Thermal performance and thermal transmittance prediction of novel light-gauge steel-framed straw walls

Thermal bridges in light-gauge steel-framed exterior walls lead to more winter heat loss, summer heat gain, and building pathologies generated by moisture condensation and mould growth, which are unfavorable for building energy efficiency and service life of sheathing panels. A novel light-gauge steel-framed straw wall with a nonmetallic broken bridge layer was proposed, which consists of two sheathing panels on both sides of the wall, two frames, a middle sandwich layer, and two kinds of fasteners. The environmentally friendly straw boards including paper straw board (PSB) and wheat straw strand board (WSSB) were used as middle sandwich layer and exterior panel respectively. This study focuses on the thermal performance and the prediction method of thermal transmittance (U_m-value) for such walls. Tests using the temperature control box-heat flux meter method were carried out for three representative walls with different configurations. Based on analyses of the temperature and heat flux density distributions, it is found that the broken bridge layer and the airspaces between the sheathing panel and the track or the bracing can effectively reduce the thermal bridge effect, with the relative effects less than 5%. Based on the test results of the U_m-values, different wall configurations were matched with the feasible climate zones on the basis of energy efficiency design. After the validation by comparing with the test results in terms of temperature, heat flux density, and U_m-value, the numerical study was carried out to broaden the feasible climate zones, and walls with improved configurations were proposed and analyzed. The results show that, by replacing the unventilated airspaces with the expanded polystyrene (EPS) boards, a maximum improvement in thermal performance reaches 74.89%. Thus, the feasible climate zones of the walls have been extended to the severe cold zone. Finally, a thermal transmittance prediction methodology based on the calculation method specified in European Standard EN ISO 6946-2017 in conjunction with the modified method considering thermal bridge effect was used to predict the U_m-‍values of the improved walls. This study provides a new design strategy with nonmetallic broken bridge layer to reduce the unfavorable influence of thermal bridges in light-gauge steel-framed walls. Such a design strategy contributes to less energy consumption in addition to making full use of agricultural waste straw, which plays a positive role in the development of green buildings.