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THERMAL PERFORMANCE ANALYSIS OF PRE-INSULATED CONCRETE MASONRY W ALLS

Ossama A. Abdou Assistant Professor Dept. of Civil & Arch. Engineering Drexel University Philadelphia, PA 19104, USA

ABSTRACT

Ahmad A. Hamid Professor Dept. of Civil & Arch. Engineering Drexel University Philadelphia, PA 19104, USA

Thermal analyses of pre-insulated masonry wall systems were performed to evaluate their energy efficiency. Six concrete block wall types were thermally modelled using computer simulation, in which combinations of insulation thickness, location and grouting options were investigated. The dynarnic thermal petformance of pre-insulated concrete masonry walls is compared with the performance of both uninsulated and extemally insulated wall systems. Hourly climatic data for the localities under consideration were employed to predict energy needs in all cases simulated. The results show that pre-insulated hollow block walls significantly reduce energy requirements under dynarnic conditions and that no significant improvement is realized when adding thermal mass to pre-insulation. Moreover, it was concluded that there is no appreciable difference in the thermal behavior of pre-insulated and extemally insulated walls, the latter being superior especially when additional thermal mass (i.e. grout) is incorporated in the wall system. Furthermore, it was demonstrated that differentials in heating and cooling loads between 25mm and 50mm pre-insulated walls were insignificant.

INTRODUCTION

A number of ways exist to improve the thermal performance of walls constructed with concrete blocks. These may be broken down into three categories: (1) Adding insulation layers to the concrete block wall , (2) altering the block-making material, and (3) incorporating insulating material into the core of the concrete blocks. One of the most common approaches of the latter category is the use of loose fill (such as verrniculite or perlite) which is poured into the block at various stages of completion of the block wall. However, more recently, there has been an interest in using pre-insulated masonry wall systems employing rigid insulative liners or inserts imbedded in the cores. The integral insulative liners are either flat or molded according to the shape of the cores and are placed accross the entire block unit. Typically, these inserts are made of lightweight polystyrene of various thicknesses and are designed in such a way as to fit into the cores of each block with their outside surface fitting flush against the inside wall of the core. Altematively, a small air gap (of approximately 20mm) is left between the insulation surface and the core surface for ease of block handling. This in tum has the added advantage of increasing the insulative value of the block wall composite. The block cells are left either hollow, thereby facilitating insertion of pipes and ducts, or they are grouted for structural andlor thermal purposes. This research effort is geared toward performing thermal analyses of pre-insulated masonry walL systems to evaluate their energy efficiency and to compare their performance with extemally insulated masonry wall systems under two distinctly different clirnatic conditions.

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INVESTIGATIVE PROCEDURE

Simulatión Model

A computer simulation was performed in order to test the thermal performance of exterior wall concrete blocks making use of pre-insulated inserts against other types of exterior concrete block construction. A typical residential space 3.6 m wide, 4.8 m long and 2.7 m high occupying a peripherallocation of a larger multi-story residential building was used as a model for thermal analysis. It was assumed that the modelled space was sUITounded by other spaces at three sides and that it had a single exterior exposure facing south. A double-glazed shaded window of the size 1.2 m x 1.2 m was located in the middle of the exterior wall, with a sill height of 0.9 m. AlI interior partitions of the space were of 200 mm lightweight hollow concrete block construction. AlI adjacent space temperatures were assumed to be similar to the temperature of the simulated space model to keep any losses or gains through the interior partitions, floor slab and ceiling (reinforced concrete) negligible. AlI building parameters were kept constant in alI simulation runs. The only parameter varied in this study was the exterior concrete block wall system.

WaIl Types

Six wall types are considered in this study: - WI: Hollow concrete block; - W2: 25-mm pre-insulated hollow concrete block; - W3: 50-mm pre-insulated hollow concrete block; - W4: Hollow concrete block with 50 mm exterior insulation; - W5: Fully grouted concrete block with 50 mm pre-insulated liners; and - W6: Fully grouted concrete block with 50 mm exterior insulation. AlI of the above wa1ls have a 200 mm nominal thickness . The pertinent thermal properties of the individual constituents making up the wall assemblies are shown in Table 1. Arrangement of the constituents within each wall showing their relative location within the walI assembly along with the thermal conductance values of each assembly is given in Table 2.

Table 1. Thermal properties of wall constituents

Wall Constituent

200 mm HoUow Concrete Block

Insulation (Polystyrene)

Grout

Thermal Analysis

Conductivity [W/(m*K)]

0.028

1.730

Conductance [W/(m2+K)]

3.573

Density [kg/m3]

1600

56

2240

Specific Heat [KJ/(kg*K)]

0.837

1.213

0.837

The objective of the thermal analysis was to quantify the long-term thermal performance of the concrete block wall systems indicated above. The analysis was performed by deterrnining the hour\y heat transfer of ali energy transfer components. Total space heat balance over the long term was then deterrnined by summing the results of each individual component's hourly heat transfer. The analysis was performed using the Building Loads Analysis & System Thermodynamics (BLAST) microcomputer software [1] . The computer program contains a loads predicting sub-program in which the dynamic thermal performance of a building space

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can be predicted. One of the benefits of the dynamic analysis method is that it accurately predicts the effects of heat storage capacity of building elements which, in turn, results in a more precise calculation of indoor temperature leveIs and hence the calculation of maximum thermal loads.

Typical Meteorological Year (TMY) weather data were used from two locations, Phoenix, Arizona (a typical hot, dry climate), and Philadelphia, Pennsylvania (a typical moderate climate), to assess clirnatological effects. Hourly weather observations containing real weather sequences that represent the long-term climatic mean conditions of the climates under consideration were employed to simulate monthly and annual heating and cooling loads in an effort to determine the effectiveness of pre-insulated masonry walIs (as compared to the other masonry walI systems) in reducing energy needs.

In order to calculate thermalloads accurately, control strategies for heating and cooling were defined by dual throttling ranges with night setback, specifying the fraction of fulI heating or cooling available at a specified room temperature. Linear throttling ranges and deadbands were set at specified points. It is to be noted that these control points varied according to the climate. For the hot, dry climate, heating was on any time the room temperature was 18'C or below. At 16'C the heating was operating at fulI capacity and a linear throttling range was used between 16' C and 18' C. No heating or cooling was accomplished (i.e. , a deadband was constructed) between 18'C and 28 'C. This was assumed to be the comfort range for the hot, dry climate [2] . A linear cooling throttling range was in effect between 28'C and 30'C.

For the moderate climate a similar control strategy was implemented, except that the values changed to more accurately mirror the cornfort range of this climate. The heating throttling range was between 18'C and 20'C, the deadband (i.e., comfort range) between 20'C and 26'C [31 and the cooling throttling range between 26'C and 28' C. In both climates the heat balance point occurred at the temperature where the heat gains or losses to the room air exactly equalled the cooling or heating capacity available at that temperature.

RESUL TS AND DISCUSSION

The thermal performance of six concrete block masonry wall construction types in both heating and cooling mode in two di verse climates was investigated. Table 2 illustrates the monthly and total annual results of both climates for alI simulated walI systems, based on hourly calculations of heating and cooling loads. The figures indicate relatively large month-to­month variations, in both heating and cOQling loads within each wall. Between the wall types, however, the monthly variations are less pronounced, indicating only a limited impact of the wall assembly. Comparisons for heating and cooling loads were performed at various level~, monthly, seasonally and annually . In addition, overalI annual thermalloads were evaluated based on a weighted average assessment of both types of loads based on their prorated occurrence with respect to time.

Figures 1 and 2 ilIustrate graphicalIy the results of both simulation modes in which individual comparisons are performed and juxtaposed against the base case (uninsulated hollow walI). Overall comparisons for both climates are shown graphically in Figures 3 and 4. It is quite clear that the magnitudes of cooling and heating loads, with the exception of the simple holIow block walI system, are only modestly sensitive to the variations in the walI assemblies. The variation is more pronounced in the less severe (i.e. moderate) climate than it is in extreme climates (e.g., the hot, dry climate). The difference in the magnitude of the entries within each simulation scheme can be taken as a measure of the rei ative contribution of a walI's constituents to the energy budget of the building, given that all other building aspects remain constant.

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Table 2. Summary of heating and cooling loads (K wh)

5 em ExL Insul. Grouted BIocks

The entries in the table suggest that variation in the thennal performance among ali wall types is higher in the intermediate seasons (spring, fali) in the hot, dry climate, than it is in the erxtremes (winter, summer). In the modera te climate, however, there is evidence that the highest variations among the walls occur in the summer time (June through September) . Further, it seems, that insulation has a more pronounced effecl in the intermediate season for the hot dry climate and is superior during the overheated period in the moderate climate (i .e., in the least criticai season).

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400 +-------------

350+-~-----------

2~~~----------__ 1800 ~ __ -----------

300

:t 250 ~ ~ 200

150

100

50

(a) Heating Load

1600

1400

~ 1200 ~1~

800 600

400

200 O 1.--I __ .lillL......IiO~_

(b) Cooling Load

Figure 1. Comparative energy load analysis between wall types in the hot dry climate

• Wl: Hollow Blocks II1II W4: Hollow Blocks with 50 mm Exterior Insulation

o W2: 25 mrn Pre-Insulated Hollow Blocks [§I W5: 50 mrn Pre-Insulated Grouted Block

[jj W3: 50 mm Pre-Insulated Hollow Blocks ~ W6: Grouted Block with 50 mm Exterior Insulation

~+---------------------------- 350+-~-----------------------

5~+-~------------

(a) Heating Load (b)Cooling Load

Figure 2. Comparative energy load analysis bctween wall types in the moderate climate

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Although, as indicated above, the thermal performance of the walls varies more with the season, the "yearly needs" is a more proper measure, because there is no monthly or seasonal use or application of any of the walls; therefore, analysis of the annual performance deems more relevant. Since the hollow block wall is the only wall system which has no insulation, it can appropriately be taken as a reference against which ali other wall systems will be compared. Hence, the annual heating and cooling loads, respectively, for the hollow wall case was set equivalent to unity (100%). All other entries were normalized accordingly. This facilitated comparison of the relative heating and cooling energy needs for the space when applying different exterior masonry wall types. For example, while a hollow block wall in a hot, dry climate requires heating energy of the magnitude of 100, the 25-mm pre-insulated hollow block wall system would require approximately 81 % of that amount. Further, the same latter wall system would require 86.3% of the cooling energy that would be necessary to be supplied to the space if a holJow block wall was used. A similar analysis is performed for the moderate clirnate. According to the figures, the highest energy savings will be realized with fully grouted block walls with exterior insulation for both climates. The lowest candidate for energy conservation, other than the base case (hollow block wall system), is the 25 mm pre-insulated hollow block wall system for both climates alike.

A number of parameters related to the insulation material were investigated. These included insulation location relative to the wall block, insulation thickness, and interaction with thermal mass (grouting option). As regards insulation location (see W3 and W4) the difference between the two walls indicate the effect of the location of the insulation with respect to the masonry block. Thermal insulation within the wall (as in the pre-insulated wall system) has the advantage that the thermal insulation material need not have any particularly high strength, as it is protected against both atmospheric attack and damage from the inside of the building. Exterior insulation helps damp the heating effect of solar radiation absorbed in the outer surface and of steep rises in air temperature.

As verified in the literature, exterior insulation is more effective in cutting down heating and cooling loads, however, integral insulation is quite comparable with exterior insulation in the 'moderate cJimate especially when one looks at it in a more global fashion; i.e.,weigh advantages vs disadvantages from different viewpoints. For example, integral insulation does not take up any additional building space, and may constitute savings from the point of view of construction economics (construction time, labor union construction specialities, etc.). The range of savings in cooling loads for wall (W4) was only 1.5% compared to a loss of 0.2% for the heating load in the hot, dry climate. In the moderate cJimate a maximum of 3.3% in the cooling load over integral insulation can be expected, while actually a loss of approximately 0.4% can be expected when using exterior insulation with hollow concrete blocks.

Figures 1 and 2 cJearly indicate the effectiveness of thermal mass in reducing energy loads. Note that although the occurrences of high energy needs is present in all cases, the superiority of the more massive walls is maintained, and is in fact even more evident. Grouting has virtually no effect during heating season in the moderate cJimate, however, during the cooling season it helps cut-down the load by approximately 5% in the hot, dry climate in combination with exterior insulation.

Comparing the results of walls Wl , W2 and W3 clearly indicates the significance of pre-insulation in reducing heating and cooling loads. With an insulation thickness of 25 mm (W2) heating loads were reduced by 19% for the hot, dry cJimate and 14.3% for the modera te cJimate. Cooling loads were reduced by 13.7% for the hot, dry cJimate and 16.5% for the modera te climate. The results presented in Table 2 and Figures 1 and 2 show that doubling the thickness of insulation in pre-insulated walls only reduces heating loads by 2.9% and cooling loads by 2.2% in the hot dry climate (2.3% and 3.1 %, respectively, for the moderate climate).

::r: ~ :.::

400+-----------------

350+-----.-----------

(a) Heating Loads

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::r: ~ :.::

2000

1800 1600

1400

1200

1000

800

600

400 200

O

(b) Cooling Loads

Figure 3. Annual heating and cooling loads for the hot dry climate

• WI: Hollow Blocks l1li W4: Hollow Blocks with 50 mrn Exterior Insulation

D W2: 25 mrn Pre-Insulated Hollow Blocks llliI W5: 50 mm Pre-Insulated Grouted Block

DiII W3: 50 mm Pre-Insulated Hollow Blocks § W5: Grouted Block with 50 mm Exterior Insulation

6000 350

5000 300

4000 250

::r: 200 3000 ~ :.:: 150 2000

100

1000 50

O

(a) Heating Loads (b) Cooling Loads

Figure 4. Annual heating and cooling loads for the moderate climate

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The effect of the location of insulation (integrated versus external) can be studied by comparing walls W3 with W4 and W5 with W6. It is shown that the location of insulation has virtually no difference in heating and only a small difference in cooling (2.5% maximum) for hollow construction, whereas it has more effect with grouting the wall, i.e. , there exists high interaction between mass and insulation.

CONCLUSIONS AND RECOMMENDATIONS

An analyis has been presented to predict the thermal performance of pre-insulated walls and compare them with other types of masonry wall construction for predominantly residential and Iight commercial applications in which the internai heat gain component is relatively small compared to gains through the envelope. The emphasis is on the thermo-physical aspects of the walls affecting heat transfer and quantitative calculations of potential energy savings rather than building economics or cost-effectiveness. While the studies presented here are in no way comprehensive in exploring ali the design variables that might be encountered, correlations of the results yield the following:

(1) Mass, thermal resistance and insulation location ali play a part in determining the energy efficiency of building envelopes; how('ver, their magnitudes differ, and they are not additive under dynamic conditions. The addition of insulation to the hollow block walls significantly reduced energy requirements under dynamic conditions.

(2) Differentials in heating and cooling loads between 25 mm and 50 mm pre-insulated concrete block masonry appear to be minor when companng thermal performance.

(3) Grouting has no significant effect on heating and cooling loads of pre-insulated walls. The effect of grout (thermal mass) is more evident with exterior insulation.

(4) Location of insulation (externai versus integral) has no appreciable difference in heating loads of hollow masonry walls in both the hot, dry and the moderate c1irnates. The difference is more significant in the cooling period of both c1imates.

As a result of these performance predictions, further study of the cost-effectiveness of concrete block rnasonry wall assemblies not only from the energy point of view, but also from the structural and constructional standpoints is warranted. There also appears to be a high potential for significant pay-off through energy savings by refining the design of the wall assembly.

REFERENCES

(1) The Building Loads Analysis & System Thermodynamics (BLAST) Computer Program, Blast Support Office, Urbana, Illinois 61801.

(2) Abdou, O. A., The Impact of Passive Solar Energy Utilization on Multi-story Apartment Houses in Hot Dry Climates, Doctoral Dissertation, University ofMichigan, 1987.

(3) Olgyay, V., Design with Climate: Bioclimatic Approach to Architectural Regionalism, Princeton University Press, Princeton, (1963).