Hot box method to investigate U-values for straw bale walls with various structures

Abstract

Straw bales are used increasingly as rural building material due to their relatively low thermal conductivity and availability as a local product. The U-value is a key parameter for evaluating thermal performance of building walls. The present study investigated the effects of different structural details concerning straw bale joints (SBJs), rebar arrangements, and various finishes on U-values for SBWs using the guarded hot box method. Firstly, modification coefficient was proposed to qualitatively evaluate SBJs, results showed that R-value for straw bales without SBJs was more than three times that of straw bales with SBJs, the optimal U-value was achieved for straw bale wall (SBW) when the inner pressure was 1386 N. Secondly, for rebar arrangements, the SBW with three bars yielded the minimum U-value of 0.53 W·m−2·K−1. Lastly, we found that plastering improved thermal performance of SBW by increasing the air tightness of the wall, and U-values for SBWs with plate tended to be larger than those with plastering attributed to that the uneven surface of the straw bales resulted in an incomplete contact between the plate and the straw bales, causing convective heat transfer, giving rise to U-values of SBWs. Thus, we recommend that clay or lime plaster should be attached to the straw bales first, before installing an exterior plate.