STEADY STATE THERMAL PERFORMANCE OF CONCRETE MASONRY UNIT WALL SYSTEMS

STEADY STATE THERMAL PERFORMANCE OF CONCRETE MASONRY WALLS

For clear wall thermal analysis, six types of masonry wall units were considered during computer modeling. For each shape of CMU thermal efficiency of insulation (TE) and clear wall R-value were computed as a function of thermal resistivity of concrete used in block production. A reduction of wall R-value caused by using mortar was discussed as a function of thermal resistivity of block concrete for uninsulated and insulated 2-core units. For uninsulated 2-core units, insulated 2-core units, cut-web units, uninsulated multicore units, and insulated multicore units, a reduction of the wall R-value caused by grout was computed as a function of thermal resistivity of block concrete. Overall wall thermal analysis was performed for uninsulated 2-core units, insulated 2-core units, cut-web units, uninsulated multicore units, and insulated multicore units. Structural drawings of the wall details for solid CMU and for solid CMUs with the serpentine and interlocking insulation inserts were not available for the author, so they were not included in the overall wall analysis.

As shown in Figure 3, the thermal efficiency (TE) of the insulation material in two-core, cut-web, and multicore units made of normal density concrete varies from 20-40%. For solid units with interlocking insulation inserts - shape B, TE varies from 30%-80%, and for shape A units - 70%-90%. It can be observed that if CMUs are made of lightweight concrete, the thermal efficiency of the insulation is higher. For most of the insulated blocks made of lightweight concrete (except insulated multicore CMUs), TE can reach 60%-90%. Insulation in multicore units is very ineffective. For normal density concrete, it is below 20%. The maximum TE value for these multicore units made of very lightweight concrete will not likely exceed 65%.

Thermal resistances of six considered shapes of CMUs are depicted in Figure 4 as a function of thermal resistivity of block concrete.

Solid CMUs are normally produced of the lightweight concretes. For 12-in. thick units R-value varies from about 5 to 10 hft²F/Btu (0.8 - 1.7 m²K/W).

As shown in Figure 4, the thermal performance of two-core units made of normal-density concrete is very low; for an uninsulated 12-in. (30-cm) thick unit, the R-value is below 2 h•ft²F/Btu (0.35 m²K/W). Because of this, several companies offer many types of insulation inserts that are supposed to improve the block’s thermal performance. Unfortunately, because the inserts are located only in air cavities portioned with the concrete webs, they cannot eliminate the thermal shorts through the transversal concrete webs. For insulated units, the R-value remains below 3.5 h•ft²F/Btu (0.62 m²K/W). If two-core units are made of lightweight concrete (not a common practice in the U.S.), their R-values may be higher - about 4 h•ft²F/Btu (0.7 m²K/W) for uninsulated units, and 8 h•ft²F/Btu (1.4 m²K/W) for insulated units.

Cut-web CMUs were designed to reduce heat losses caused by transversal concrete webs in two-core units. Many types of the insulation inserts for the cut-web units are available in the U.S. market. Even if the concrete web height is radically reduced (about 40% in simulated cut-web units), heat losses still occur through the transversal concrete webs. It can be observed in Figure 4 that the increase of the thermal resistance caused by the reduction of concrete webs is minimal for units made of normal density concretes (comparison of R-value between insulated two-core and cut-web units). For the insulated 12-in. thick cut-web unit made of normal density concrete, the R-value is below 5.4 h•ft²F/Btu (0.95 m²K/W). R-values of the cut-web units made of lightweight concrete could exceed 11 h•ft²F/Btu (1.94 m²K/W).

As shown in Figure 4, for multicore units made of normal density concretes, the R-value of an uninsulated 12-in. (30-cm) thick unit is below 3.5 h•ft²F/Btu (0.62 m²K/W) and for an insulated unit it is about 6.8 h•ft²/Btu (1.2 m²K/W). It is interesting that the R-value of an uninsulated multicore unit is as high as the R-value of an insulated two-core unit. For insulated multicore units made of lightweight concrete, the R-value could exceed 19 h•ft²F/Btu (3.35 m²K/W).

Solid blocks with interlocking insulation inserts are usually made of lightweight concretes. As shown in Figure 4, for solid units with integral insulation inserts -shape A, the R-value can exceed 18 h•ft²F/Btu (3.17 m²K/W). For shape B unit, R-value can reach 20 h•ft²F/Btu (3.52 m²K/W).

Note that all presented above R-values account only for blocks themselves, they do not account for mortar or grout.

Mortar effect

The mortar joint area usually covers 4%-10% of the total wall area. Mortar may generate additional wall heat losses in masonry walls. Because of the complicated 3-dimmensional character of the heat transfer in areas of mortar joints, the reduction of the wall thermal resistance is seldom incorporated in the R-value calculations. As shown in Figure 5, the R-value reduction can exceed 12% for two-core units. The mortar effect increases when the thermal resistivity of block concrete increases. A reduction of the influence of the heat losses through the mortar on the wall R-value can be achieved by using less-conductive mortars or decreasing the area of mortar joints. In many CMUs, side mortar is being replaced by the interlocking means to connect adjacent units without the usage of mortar.

Grout effect.

Construction of load-bearing walls made of hollow-core blocks frequently requires installing additional reinforcement and filling air cores with the grout. For all CMUs, grout effect decreases when the concrete thermal resistivity increases. For the grout of thermal resistivity 0.11 h•ft²F/BTU per in (0.77mK/W), the grout effect was depicted as a function of the block concrete thermal resistivity. It can be observed in Figure 6, that cut-web units are less sensitive to the grout effect (grout effect varies from 3%-7%). For two-core units made of normal density concretes, reduction of the R-value caused by the grout poured into the cores is about 10%. For two-core units made of lightweight concrete, the grout effect is about 5%. For uninsulated multicore CMUs, the grout effect remains in the 6%-12% range. The R-value of insulated multicore units is very sensitive to the local thermal bridges caused by cores filled with grout. Reduction of the R-value for these units may reach 30% for normal density concretes and 25% for lightweight concretes.

Overall wall thermal analysis

Walls are not homogeneous thermal barriers made from uniform components. Wall details, such as corners or structural connections between wall and ceiling, behave very differently from the clear wall. For example, in walls constructed of cut-web units, standard two-core units have to be used for corners and wall openings [Insul Block 1992]. In all masonry systems, U-blocks are frequently used in bond beams and cast in place lintels [Hoke 1988, Sparfil 1991]. Steel lintel angels are in common use for window and door headers [Sparfil 1991]. Also, for most of masonry wall systems, beams, girders and other concentrated loads shall bear on the grouted blocks [Hoke 1988, NCMA 1975]. A concentration of highly conductive materials (steel, concrete, grout) creates thermal shorts in locations of the construction details in masonry walls. At present, the impact of the construction details on the overall wall thermal performance is often overlooked. This simplification can lead to errors in predicting the energy efficiency of building envelopes.

Results of the whole wall thermal analysis for uninsulated 2-core units, insulated 2-core units, cut-web units, uninsulated multicore units, and insulated multicore units are summarized in Figure 7. 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. Detailed description of wall details used for computer modeling is presented in Tables 2, 3, and 4.

Table 2. Wall details for uninsulated and insulated two-core wall systems

Wall detail	Description of wall detail	References
clear wall	Thermal effect of mortar 0.2 h•ft2F/Btu per in. ( 1.4 mK/W) and grout 0.11 h•ft2F/Btu per in. ( 0.77 mK/W) installed with 16-in. ( 40-cm.)o.c., included. In case of insulated units: 1-7/8-in. ( 4.76-cm.) thick EPS inserts.
corner	standard 12-in. (30-cm.) corner units	NCMA 1975 p. 28.
wall/ceiling	two courses of blocks with cores filled with grout, 4-in. ( 10-cm.) wood sill plate, 9-in. ( 22.5-cm.) joists, R-30 insulation.	Hoke 1988 p.189,
wall/floor	two courses of units with cores filled with grout, 6-in. ( 15-cm.) joists, plus 8-in. (20-cm.) two-core block with cores filled with grout in case of uninsulated wall, or in case of insulated wall, 6-in. (15-cm.) solid block with 2-in. ( 5-cm.) thick EPS plate between block and joist.	NCMA 1975 p. 29
window, door header	2 two-core 6-in. ( 15-cm.) units, plus 2 steel lintels 3-1/2x5x1/4-in. ( 8.9x12.5x.6-cm.)	Hoke 1988 p. 90
window, door side	standard two-core units
window sill	one row of two-core blocks with cores filled with grout

Table 3. Wall details for uninsulated and insulated cut-web wall systems

Wall detail	Description of wall detail	References
clear wall	Thermal effect of grout 0.11 h•ft2F/Btu per in. ( 0.77 mK/W) installed with 16-in. ( 40-cm.) o.c., included. Insulated units with 2-1/2-in. (6.4-cm.) thick EPS inserts
corner	standard 12-in. (30-cm.) two-core units with 2-1/2-in. ( 6.4-cm.) thick EPS inserts	Insul Block 1992 p. 4
wall/ceiling	One row of U-blocks with cores filled with grout, with 2-in. ( 5-cm.) thick EPS inserts, 4-in. ( 10-cm.) wood sill plate, 9-in. ( 22.5-cm.) joists, R-30 insulation	Hoke 1998 p. 189
wall/floor	One row of U-blocks with cores filled with grout, with 2-in. ( 5-cm.) thick EPS inserts, 6-in. ( 15-cm.) joists, 6-in. (15-cm.) solid block with 2-in.( 5-cm.) thick EPS plate between block and joist	NCMA 1975 p. 29
window, door header	2 two-core 6-in. ( 15-cm.) units with 2-in. ( 5-cm.) thick EPS inserts, plus 2 steel lintels 3-1/2x5x1/4-in. ( 8.9x12.5x.6-cm.)	Hoke 1988 p. 390
window, door side	standard two-core units with 2-in. ( 5-cm.) thick EPS inserts
window sill	one row of two-core blocks with cores filled with grout

Table 4. Wall details for uninsulated and insulated multicore wall systems

Wall detail	Description of wall detail	References
clear wall	12-in. ( 30-cm.) multicore units with 3 rows of air cores. In case of insulated units all cores filled with EPS inserts.
corner	12-in. (30-cm.) corner units with one row of air cores.	Sparfil 1991 p. 22
wall/ceiling	one course of multicore units with two rows of air cores filled with grout, 4-in. ( 10-cm.) wood sill plate, 9-in. ( 22.5-cm.) joists, R-30 insulation.	Sparfil 1991 p. 13
wall/floor	one course of multicore units with two rows of air cores filled with grout, 3/8-in. ( 1-cm.) concrete pillow, 6-in. ( 15-cm.) joists, plus 8-in. (20-cm.) multicore block with cores filled with EPS inserts.	Sparfil 1991 p. 14, Sparfil 1989 p. 11
window, door header	multicore units 12-in. ( 30-cm.) with center webs routed to fit steel lintels, 2 steel lintels 3-1/2x5x1/4-in. ( 8.9x12.5x.6-cm.).	Sparfil 1991 p. 12
window, door side	standard multicore units
window sill	standard multicore units

For all considered wall systems, except an uninsulated two-core unit wall, the R-values of the wall details are 20%-50% lower than the R-value of the clear wall. For the uninsulated two-core CMU system, the R-value of the clear wall area is so low [1.56 h•ft²F/Btu (0.27 m²K/W)] that the thermal performance of the wall details can actually increase the R-value of the overall wall area. In the cut-web unit wall system, two-core units are commonly used for the wall details. For the cut-web unit wall, the R-value of the whole wall is about 12% less than that of the clear wall. For uninsulated multicore units, the clear wall R-value is almost equal to the overall wall R-value. For insulated multicore units, the whole wall R-value is 24% lower than the clear wall R-value. It was observed that for walls made of cut-web or insulated multicore units, R-values of the three most significant wall details (corner, wall/ceiling, and wall/floor details) are 25%-50% lower than the clear wall R-value. The wall/ceiling detail has the most lowering impact on the overall wall R-value.