J. Cent. South Univ. (2012) 19: 1645-1656

DOI: 10.1007/s11771-012-1188-x

Analyses on performances of heat and multilayer reflection insulators

LEE Moo-jin¹, LEE Kang-guk², SEO Won-duck³

1. School of Architecture, Kyungil University, Gyeongsan 702 701, Korea;

2. Research Center for Urban Affair, Kyungil University, Gyeongsan 702 701, Korea;

3. Department of Architectural Design, Joongbu University, Geumsan-gun 701 140, Korea

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: This research was conducted to study the performances of the heat and multilayer reflection insulators used for buildings in South Korea to realize eco-friendly, low-energy-consumption, green construction, and to contribute to energy consumption reduction in buildings and to the nation’s greenhouse gas emission reduction policy (targeting 30% reduction compared to BAU(business as usual) by 2020). The heat insulation performance test is about the temperatures on surfaces of test piece. The high air temperature and the low air temperature were measured to determine the overall heat transfer coefficient and thermal conductivity. The conclusions are drawn that the heat transmission coefficients for each type of existing reflection insulator are: A-1       (0.045 W/(m·K)), A-2 (0.031 W/(m·K)), A-3 (0.042 W/(m·K)), A-4 (0.078 W/(m·K)), and the average heat conductivity is 0.049 W/(m·K); The heat conductivity for each type of Styrofoam insulator are 0.030 W/(m·K) for B-1, 0.032 W/(m·K) for B-2, 0.037 W/(m·K) for B-3, 0.037 W/(m·K) for B-4, and the average heat conductivity is 0.035 W/(m·K) regardless of the thickness of the insulator; The heat conductivity values of the multilayer reflection insulators are converted based on the thickness and type C-1 (0.020 W/(m·K)), C-2 (0.018 W/(m·K)), C-3 (0.016 W/(m·K)), and C-4 (0.012 W/(m·K)); The multilayer reflection insulator keeps the indoor-side surface temperature high (during winter) or low (in summer), enhances the comfort of the building occupants, and conducts heating and moisture resistance to prevent dew condensation on the glass-outer-wall surface.

Key words: heat reflection insulator; multilayer reflection insulator; overall heat transfer coefficient; thermal conductivity; nonflammability; vapor permeability; eco-friendly construction

1 Introduction

South Korea should to some extent reduce its greenhouse gas emission and energy consumption to obtain the status of an advanced country in preparation for the evaluation of the effectiveness of Second Climate Change Treaty in 2013. In 2009, South Korea, a country with limited natural resources that relies on imports for a large part of its national energy consumption, had the following energy consumption distribution: 55% for the industrial sector, 22.3% for buildings, and 22.7% for the traffic sector [1-2]. The reduction of the energy consumption of buildings directly affects the overall reduction of carbon dioxide (CO₂) as the total greenhouse gas emission from the South Korean buildings accounts for 35%-40% of the total emission [3]. Therefore, the greenhouse gas emission in the building sector is targeted to be reduced by 31% against the expected emission value by 2020 [2]. Moreover, as South Korea produces about 140×10⁶ m² of new buildings every year, and the total building floor area of the country has reached approximately 2.2×10⁹ m² by the end of 2009, there is an urgent need to prepare a concrete energy consumption and CO₂ emission reduction plan. In particular, the heat loss and acquisition through the outer skins of buildings account for the largest portion of the building energy consumption and for more than 40% of the cooling and heating loads of buildings on average. Therefore, design and construction with optimal insulation should be considered as a prerequisite and a top priority for energy savings in buildings [2-3]. Technological researches are actively being carried out in the related fields in the international insulation technology sector, in response to the common risk situation of fossil energy exhaustion and global climate change. Moreover, researches related to the insulator comprise developments and studies to enhance the energy effectiveness of buildings in the detailed fields of new-material insulator models and assessment tools for insulation performances, and relevant researches are being conducted according to the national differences in climate, energy-related policy, resources, economy, industry, and trading policy. The adoption of nonflammable mineral materials and related laws and regulations is being accelerated, however, the existing organic-substance and multiple insulators have limitations due to their toxic-gas emission (fire stability).

Particularly, in South Korea, according to the guidelines of the overall heat transmission coefficient for building parts by region specified in the Partial Revisions (Proposal) of the Rules for the Equipment and Facilities of Buildings, which was announced in April 2010 and has been in force since January 1st, 2011, the outer wall of a living room directly facing the open outdoor air in the central district of South Korea requires less than    0.33 W/(m²·K), and the side walls of the apartments  0.25 W/(m²·K), which can be achieved through the use of insulation materials thicker than 85 and 125 mm, respectively, based on the class A heat protection panels processed by extrusion. Moreover, the amendment (proposal), which is now in force, requires that the thickness of the insulation materials should be strengthened by 30%-40% as the energy-saving standard of buildings is required to be reinforced by about 20%-30% by July 1st, 2011. The insulation of the outer skins of buildings in South Korea has been conducted mainly with the use of heat resistance insulators such as Styrofoam and glass fiber. The use of reflection insulators is increasing for buildings with stone outer-skin finishing. The preceding researches and technological studies in relation to the facts are: Impact of the reinforced standard for envelope insulation on heating and cooling energy consumption in a Korean detached house as well as identifying the effect of regionally subdivided standards [4]; The possibility of complex insulating wall combination using reflective insulation [5]; The magnitude of energy savings as a result of using thermal insulation varying according to the building type; The climatic conditions at which the building is located as well as the type of the insulating material used [6]; Thermal insulation material for building applications is a promising technique but further investigations are needed in order to enhance fire prevention [7]. Aerogels are regarded as one of the most promising high performance thermal insulation materials for building applications today [8]. Vacuum insulation panels (VIPs) are regarded as one of the most promising high performance thermal insulation solutions on the market. Thermal performances are three to six times better than still-air are achieved by applying a vacuum to an encapsulated micro-porous material, resulting in a great potential for combining the reduction of energy consumption in buildings with slim constructions [9]. Examples of these may be mineral wool, expanded polystyrene, extruded polystyrene, polyurethane, vacuum insulation panels, gas insulation panels, aerogels, and future possibilities like vacuum insulation materials, nano-insulation materials and dynamic insulation materials. Various properties, requirements and possibilities have been compared and studied [10].

As South Korea will become a subject nation in 2013, and it assumes the full-fledged responsibility of reducing its greenhouse gas emission, there is a need to develop an eco-friendly insulator. At present, it is apparent that the realization of the 21st century low-CO₂ society is one of the highest-priority issues, and a sustainable building energy system can be realized through the considerable reduction of environmental pollutants like greenhouse gases if the multilayer reflection insulation technology is introduced to the insulators and claddings of buildings. The destructive method instead foresees the acquisition of a sample by the use of a hollow drill, the building envelope layers thickness measurement and the thermal properties assignment to each different material. The wall R-value is the sum of each layer thermal resistance [11]. The use of vacuum insulation panels in buildings has gradually increased in the past few years and developments in their production have occurred in parallel. This has mainly led to an optimization of different hygro-thermal properties of the core material and envelope. The issue of thermal bridges caused by the 300 nm thin metallic layers of barrier envelope and by the joints between two adjacent panels remains [12]. Three methods for the analysis of in situ data are also presented to determine the thermal resistance of buildings although the R-values evaluated by these methods have smaller values than those of design due to the limitation of field conditions [13]. The existing heat reflection insulators, which have very low insulation capacity, are inappropriate for use as the outer-wall insulators of buildings, and cannot protect life and property during a fire due to their toxic-gas emission. The multilayer reflection insulators are considered as eco-friendly green insulators which meet the rules covering the overall heat transmission standard with their high insulation capacity, and are not harmful to the human body. Moreover, the existing insulators, such as Styrofoam, urea form, and the heat reflection insulator, have caused much damage to life and property due to combustion and toxic-gas emission during a fire, but the multilayer reflection insulators have high insulation capacity and other merits, such as nonflammability, vapor permeability, and shortening of the construction period [14].

Therefore, this research was conducted to study the performances of the heat and multilayer reflection insulators used for buildings in South Korea, to realize eco-friendly, low-energy-consumption, and green construction, and to contribute to energy consumption reduction in buildings and the nation’s greenhouse gas emission reduction policy (targeting 30% reduction compared to BAU by 2020).

2 Summary of heat transmission and experiment

2.1 Necessity of study

KS L9016 (KS L9016 is the quality test standard that states the overall heat transfer coefficient standard for construction materials in Korea) specifies the method for measuring the thermal conductivity of heat insulators. KS F2277 is the quality test standard that states the heat resistance or overall heat transfer coefficient measurements for the heat insulation of construction materials in Korea, i.e., for walls, floors, and ceilings, on the other hand, is the method for measuring the heat insulation (overall heat transfer coefficient) of building component materials. The heat insulation performance (thermal conductivity) of volume type insulators (e.g., Styrofoam and glass wool) can be evaluated according to KS L9016. The heat insulation performance of the heat reflection insulator (6-13 mm thick) for Building, however, is not indicated on the product; only that of the building component, which contains the interior heat reflection insulator, is indicated. If it is assumed that the heat insulation of the reflective sheet (i.e., aluminum sheet) in the air under normal condition is similar to that of the heat insulator with a  10 mm-thick glass wool, the heat insulation of the heat reflection insulator is 1/7 of the 75 mm-thick Styrofoam (bead method heat insulation plate No. 3). This is based on the building skin heat insulator thickness standard (Enforcement Decree of the Building Act for the central district of Korea) which is unsuitable as a building skin insulator [14].

Therefore, the insulation of the heat reflection insulator, as well as the volume type heat insulators must also be evaluated to determine whether it is a suitable building skin insulator. The overall heat transfer coefficient and thermal conductivity of the multilayer reflection insulator in this work are also measured and evaluated to analyze the heat insulation performance. Thermal insulation of buildings (R_o) is defined by     Eq. (1), in which,  is the quotient of the D-value of the mean temperature between two surfaces of a wall and the mean heatflow rate through an identical wall (as given in Eq. (2)); R_e and R_i are the reciprocals of coefficient of heat transfer of the outside and inside insulator building, respectively (as given in Eqs. (3) and (4)).

                                                           (1)

                                (2)

                                    (3)

                                    (4)

2.2 Method of heat insulation performance test

The heat insulation performance test in this work is summarized as follows. The temperatures on surfaces of the test piece (θ_i, θ_o), the high air temperature (t_i), and the low air temperature (t_o) were measured to determine the overall heat transfer coefficient and thermal conductivity. The most important factor to ensure the measurement reliability of the overall heat transfer coefficient and thermal conductivity is to keep the air temperatures as constant on both sides. In this experiment, the air temperature on the high-temperature side was kept as constant at ±0.1 ℃ by supplying a constant current to a plane heater. The low-temperature side was kept as constant at ±0.05 ℃ in the environmental chamber with sufficient heat insulation on the floor, wall and ceiling.

As shown in Fig. 1, the low-temperature side (A, constant temperature environmental chamber) and the high-temperature side (C, chamber heated by the plane heater) were positioned on each side of the test piece (B). The air temperatures on both sides were kept constant. If the heat transfer occurs through the test piece from the high-temperature side to the low-temperature side, the overall heat transfer coefficient and thermal conductivity are then calculated using the differences in air and surface temperatures on both sides.

Fig. 1 Overall heat transfer coefficient test equipment: A- Low temperature unit; B-Test piece for overall heat transfer coefficient; C-High temperature unit

Figure 2 shows the test chamber to measure the overall heat transfer coefficient. A plane heater was installed on the floor inside the chamber to maintain the high-air temperature (C). Four test pieces were mounted on the upper part of the chamber to evaluate the heat insulation performance of each test piece under the same temperature condition. Figure 3 shows the temperature measurement equipment (i.e., with the data logger, infrared thermal imaging device, and infrared thermometer). The channel with the allowable temperature sensor error of 0.05 ℃ or less was selected for the data logger, and a copper-constantan (cc) thermocouple with 0.2 mm in diameter, was used as the temperature sensor. The positive and negative electrodes have the same materials. The measurement temperature range is from -200 ℃ to 300 ℃.

Fig. 2 Overall heat transfer test chamber

Fig. 3 Data logger and thermal imaging device

The difference between the air temperature on the low-temperature side (A) and that on the high- temperature side (C) was kept at (30±1) ℃, considering the indoor/outdoor temperature difference in winter in Korea. The change in the air temperature on the low-temperature side (A) and high-temperature side (C) was 1% or less per hour. The overall heat transfer coefficient and thermal conductivity were determined as the average values when the change in the measurements was within 1% for at least 3 h. To measure the thermal conductivity and overall heat transfer coefficient, the heat transfer coefficient for both sides of the test piece was determined to be 8.141 W/(m²·K), considering the temperature and the heat transfer direction [15].

3 Insulation capacity and problems of heat reflection insulator used in buildings

3.1 Insulation capacity of heat reflection insulator used in buildings

The reflection insulator became popular when the aluminum-surface reflection insulator in the market in the 1930s, since the patent acquisition of the reflection- type insulator used the bright-colored metal surface in 1804.

The reflection insulator has a very low emissivity or E value (0.03 in general; for most insulators, 0.90) and significantly reduces heat transfer by radiation. Using aluminum thin plates with low-emissivity surfaces as the core material, it has a lamination structure in which a number of thin materials (e.g., an air space layer or a non-woven fabric considering the durability, damp-proof property, transportation, loading, workability, etc.) are affixed to one another. The heat reflection insulator, which has the merits of a smaller volume and excellent site workability compared to polystyrene foam, is available in roll type and can be curved and flexed, making it suitable for columns, beams, and corners. It also has very effective characteristics for heat exchange shutting-off [16]. The reflection insulator with the aforementioned characteristics is mainly used in buildings with marble, granite, and base panel finishing (see Figs. 4 and 5).

Fig. 4 Reflection insulator used in buildings (1)

Fig. 5 Reflection insulator used in buildings (2)

3.1.1 Type of heat reflection insulator (Type A) used in buildings

The types of heat reflection insulator (Type A) used in buildings have non-woven fabric and PE foam on the back surface of the aluminum reflective film with a thickness of 6-13 mm. They are presently used as heat insulators for many new buildings. The heat reflection insulator types that were used in the heat insulation performance test (Type A) were Type A-1 (10 mm thick, manufactured by ChangJin Insulation Private company), Type A-2 (11 mm thick, manufactured by SeIl Insulation Ltd. company), Type A-3 (7 mm thick, manufactured by Ohyung Insulation Private company), and Type A-4 (6 mm, manufactured by Cheil Insulation Private company), as shown in Fig. 6.

3.1.2 Overall heat transfer coefficient and thermal conductivity of heat reflection insulator

Figure 7 shows the aging change of the overall heat transfer coefficient of the heat reflection insulator. The air temperature difference between both sides of the test pieces was kept constant at about 30 ℃ (high air temperature side, 43.3 ℃; low air temperature side,  13.6 ℃).

Fig. 6 Heat reflection insulator type (Type A) (Unit: mm): (a) Type A-1; (b) Type A-2; (c) Type A-3; (d) Type A-4

Fig. 7 Aging change in overall heat transfer coefficient of heat reflection insulator by insulator type
 

The overall heat transfer coefficient and thermal conductivity were determined as the average values for  9 h (from 19:30 March 27, 2010 to 11:40 March 28, 2010) when the change in the air temperature on the low- and high-temperature sides was 1% or less in 1 h, and the change in the measurements was 1% or less in at least  3 h.

The ascending order of overall heat transfer coefficients of heat reflection insulators was        1.88 W/(m²·K) for Type A-2; 2.19 W/(m²·K) for Type A-3; 2.30 W/(m²·K) for Type A-1; and 3.13 W/(m²·K) for Type A-4. Type A-2 (11 mm thick, manufactured by SeIl Insulation Ltd. company) had the highest heat insulation performance, and Type A-4 had the lowest  (6 mm thick, manufactured by Cheil Insulation Private company).

In the comparison of the surface temperatures of the existing Type A heat reflection insulators (see Fig. 8), the lowest surface temperature was found to be that of A-2 (16.5 ℃), and the highest was that of A-4 (22.4 ℃). which indicates that the heat reflection insulator has less heat insulation performance than the Styrofoam with the same thickness. Demand for energy efficient buildings has increased drastically in recent years and this trend will continue in the future. Insulating building elements will play a key role in meeting this demand by reducing heat losses through the building fabric [17].

Fig. 8 Surface temperatures of existing Type A heat reflection insulators

3.1.3 Styrofoam (bead method) insulator type (Type B)

To analyze the heat insulation measurement reliability for the heat reflection insulator and multilayer reflection insulator, the overall heat transfer coefficient and thermal conductivity of Styrofoam (bead method), which are most widely used as the heat insulator in Korea, were measured. Bead method heat insulator No. 3 was selected as the test piece. The test piece types were Type B-1 (20 mm thick), Type B-2 (38 mm thick), Type B-3 (58 mm thick), and Type B-4 (77 mm thick), as shown in Fig. 9.

Fig. 9 Styrofoam insulator type (Type B) (Unit: mm): (a) Type B-1; (b) Type B-2; (c) Type B-3; (d) Type B-4

3.1.4 Aging change of overall heat transfer coefficient of Styrofoam

Figure 10 shows the aging change of overall heat transfer coefficient of Styrofoam insulator (Type B). The air temperature difference between both sides of test pieces was kept as constant at about 30 ℃ (high air temperature side, 44.2 ℃; low air temperature side,  14.3 ℃). The overall heat transfer coefficient and thermal conductivity were determined as the average values for 11 h and 50 min (22:00, March 24, 22:00 2010 to 09:50, March 25 2010) when the change in the air temperature on the low- and high-temperature sides was 1% or less per hour, and the change in the measurements was 1% or less in at least 3 h. The descending order of the overall heat transfer coefficients for Styrofoam was Type B-1 (1.07 W/(m²·K); Type B-2 (0.70 W/(m²·K)); Type B-3 (0.57 W/(m²·K)); and Type B-4 (0.44 W/(m²·K)), as shown in Fig. 11.

As for the comparison of surface temperatures of Type B Styrofoam (bead method) insulators, B-4 shows the lowest surface temperature (15.5 ℃), and B-1 the highest (17.3 ℃).

3.2 Problems and present status of heat reflection insulator used in buildings

Based on the test results, the heat insulation characteristic of heat reflection insulator and Styrofoam (bead method) insulator is summarized as follows:

1) Heat insulation performance

The heat insulation performances (overall heat transfer coefficient) of four heat reflection insulators (6-13 mm thick) were similar to or less than those of the same thickness of Styrofoam. It was 1/7 of the heat insulator thickness standard in the enforcement decree of the Building Act (exterior wall in the central district of Korea, 75 mm or more thick Class B Styrofoam), which indicates that the heat insulation performance of the insulator itself is very low. Therefore, this insulator is an unsuitable heat insulator for the building skin to meet the standard of the building act. The building with this type of insulators involves very large heat loss though its exterior; it therefore consumes a lot of energy.

2) Degradation of heat insulation performance according to heat transfer direction

The heat reflective layer of the insulator (Al foil layer) must have air layers on both sides to block 98% or more of radiant heat. Only one side of the heat reflection insulator faces the air because its surface is the heat reflective layer which has a heat insulation material such as non-woven fabric on the back. Its radiant heat blocking capability is thus reduced. In addition, this type of heat insulator is attached to building skin with its heat reflective surface facing air layer and exterior materials (e.g., stones, bricks, and exterior panels, among others) are constructed with an air layer of about 50 mm. Therefore, the exterior wall thickness increases while the available space decreases.
 

Fig. 10 Aging change of overall heat transfer coefficient of Styrofoam insulator

Fig. 11 Surface temperatures of Type B Styrofoam (bead method) insulators

3) Degradation of heat insulation performance according to damage and contamination of surface

The surface of the Al foil must be kept with an emissivity of 0.05 or less to efficiently block the radiant heat. This blocking effect of heat reflection insulator is reduced when its heat reflective surface is scratched or damaged during transport. In addition, the dusts that have accumulated on its surface for a long time will also degrade the radiant heat insulation performance.

4 Insulation capacity of multilayer reflection insulator and performance of heat reflection insulator used in buildings

4.1 Heat movement and types of multilayer reflection insulator

4.1.1 Heat movement cut-off mechanism (radiation, conduction, and convection)

Bulky insulators such as glass wool, Styrofoam, and cellulose can decrease the thermal conductivity by reducing the conduction and convection phenomena using the principle of confined air, but they show very low radiant heat cut-off effects. The metal thin panels (Al foil) of reflection insulator, however, are regarded as blocking off approximately more than 90% of the radiant heat conduction, owing to its low emissivity (less than 0.05).

1) Radiant heat: More than 98% of the radiant heat is blocked off by each layer of 2-8 Al foils.

2) Conductive heat: The movement of the conductive heat is effectively blocked off as each layer of 2-8 Al foils is separated by the insulated isolation material.

3) Convective heat: The movement of convective heat is effectively blocked off as each layer of 2-8 Al foils is separated with every 10 mm gap by the insulated isolation material, which is arranged in every 100 mm gap horizontally and longitudinally to block the air in between.

Figure 12 shows the basic shape of multilayer reflection insulator. Between both surface layers, multiple Al foil layers (two to eight layers) are placed on air layers with constant gap on both sides, spaced by separators. Al foils are also attached to the back of the surface layers. To increase the heat insulation effect of the multilayer reflection insulator for radiation and convection, the air gap between Al foil layers were set at 10-15 mm. Separators were placed to keep the air layer gaps as constant. Separators were positioned at 100-  150 mm intervals horizontally and vertically to maximize the heat insulation by blocking the radiation, convection, and transfer of heat. When the surface material of the multilayer reflection insulator is nonflammable, the product likewise becomes a nonflammable high- performance insulator because Al foil is nonflammable. If glass, metal, and cement panels are used as the surface material, the product becomes a nonflammable high- performance insulation exterior material. This high- performance heat insulation material does not require additional heat insulation and it can be used as the exterior heat insulation material on the exterior wall, which is a precondition for eco-friendly construction.

Fig. 12 Structure of heat reflection insulator (Unit: mm)

4.1.2 Multilayer reflection insulator type (Type C)

To analyze the heat insulation performance of the multilayer reflection insulator, the Al foil layers were placed at 10 mm intervals between surface layers; the separators (5 mm×10 mm) were installed at 100 mm intervals horizontally and vertically to keep the air layer gap (10 mm) between the Al foil layers. The test piece types were Type C-1 (20 mm thick), Type C-2 (30 mm thick); Type C-3 (40 mm thick), and C-4 type (50 mm thick) as shown in Fig. 13. The Al foil thickness was  16 μm, and the emissivity was 0.04.

4.1.3 Overall heat transfer coefficient and thermal conductivity of multilayer reflection insulator

Figure 14 shows the aging change in the overall heat transfer coefficient of the multilayer reflection insulator. The air temperature difference between both sides of test pieces was kept at about 30 ℃ (high air temperature side, 44.2 ℃; low air temperature side: 13.9 ℃). The overall heat transfer coefficient and thermal conductivity were determined as the average values for 9 h and 30 min (from 21:30 March 21, 2010 to 08:50 March 22, 2010) when the temperature change on the low- and high-temperature sides was 1% or less per hour, and the change in the measurements was 1% or less for in least 3 h.

Fig. 13 Multilayer reflection insulator type (Type C): (a) Type C-1; (b) Type C-2; (c) Type C-3; (d) Type C-4

Fig. 14 Aging change in overall heat transfer coefficient of multilayer reflection insulator by insulator type

The descending order of overall heat transfer coefficients for multilayer reflection insulators was Type C-1 (0.80 W/(m²·K)), Type C-2 (0.53 W/(m²·K)), Type C-3 (0.36 W/(m²·K)), Type C-4 (0.23 W/(m²·K)), as shown in Fig. 15. Type C-4 has the highest heat insulation performance which corresponds to the     150 mm-thick Styrofoam. However, the multilayer reflection insulator had a lower thermal conductivity as it became thicker. For the surface temperature of the C-type multilayer reflection insulators, C-4 showed the lowest (13.9 ℃) and C-1 the highest (15.3 ℃).

Fig. 15 Surface temperature of C-type multilayer reflection insulators

4.2 Heat insulation performance of heat reflection insulator and existing insulator

A significant element in the cost of a new building is devoted to fire safety. Energy efficiency drives the replacement of traditional building materials with lightweight insulation materials, which can contribute to the fire load if flammable. Most fire deaths arise from inhalation of toxic gases. The fire toxicity of six insulation materials (glass wool, stone wool, expanded polystyrene foam, phenolic foam, polyurethane foam and polyisocyanurate foam) was investigated under a range of fire conditions. Two of the materials, stone wool and glass wool failed to ignite and gave consistently low yields of all of the toxic products [18]. The use of thermal insulation materials for building applications is a promising technique but further investigations are needed in this area in order to enhance fire prevention. The organic foamy materials, polyethylene foam and polyurethane foam, which did not meet the requirements of the low fire hazard material and were unfavorable in the building’s fire prevention [19].

Figure 16 shows the overall heat transfer coefficient of the existing insulators (heat reflection insulator (Type A) and Styrofoam (Type B) and multilayer reflection insulator (Type C), compared under the same air temperature difference between both sides of test specimen (30 ℃) and heat transfer coefficient (α). If the heat insulation performance (overall heat transfer coefficient) of the reflection insulator was compared with that of Styrofoam, with the average heat transfer coefficient of 0.035 W/(m·K) as the reference, Type A-1 (10 mm thick) corresponded to 6 mm-thick Styrofoam; Type A-2 (11 mm thick), 10 mm-thick Styrofoam; Type A-3 (7 mm thick), 7 mm-thick Styrofoam; and Type A-4 (6 mm), 6 mm-thick Styrofoam.

The reflection insulator generally has a high overall heat transfer coefficient with a very low heat insulation performance. The heat reflective performance is degraded by the high emissivity of the aluminum film layer on the reflection insulator surface; the materials (e.g., woven fabric and PE foam attached to the back of the aluminum film) have lower heat insulation property than Styrofoam. Next, the heat insulation performance of the multilayer reflection insulator (Type C) was compared with the Styrofoam of diverse thicknesses. Type C-1 (20 mm thick) had the same heat insulation performance as the 35 mm-thick Styrofoam (1.8 times); Type C-2 (30 mm thick), the 58 mm-thick Styrofoam (1.9 times); Type C-3 (40 mm thick), the 88 mm-thick Styrofoam (2.2 times); Type C-4 (50 mm thick), the  150 mm-thick Styrofoam (3 times), a characteristic that has a highly efficient heat insulation. Thus, with the increase in the insulator thickness, the overall heat transfer coefficient of the volume type heat insulator decreased in the form of power series, but that of the multilayer reflection insulator increased in the form of an exponential function. Accordingly, the 40 mm-thick multilayer reflection insulator (Type C-3, 0.36 W/(m²·K)) meets not only the overall heat transfer coefficient standard for the exterior wall (enforcement decree of the building act for the central district) of 0.47 W/(m²·K), but also the 2012 standard (0.42 W/(m²·K)) and 2017 standard (0.38 W/(m²·K)).

The thermal conductivities of the reflective heat insulator and multilayer reflection insulator were converted, assuming that they were the single insulators like Styrofoam, under the same condition of the air temperature difference between both sides of the test piece (30 ℃) and the heat transfer coefficient. Figure 17 shows the converted thermal conductivities in the order of the heat reflection insulator (Types A1 to A4), Styrofoam (Types B1 to B4), and multilayer reflection insulator (Types C1 to C4).

With the average thermal conductivity of Styrofoam (bead method) of 0.035 W/(m·K) as reference, the thermal conductivity of Type A-2 (Manufacturer A) was lower than that of Styrofoam; but those of Type A-1, A-3, and A-4 were higher than that of Styrofoam. These results show that Type A-2 had a heat insulation which was a little higher than the same thickness of Styrofoam; however, Type A-1, A-3, and A-4 had lower heat insulation.
 

Fig. 16 Overall heat transfer coefficient of existing insulator and multilayer reflection insulator

Fig. 17 Thermal conductivity of existing insulator and multilayer reflection insulator
 

In the case of multilayer reflection insulator (Type C), its thermal conductivity decreased in the form of polynomial function with the increase in the thickness, which indicates that it has 1.8-3.0 times higher heat insulation than the same thickness of Styrofoam. It was proved that the 50 mm-thick multilayer reflection insulator (thermal conductivity, 0.012 W/(m·K)) had a similar heat insulation performance with that of aerogel (a new insulation material known for its best heat insulation performance with a thermal conductivity of 0.013 W/(m·K)) having the same thickness; nonetheless, the multilayer reflection insulator that is thicker than this ultra-high performance insulator has a much higher heat insulation performance than the aerogel insulator of the same thickness.

These results indicate that the multilayer reflection insulator, which has many Al foil layers that are placed separately by air layers between both surface layers, has a high heat insulation performance with the following characteristics:

1) No flammability

Organic heat insulators (including the reflective heat insulator, Styrofoam, and urea foam) are burnt in fire and produce toxic gas that causes damage to human lives and properties. The multilayer reflection insulator can minimize the damage of fire because its surfaces and inner Al foil layers are nonflammable.

2) Vapor permeability resistance

The heat insulation performance of the highly absorptive heat insulators (including glass wool and rock wool) is degraded by water absorption. The absorbed water corrodes the surfaces of interior and exterior materials that are in contact and generates condensation. The multilayer reflection insulator, however, is made of highly vapor-permeability-resistant materials that include Al foil and it does not involve the decrease in heat insulation due to humidity as well as condensation.

3) Eco-friendliness

Glass wool and rock wool are harmful to human beings and may cause diverse skin and lung diseases. The multilayer reflection insulator, however, does not contain harmful fiber, form aldehyde, and volatile organic compounds (VOC), among others. It can be used as eco-friendly construction insulator that does not affect the indoor air quality.

4) Multilayer reflective exterior insulator

If the multilayer reflection insulator (50 mm thick with an overall heat transfer coefficient of 0.22W/(m²·K)) is mounted inside the double glazed glass or exterior metal panels, it becomes a nonflammable exterior insulator that has a high efficiency insulation performance equal to that of 150 mm-thick Styrofoam. Because no additional insulation is required, the wall thickness can be made thinner and the construction cost and period can be reduced. The exterior insulation on the building walls can significantly reduce the heat accumulated by solar radiant heat. The decrease in the accumulated heat from the building skin can reduce building cooling load and significantly save energy.

4.3 Performance of heat reflection insulator used in buildings

The performances of heat reflection insulators used in buildings, evaluated in this work, are listed in Table 1.

As the multilayer reflection insulator (C-1 type: 1.764 kJ/(m²·h·℃) heat transmission coefficient) showed a higher heat transmission coefficient than the existing heat reflection insulator (A-1 type: 7.980 kJ/(m²·h·℃) heat transmission coefficient), as much as 4.5-fold energy can be saved if the multilayer reflection insulator will be used in the outer walls of buildings compared to that using the existing heat reflection insulator.  Therefore, if the outer-wall-purpose high-insulation glass will be used for the outer walls of buildings, heat losses can be eliminated more than two-fold through the outer walls of buildings.
 

Table 1 Performances of heat reflection insulators used in buildings

Moreover, the high-insulation glass with nonflammable composition materials (a glass surface layer, an aluminum foil layer, and isolation layers) cannot be burnt, does not produce toxic gases in case of fire, is nonflammable, and can prevent damage to life and property. As high-insulation glass consists of materials like an outer-surface glass material, an aluminum foil layer, and an isolation material, which have large moisture resistance capacity, it is regarded as having the capacity to increase the moisture resistance opening effect by preventing the decrease of the insulation capacity through moisture absorption, thanks to its low moisture absorptivity, dew condensation, and molds.

5 Conclusions

1) The reflection insulator, which has a very low radiation coefficient or E value (0.03 in general; for most insulators, 0.90) and significantly reduces the heat conductivity by emission, is used widely mainly in buildings with marble, granite, and base panel finishing.

2) The heat transmission coefficient of existing Type A-2 heat reflection insulator is found to be    1.88 W/(m²·K) less than A-3; 2.19 W/(m²·K) less than A-1; 2.30 W/(m²·K) less than A-4, 3.13 W/(m²·K), in that order, and A-2 (11 mm thick) shows a slightly higher insulation performance while A-4 (6 mm thick) shows the lowest. As for the results of the conversion of heat transmission coefficient per type of existing reflection insulator, they are as follows: A-1, 0.045 W/(m·K); A-2, 0.031 W/(m·K); A-3, 0.042 W/(m·K); and A-4, 0.078 W/(m·K). The average heat conductivity is 0.049 W/(m·K).

3) The heat transmission coefficients of existing Type B-1 heat reflection insulators are found to be   1.07 W/(m²·K) greater than B-2; 0.70 W/(m²·K) greater than B-3; 0.57 W/(m²·K) greater than B-4, 0.003 0 W/(m²·K), in that order. As the conversion results of heat conductivity per type of Styrofoam insulator are 0.030 W/(m·K) for B-1, 0.032 W/(m·K) for B-2, 0.037 W/(m·K) for B-3, and 0.037 W/(m·K) for B-4, the average heat conductivity is 0.035 W/(m·K) regardless of the thickness of the insulator.

4) The heat transmission coefficients of the Type C-1 multilayer reflection insulators are 0.80 W/(m²·K) greater than C-2; 0.53 W/(m²·K) greater than C-3;  0.36 W/(m²·K) greater than C-4, 0.23 W/(m²·K), in that order, indicating that it is a high-performance insulator appropriate for the 150 mm-thick Styrofoam. The heat conductivity values of the multilayer reflection insulators were converted based on the thickness and type: C-1, 0.020 W/(m·K); C-2, 0.018 W/(m·K); C-3, 0.016 W/(m·K); and C-4, 0.012 W/(m·K).

5) The Type C-1 multilayer reflection insulator, which has a higher heat transmission coefficient    (0.42 kcal/(m²·h·℃)) than the existing heat reflection insulator (A-1; 1.90 kcal/(m²·h·℃)), can save energy approximately 4.5-fold if the multilayer reflection insulator will be used for the outer walls of buildings compared to that using the existing heat reflection insulator. Moreover, the multilayer reflection insulator keeps the indoor-side surface temperature high (during winter) or low (in summer), enhances the comfort of the building occupants, and conducts heating and moisture resistance to prevent dew condensation on the glass/ outer-wall surface.

References

[1] LEE E J. Technology analysis of zero energy solar buildings and their examples [R]. Korea Institute of Energy Research, 2006. (in Korean)

[2] Ministry of Knowledge Economy. A law for rationalization of energy consumption, Ministry of Knowledge Economy [EB/OL]. [2011-09]. http://www.mke.go.kr. (in Korean)

[3] KIM J J, MOON J W. Energy effects of insulation for a single-family home [C]// 11th International Building Performance Simulation Association Conference, Building Simulation. Glosgow: University of Strathclyde, 2009: 674-680.

[4] JIN Woo-moon. Impact of reinforced standard for envelope insulation on the regional heating and cooling energy consumption [J]. Korean Journal of Air-Conditioning and Refrigeration Engineering, 2011, 23(6): 646-655. (in Korean)

[5] KWON Young-cheol, UM Sang-kweon. A study on the design of the complex insulating wall including reflective insulation [J]. Journal of KIAEBS, 2010, 4(2): 83-89. (in Korean)

[6] AL-HOMOUD M S. Performance characteristics and practical applications of common building thermal insulation materials [J]. Building and Environment, 2005, 40(3): 353-366.

[7] LIANG Han-Hsi, HO Ming-Chin. Toxicity characteristics of commercially manufactured insulation materials for building applications in Taiwan [J]. Construction and Building Materials, 2007, 21(6): 1254-1261.

[8] BAETENSA R, JELLE B P, GUSTAVSEND A. Aerogel insulation for building applications: A state-of-the-art review [J]. Energy and Buildings, 2011, 43(4): 761-769.

[9] BAETENS R, JELLE B P, THUE J V, TENPIERIK M J, GRYNNING S. Vacuum insulation panels for building applications: A review and beyond [J]. Energy and Buildings, 2010, 42(2): 147- 172.

[10] JELLE B P. Traditional, state-of-the-art and future thermal building insulation materials and solutions-Properties, requirements and possibilities [J]. Energy and Buildings, 2011, 43(10): 2549-2563.

[11] DESOGUS G, MURA S, RICCIU R. Comparing different approaches to in situ measurement of building components thermal resistance [J]. Energy and Buildings, 2011, 43(10): 2613-2620.

[12] WAKILI K G, STAHL T, BRUNNER S. Effective thermal conductivity of a staggered double layer of acuum insulation panels [J]. Energy and Buildings, 2011, 43 (6): 1241-1246.

[13] PENG Chang-hai, WU Zhi-shen. In situ measuring and evaluating the thermal resistance of building construction [J]. Energy and Buildings, 2008, 40 (11): 2076-2082.

[14] CHOI Jeong-min, CHO Sung-woo. A study on the variation of indoor heating and cooling load according to insulation thickness and infiltration rate [J]. Journal of Korea Institute Architectural Sustainable Environment and Building Systems, 2011, 4(3): 111-117. (in Korean)

[15] HONG Won-kee, KIM Sun-kuk, KIM Hyung-geun, YOON Tae-ho, YUNE Dai-young, KIM Seung-il. A feasibility study of green frame (GF) for the implementation of low-carbon emissions & long-life housing [J]. Journal of Korea Institute Building Construction, 2010, 10(2): 145-150. (in Korean)

[16] JANG Cheol-yong, LEE Jin-sook, HAN Hye-sim. Evaluation of building energy rating system according to the change of building elements thermal insulation performance in apartment [J]. Journal of Korean Solar Energy Society, 2010, 30(5): 32-37. (in Korean)

[17] ALAM M, SINGH H, LIMBACHIYA M C. Vacuum Insulation Panels (VIPs) for building construction industry-A review of the contemporary developments and future directions[J]. Applied Energy, 2011, 88 (11): 3592-3602.

[18] ANNA A, STEC T, HULL R. Assessment of the fire toxicity of building insulation materials [J]. Energy and Buildings, 2011, 43(2/3): 498-506.

[19] LIANG Han-hsi, HO Ming-chin. Toxicity characteristics of commercially manufactured insulation materials for building applications in Taiwan [J]. Construction and Building Materials, 2007, 21(6): 1254-1261.

(Edited by DENG Lü-xiang)

Foundation item: Project(NRF-2010-0024155) supported by the National Research Foundation of Korea

Received date: 2011-09-03; Accepted date: 2012-04-13

Corresponding author: LEE Kang-guk, Professor, PhD; Tel: +82-10-85974638; E-mail: ggyi@naver.com