THERMAL PERFORMANCE OF WALL-ROOF INTERSECTION AREAS IN THE EXTERNAL ENVELOPE OF RESIDENTIAL BUILDINGS M.C. ALTUN

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1 CIB World Building Congress, April 2001, Wellington, New Zealand Page 1 of 9 THERMAL PERFORMANCE OF WALL-ROOF INTERSECTION AREAS IN THE EXTERNAL ENVELOPE OF RESIDENTIAL BUILDINGS M.C. ALTUN Faculty of Architecture, Istanbul Technical University, Istanbul, Turkey E. ÖZKAN Faculty of Architecture, Beykent University, Istanbul, Turkey ABSTRACT In Turkey nearly all residential buildings do have a reinforced concrete (RC) skeleton structural system with brick walls and hipped roofs. Components of the structural system like, beams, columns or cantilevered slabs in the external envelope mostly break the continuity of the thermal insulation layer or the self-insulated wall, causing thermal bridges. In addition junctions between the exterior wall and other building elements like other walls, roofs, windows or doors are creating areas where the heat transfer is higher than a part of a wall free from thermal anomalies. Such thermal bridges not only increase heat transfer but also are potential cold surfaces for moisture accumulation. In evaluating thermal performance criteria of the external envelope, those areas of intersection should also be taken into consideration to assure a performance at the required level for the whole system. In this context, the intersection of exterior wall - hipped roof is studied, considering the heat flow through the combined construction. Various architectural details in different construction are analysed and evaluated. Alternative wall and roof constructions with and without thermal insulation are taken into consideration. In the analysis, the selected materials for construction of walls and roofs, placement and thickness of the thermal insulation are utilised as variables. The thermal performance is determined with the PC-program Heat 3. The program works with a transient, three dimensional, finite difference model for the heat transfer. The thermal performance of the alternative wall - roof combinations are evaluated with different criteria and compared to the overall thermal performance of the envelope systems. The thermal performance criteria that might be applied during the architectural detail design process is also discussed under the consideration of the integrated results of the analysis. KEYWORDS: Thermal performance; wall-roof intersection; thermal bridges; reinforced concrete structure; residential building. INTRODUCTION In the last decades of the twentieth century the overall thermal performance of buildings became a significant criterion in the design process, affecting the energy consumption, energy costs, carbon dioxide emission and environmental pollution, besides indoor comfort conditions. The external envelope of buildings is an important tool in achieving energy conservation and thermal comfort at the required levels with passive methods. As the energy conservation potential is fairly large in space heating of residential buildings, the evaluation of the external envelope s thermal performance is important in those kind of buildings, (Özkan, 1997).

2 CIB World Building Congress, April 2001, Wellington, New Zealand Page 2 of 9 In Turkey it is common practice to design and build nearly all residential buildings, from bungalows to high rise blocks, with a reinforced concrete (RC) skeleton structural system. The external envelope consists of hollow brick infill walls and hipped roofs with clay tile finishing. Components of the structural system like, beams, columns or cantilevered slabs in the external envelope mostly break the continuity of the thermal insulation layer or the self-insulated wall, causing thermal bridges. In addition junctions between the exterior wall and other building elements like other walls, roofs, windows or doors are creating areas where the heat transfer is higher than a part of a wall free from thermal anomalies. Such thermal bridges not only increase heat transfer but also are potential cold surfaces for moisture accumulation. In evaluating thermal performance criteria of the external envelope, those areas of intersection should also be taken into consideration to assure a performance at the required level for the whole system. A research project is set to develop building element coupling details with optimum thermal and moisture performance, (Özkan, 2000). In this context, the intersection of exterior wall - hipped roof is studied, considering the heat flow through the combined construction. Various architectural details in different construction are analysed and evaluated. Alternative wall and roof constructions with and without thermal insulation are taken into consideration. In the analysis, the selected materials for construction of walls and roofs, placement of the thermal insulation are utilised as variables. The thermal performance is determined with the PC-program HEAT 3. The program works with a transient, three dimensional, finite difference model for the heat transfer. METHODOLGY Various roof and wall construction type combinations are thermally analysed with the Computer program HEAT 3 under steady-state conditions and their thermal performance is evaluated with different criteria. Construction Types Fourteen construction alternatives for the intersection area of exterior wall - hipped roof are studied. Those are the combination of four different wall construction types and five different roof construction types. For all alternatives the type and dimensions of RC structural system components are taken as constant. The square shaped column s dimensions are 40/40 cm, the beams are 25/50 cm and the floor slab s thickness is 12 cm. The roof construction material is taken either as wood or reinforced concrete. The alternatives with the wooden roof construction do have either a thermal insulation layer placed on the RC slab (R1.1) or are without any thermal insulation (R 1.2). The alternatives with the sloped RC slab roof construction have either a thermal insulation on the external surface (R 2.2) or the internal surface (R 2.1). A third alternative is RC slab roof construction without any thermal insulation (R 2.3). The wall construction consists either of 13.5 cm thick hollow bricks or 25 cm thick aerated concrete blocks. Wall types with hollow bricks are either insulated internally (W1) or externally (W2). Constructions with the aerated concrete blocks; either have a thermal insulation layer on the external surface of the RC components, columns and beams namely (W3) or do not have any thermal insulation (W4). The insulation thickness for the wall and roof construction is taken constant as 5 cm and 8cm, respectively. The U-value for all wall constructions is approximately 0.6 W/m 2 K and for all roof constructions approximately 0.4 W/m 2 K, fulfilling the limitations in the Turkish Standard for the region of Istanbul (TS 825, 1998), with the exception for the roof constructions without any thermal insulation. For all construction alternatives the roof ends with RC eaves of 40 cm. The examined construction alternatives for the wall- roof intersection areas are given Figure 1. The properties of the materials used in the constructions are given in Table 1.

3 CIB World Building Congress, April 2001, Wellington, New Zealand Page 3 of 9 Table 1. Properties of the materials used in the constructions. Density Thermal Conductivity λ, [W/mK] Specific Heat Capacity c,[j/kgk] Volumetric Heat Capacity C,[J/m 3 K] γ, [kg/m 3 ] RC E6 Hollow Brick E6 Aerated Conc. Block E6 Polystyrene E4 Wood E6 Plaster, cement-lime E6 Gypsum Board E6 Calculation Method The thermal performance is determined with the PC-program Heat 3 which is using a transient, three dimensional, finite difference model for heat transfer (Blomberg, 1999). In the PC-program, all building parts to be analysed have to be described through the corner coordinates of rectangular prisms, positioned parallel to the x-, y- or z- directions of a Cartesian coordinate system. In this study, some components of the examined building parts are geometrically modified in order to make the description possible. Those are mainly the conversion of sloped roofs into horizontal constructions. The model selected for the study comprises hollow brick walls, RC structural components such as beams, a column and a slab that meet with a part of the roof in a corner. Cut off planes for the corner 3-D model are located 1 m in distance from the column and beam surfaces in every direction as recommended in (ISO , 1995). Under the assumptions for the airspace in the attics; to be slightly ventilated, to be bounded by opaque surfaces of an emissivity KDYLQJ D PHDQ WHPSHUDWXUH RI ƒ& WR EH KRUL]RQWDOO\ SRVLWLRQHG and for an upward direction of heat flow the thermal resistance is taken as 0.08 m 2 K/W (ASHRAE, 1989), (ISO , 1995). The equivalent thermal conductivity of air cavities is calculated according its thickness for each alternative. Heat flows and surface temperatures for both wall and roof are calculated for steady-state conditions. Climatic Conditions In the simulations, both the indoor and outdoor temperatures are considered to be constant; 21 C and 0 C respectively. The indoor and outdoor wind velocity is also considered to be constant and so the film coefficients are taken as 0.13 W/m 2 K for the inner and 0.04 W/m 2 K for the outer surface.

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6 CIB World Building Congress, April 2001, Wellington, New Zealand Page 6 of 9 Thermal Performance Evaluation For the evaluation of the thermal performance of the designed constructions two criteria are used; the heat flow rate per square meter and the inner surface temperatures. As the heat flow rate of the external envelope is affecting the energy consumption, energy costs, carbon dioxide emission and environmental pollution, it is an important thermal performance criterion. Heat flow rates of each examined construction is compared with a wall-roof heat flow rate as recommended in local regulations. The regulatory heat flow rate is calculated with an area weighted method, using the recommended U-values of 0.6 and 0.4 W/m 2 K respectively for the opaque components of external walls and roofs in the Turkish Standard for the region of Istanbul (TS 825, 1998). For the area-weighted method, the dimensional properties of a sample five storey high residential building, 24.0x11.0x15.0m in size, are used. The characteristics of the sample building can be summarised as; roof area: 264 m 2, opaque wall area (including RC structure and wall construction): 735 m 2, window/door area: 315 m 2. The total opaque external envelope above ground consists of 26.4 % roof area and 73.6 % opaque wall area. The regulatory wall-roof heat flow rate, comprising the opaque components of the sample building s external envelope, is calculated as W/m 2, for an indoor temperature of 21 C and an outdoor temperature of 0 C. The inner surface temperature is affecting both the thermal comfort and risk of surface condensation. Where as local low surface temperatures, in small areas of the opaque wall component, are negligible from the thermal comfort point of view, they can still cause surface condensation and related harmful effects to the user. Inner relative humidities which might cause condensation at the surfaces are determined according to the calculated lowest inner surface temperature values and psychrometric charts (ASHRAE, 1989). The mean inner relative humidity value varies from room to room in residential buildings (60%-46%) affected various factors (Garratt, 1991). A mean inner relative humidity value of 50 %, which satisfies also the indoor climatic comfort conditions for an indoor temperature of 21 C, is taken as a limit value. Constructions with an inner surface temperature value, which allows surface condensation at an indoor R.H., lower than 50 % are classified as riskfull. THERMAL ANALYSIS OF THE CONSTRUCTIONS Heat flow per square meter, maximum / minimum inner surface temperatures and inner relative humidities which might cause condensation at the surfaces of each wall-roof intersection alternative s corner 3-D model are determined. The heat flow rates for the corners with a roof with wooden construction, thermal insulation and attic space (R1.1), varies according to the wall construction type. Those are W/m 2, W/m 2, W/m 2 and W/m 2 for the wall types with hollow bricks internally insulated (W1) or externally insulated (W2), aerated concrete blocks with thermal insulation on the external surface of the RC components (W3) and aerated concrete blocks without any thermal insulation (W4) respectively. The heat flow rates for the corners with a roof of an internally insulated RC construction (R 2.1) are W/m 2, W/m 2, W/m 2 and W/m 2 for the wall types with hollow bricks internally insulated (W1) or externally insulated (W2), aerated concrete blocks with thermal insulation on the external surface of the RC components (W3) and aerated concrete blocks without any thermal insulation (W4) respectively. The heat flow rates for the corners with a roof of an externally insulated RC construction (R 2.2) are W/m 2, W/m 2, W/m 2 and W/m 2 for the wall types with hollow bricks internally insulated (W1) or externally insulated (W2), aerated concrete blocks with thermal insulation on the external surface of the RC components (W3) and aerated concrete blocks without any thermal insulation (W4) respectively. The heat flow rate for

7 CIB World Building Congress, April 2001, Wellington, New Zealand Page 7 of 9 the corner with a roof with wooden construction and attic space but without thermal insulation and a wall construction of aerated concrete blocks again without any thermal insulation (W4R1.2) is W/m 2, where as the heat flow rate for the corner with a roof with a RC construction and a wall construction of aerated concrete blocks both without any thermal insulation (W4R2.3) is W/m 2. The heat flow rate for the corner construction W1R2.1 is less by 1.7% compared to the regulatory wall-roof heat flow rate. All other corner constructions have a higher heat flow rate, ranging from 34.5 % for W1R1.1 to % for W4R2.3 compared to the regulatory wall-roof heat flow rate. For each examined alternative, the heat flow rates and percentage of the difference between the overall wall-roof heat flow rate of the sample building and heat flow rates of the corner construction of the wall-roof intersection area are given in Table2. Table 2. Heat flow rates (HF), and percentage of the difference between the overall wall-roof heat flow rate (11.46 W/m 2 ) of the sample building and heat flow rates of the corner construction of the wall-roof intersection area for different alternatives. Construction Heat Flow [W/m 2 ] Difference [%] W1R W2R W3R W4R W4R W1R W2R W3R W4R W1R W2R W3R W4R W4R For all the examined alternatives, the minimum inner surface temperature is located at the corner of the RC column-beam-slab intersection point. The minimum inner surface temperature for the corners with a roof with wooden construction, thermal insulation and attic space (R1.1) are 8.93 C, C, 12.89, 9.86 for the wall types with hollow bricks internally insulated (W1) or externally insulated (W2), with aerated concrete blocks with thermal insulation on the external surface of the RC components (W3) and with aerated concrete blocks without any thermal insulation (W4) respectively. The minimum inner surface temperature for the corners with a roof of an internally insulated RC construction (R 2.1) C, 8.64 C, 8.20 C, 6.41 C for the wall types with hollow bricks internally insulated (W1) or externally insulated (W2), aerated concrete blocks with thermal insulation on the external surface of the RC components (W3) and aerated concrete blocks without any thermal insulation (W4) respectively. The minimum inner surface temperature for the corners with a roof of an externally insulated RC construction (R 2.2) are 9.54 C, C, C, C for the wall types with hollow bricks internally insulated (W1) or externally insulated (W2), aerated concrete blocks with thermal insulation on the external surface of the RC components (W3) and aerated concrete blocks without any thermal insulation (W4) respectively. The minimum inner surface temperature for the corner with a roof with wooden construction and attic space but without thermal insulation and a wall construction of aerated concrete blocks again without any thermal insulation (W4R1.2) is 9.77 C, where as the minimum inner surface temperature for the corner with a roof of RC construction and a wall construction of aerated concrete blocks both without any thermal

8 CIB World Building Congress, April 2001, Wellington, New Zealand Page 8 of 9 insulation (W4R2.3) is 3.68 C. The lowest indoor relative humidity value with condensation risk is with R.H.=30 %, the corner construction W4R2.3 where as the highest indoor relative humidity value with condensation risk appears for the corner construction W1R2.1 with R.H.=70%. For each studied alternative the maximum / minimum inner surface temperature and the indoor relative humidity value with condensation risk at the corresponding inner surface temperature at a corner of the wall-roof intersection area, are given in Table 3. Table 3. Heat flow rates (HF), maximum inner surface temperatures (T max ), minimum inner surface temperature (T min ) and indoor relative humidity value (RH) with condensation risk at (T min ) at a corner of the wall-roof intersection area for different construction alternatives. Construction Heat Flow [W/m 2 ] T max T min RH [%] [ C] [ C] W1R W2R W3R W4R W4R W1R W2R W3R W4R W1R W2R W3R W4R W4R CONCLUSION The thermal performance of fourteen construction alternatives for the intersection area of exterior wall - hipped roof which are the combination of four different wall construction types and five different roof construction types, in residential buildings, is calculated with the PC-program HEAT 3. HEAT 3 is using a steady-state, three dimensional, finite difference model for heat transfer. For the evaluation of the thermal performance of the examined constructions two criteria are used; the heat flow rate per square meter and the inner surface temperatures. The results of the calculations are examined and compared and the following series of conclusions are developed: The external envelope of buildings is a combination of details, subsystems and envelope area free from thermal anomalies and cannot be accurately assessed simply by considering the envelope area free from thermal anomalies from the thermal performance point of view. By evaluating the thermal performance at the building element level also the junction areas should be taken into consideration. Such areas have mostly a larger heat flow rate compared to the building element itself. Also in such areas the inner surface temperature is lower than in areas free from thermal anomalies, not affecting the thermal comfort significantly but causing surface condensation at certain points. Heat flow rate per square meter and the inner surface temperatures are criteria which can be used in the thermal performance assessment of building element coupling areas adequately.

9 CIB World Building Congress, April 2001, Wellington, New Zealand Page 9 of 9 The internally thermally insulated wall alternative has in all three insulated roof combinations the lowest heat flow rate, where as the wall alternative with aerated concrete blocks without any thermal insulation has in all three roof combinations the highest heat flow rate. The heat flow rate for the both internally insulated wall and roof alternative W1R2.1 is the lowest among the examined combinations. The minimum inner surface temperatures for the alternatives; W1R1.1, W4R1.1, W4R1.2 W2R2.1, W3R2.1, W4R2.1, W1R2.2, W4R2.3 are at a level which causes surface condensation at indoor relative humidity lower than 50 %. So that those constructions can be classified as risky from the surface condensation point of view. The minimum inner surface temperatures for the alternatives; W2R1.1, W3R1.1, W1R2.1 W2R2.2 W3R2.2 W4R2.2 are at a level which causes surface condensation at indoor relative humidity above 50 %. Those constructions do not have a risk of surface condensation under the considered conditions. In evaluating the thermal performance with both, the heat flow rate and inner surface temperature criteria the best performing alternative is the internally insulated wall and roof alternative W1R1.2 among the studied constructions. This is the only alternative with a continuity in thermal resistance at the required level. The alternatives with externally insulated walls and those with aerated concrete blocks with thermal insulation on the external surface of the RC components perform still at an acceptable level. ACKNOWLEDGEMENT The research project is supported by TÜBITAK (The Scientific and Technical Research Council of Turkey). REFERENCES ASHRAE 1989, ASHRAE Handbook Fundamentals, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., Atlanta. Blomberg, T HEAT3 - PC Program for Heat Transfer in Three Dimensions - Manual with Brief Theory and Examples, Lund Group for Computational Building Physics (ISRN LUTVDG/TVBH 98/7206), Lund. Garratt, J. Nowak, F Tackling Condensation, Building Research Establishment Report, BRE, Watford. ISO Thermal Bridges in Building Construction-Heat Flows and Surface Temperatures-Part1:General Calculation Methods, ISO, Genève. Özkan, E., Altun, M.C., Tavil, A. and Sahal, A.N Developing energy efficient external envelope by retrofitting in rehabilitation of existing residential buildings. TÜBITAK - INTAG / TOKI 223, Research Project, Final Report (in Turkish), Istanbul. Özkan, E. and Altun, M.C Building element coupling details with optimum thermal and moisture performance. TÜBITAK - INTAG 234, Research Project (ongoing), Istanbul. TS Thermal Insulation in Buildings, (in Turkish), TSE, Ankara.