Abstract
The influence of various diagonal-bracings arrangements on the heat transfer coefficient of wooden walls was studied with the goal of improving the thermal insulation performance of the walls. Through the reliability verification of the theoretical value of the heat transfer coefficient, this study found that a larger proportion of wood frame area resulted in larger theoretical and test values for the heat transfer coefficient. The heat transfer coefficient of the wall with expanded polystyrene foam sheet (EPS) was 5.90% to 6.10% higher than that with extruded polystyrene foam sheet (XPS), and the tested value was 4.75% to 8.60% higher. The maximum value of the average heat transfer coefficient of 12 diagonal-braced walls was 0.366 W·m-2·K-1, which met the thermal level of the severe cold area. The test value of the heat transfer coefficient was larger than the theoretically calculated value, and the linear correlation was up to 0.978.
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Effects of Diagonal Bracing on Thermal Insulation of Wood-frame Walls
Mingbin Liu,a Feng Lu,a,* Xuedong Zhang,a and Xiaolin Yang b
The influence of various diagonal-bracings arrangements on the heat transfer coefficient of wooden walls was studied with the goal of improving the thermal insulation performance of the walls. Through the reliability verification of the theoretical value of the heat transfer coefficient, this study found that a larger proportion of wood frame area resulted in larger theoretical and test values for the heat transfer coefficient. The heat transfer coefficient of the wall with expanded polystyrene foam sheet (EPS) was 5.90% to 6.10% higher than that with extruded polystyrene foam sheet (XPS), and the tested value was 4.75% to 8.60% higher. The maximum value of the average heat transfer coefficient of 12 diagonal-braced walls was 0.366 W·m-2·K-1, which met the thermal level of the severe cold area. The test value of the heat transfer coefficient was larger than the theoretically calculated value, and the linear correlation was up to 0.978.
Keywords: Wall; Diagonal-braced; Insulation; Heat transfer coefficient
Contact information: a: Department of Industrial Design, Anhui Polytechnic University Wuhu, Anhui, China; b: School of Architecture, Zhengzhou University Zhengzhou, Henan, China;
* Corresponding author: liumingbin2019@163.com
INTRODUCTION
Wood-frame walls are the primary structural assemblies used in low-rise wood construction for effective thermal insulation performance. In North America, several studies have evaluated the thermal insulation properties of wood-frame walls. In a comprehensive study on the design of light wood-frame buildings with respect to energy conservation and heat insulation, reasonable planning and structural design were used to improve the thermal performance of wood-frame buildings, reveal the experimental parameters for materials, and derive a theoretical formula for calculating heat loss through design, construction, and experimental tests (Sherwood and Hans 1979). In 2008, the Engineered Wood Association (APA) conducted research on insulation of light wood frame walls. They expounded the effective measures to improve the heat insulation and energy conservation of wood-frame buildings from five aspects, including the materials of the wall, the air tightness of the wall, heat insulation, the sound insulation of doors and windows, and the installation of heating equipment. Smegal and Straube (2010) carried out a systematic study on a double-row stud and high-heat wall with external thermal insulation from the perspective of climate change in cold regions. To ensure a high level of heat insulation performance of wood-frame buildings, they proposed a thermal insulation strategy between the building foundation, basement, and the wall, which would meet the thermal insulation requirements of the cold region through controlling the building’s air tightness. A study from the Oak Ridge National Laboratory (ORNL, USA) established the Zero Energy Building Research Alliance (ZEBRA) (Miller et al. 2010). They studied four newly constructed wood-frame buildings with various thermal insulation structures using structural insulated panels (SIP), optimizing frame structure (OVF), dynamic maintenance structure (DE), and exterior insulation and finish system (EIFS). In addition the study found that the thermal resistance of the external wall using SIP and OVF structures exceeded 4.4 m2·K·W-1, and the energy consumption was about half of the current American building standard (Nyers et al. 2015), which demonstrated excellent thermal insulation performance. Forestry Product Innovations had published a guide for designing energy-efficient building enclosures for wood frame multi-unit residential buildings in marine to cold climate zones in North America (Finch et al. 2013). This guide provided technical guidance and specifications for enclosure’s energy conservation, heat insulation, air tightness, and air quality of wood-frame buildings in cold areas, it also provided reference basis for the design and research of the energy conservation and thermal insulation of wood frame buildings. Kucerova et al. (2014) studied the heat transfer coefficient of wood frame walls that had been used for many years. Based on this test, the U value of the heat transfer coefficient was 0.04 W·m-2·K-1, which was slightly higher than the simulated value by the software, but it met the current technical standards for heat insulation of wood-frame buildings. Blazek et al. (2016) used the calibrated heat box method to test the thermal insulations of four passive wood-frame walls. Having used the measured surface temperature and energy consumption to calculate the wall’s heat transfer coefficient, they found that the error between the tested value, the standard value, and the empirical value was about 13%. They compared the energy consumptions of the four walls and found that the energy consumption of the optimized fourth wall structure was approximately 39% lower than that of the non-optimized wall structure. Liu et al. (2018) studied the factors affecting the heat transfer coefficient of wood frame wall as well as the method of improving thermal insulation property of the wall. Twelve walls with various structures were tested by the hot box-heat flow meter test method. It was found that the moisture content of spruce-pine-fir (SPF), insulation materials, spacing, and thickness of studs had an influence on the heat transfer coefficient of walls. The effective heat transfer coefficient values of three walls ranged from 0.325 W·m-2·K-1 to 0.398 W·m-2·K-1, which met the thermal level It of the severe cold area. With the wide application of wood-frame construction in different climate areas and the surge of new materials, thermal insulation and steady-state heat transfer properties of wood-frame walls were attracted great importance (Zarr et al. 1995; Dalgliesh et al. 2005).
This study evaluated the effect of the thermal insulation performance of wooden walls due to diagonal-brace and material proportion. The results offer scientific guidance for future design of wood-frame walls, especially with respect to anti-seismic and thermal insulation characteristics.
EXPERIMENTAL
Wall Materials and Frame Structure Design
Dimension lumber of spruce-pine-fir (SPF) was employed as the studs of wood frame walls, whose section size was 38 × 89 mm. Larch oriented strand board (OSB) and thistle board finish (TB) of 12 mm size were used as sheathings. Glass wool (GW) was chosen as insulation material; 30 mm expanded polystyrene foam sheet (EPS) or extruded polystyrene foam sheet (XPS) was applied as external insulation material. Felt (3 mm) was applied between wood frame and TB as insulation material. Wood-plastic board was applied as a waterproof layer. Figures 1 and 2 illustrate the wall frame designs and their structures, which all refer to the Canadian wood frame house construction (Burrows 2005), and Chinese standard GB 50005 (2005).
Fig. 1. The wall frame designs (the units shown are in millimetres)
Fig. 2. The wall frame structure (the numbers at the left side of the figure are the thickness of each layer of material and the units shown are in millimetres)
Wall Structures
The structures and numbers of walls are shown in Table 1. The proposed structural systems met all the requirements of technical standards in terms of stability, sound insulation, and technical properties of the shells of buildings, fire resistance, and anti-earthquake of living space.
Table 1. The Structures of Walls
Methods
The thermal insulation performance of wall was tested by guarded hot box according to the GB/T 13475 (2008) standard. The guarded hot box was made up of three parts: cold box, hot box, and specimen box, as shown in Fig. 3. The steady thermal transmission was controlled by temperatures of cold box and hot box for constant temperature difference of cold and hot wall surfaces. The temperature and heat flow data were tested and recorded.
