 1985, Kosny 1994 ].

Expanded polystyrene beads are used sometimes as lightweight aggregates for concrete production. Density of the expanded polystyrene beads concrete is in the range 25-70 lb/ft³ (400-1120 kg/m³). Proposal of the Canadian 1995 National Energy Code recommends the use of the expanded polystyrene beads concrete of density 30 lb/ft³ (480 kg/m³) the thermal resistivity of 0.89 h•ft²•EF/BTU per in. (6.17 mK/W) [NRCC 1995].

Autoclaved Aerated Concrete (AAC) is a very popular material for solid CMUs production in Europe since 1940-ties. The density of the CMUs made of AAC is in the range 30-40 lb/ft³ (480-640 kg/m³), thermal resistivity is about - 0.95 h•ft²•EF/BTU per in. (6.58 mK/W) [RILEM 1993].

The mortar joint area usually covers 4-10% of the total wall area. Mortar may generate considerable heat losses in masonry walls. Also, the construction of load-bearing walls made of hollow-core blocks very often requires installing additional reinforcement and filling air cores with the grout. The evaluation of the thermal effects generated by mortar and grout were included in the clear wall thermal analysis.

Existing methods to do thermal calculations for building wall systems are based only on the measured or calculated thermal performance of the clear wall area. Clear wall measurements are typically carried out by an apparatus such as the one described in ASTM C 236 [ASTM 1989]. A relative large (approximately 8 H 8 ft or larger) cross-section of the clear wall area of the wall system is used to determine its thermal performance. Thermal anomalies, such as concrete webs or core insulation inserts, are typically included in the test configuration. For concrete and masonry walls, building envelope intersections and opening perimeters may represent different construction than the clear wall area. Obviously, the thermal properties measured or calculated for the clear wall area may not adequately represent the total wall system thermal performance. In the past, that fact has often been omitted and, as a result, wall details have not been thermally examined and improved. As was discussed in [Kosny, Desjarlais 1994] the cases of the wood and metal frame walls, polystyrene foam wall form system, and two-core CMU wall, these simplifications can lead to errors in determining the energy efficiency of the building envelope. Investigating areas of possible heat losses in buildings and opportunities to replace highly conductive materials should aid thermal designing of future buildings.

Three masonry wall systems were considered for the overall wall analysis (two-core, cut-web, and multicore units). For each wall system, models of the clear wall area, corner, wall/ceiling (roof/wall) intersection, wall/floor intersection, window header, window sill, window edge, door header, and door edge were analyzed. For all listed above wall systems, two densities of concrete were considered during modeling:

• for two-core and cut-web units: normal density concrete, 120 lb/ft³(1,920 kg/m³) of thermal resistivity 0.19 h•ft²F/Btu per in. (1.32 mK/W), and

• for multicore units: lightweight concrete, 40 lb/ft³(640 kg/m³) of thermal resistivity 0.90 h•ft²F/Btu per in. (6.24 mK/W).

Geometries of wall details were obtained from the following standard architectural drawings or system manufacturers' design guides [NCMA 1975, Hoke 1988, Insul Block 1992, Sparfil 1989, Sparfil 1991].

The temperatures used in all of the modeling runs were 70EF (21EC) for the interior space and -20EF (6.6EC) for the exterior environment. The resultant temperature maps were used to calculate average heat fluxes, and the wall system R-values. Using a standard building elevation, these results have been combined to compute the amount of the clear wall area and to determine the overall wall system thermal performance for a typical single-story ranch house.