Acoustic performance and measurement challenges of loose blown-in insulation materials in wall constructions: a focus on straw

In this study, loose blown-in sound-absorptive materials were used as insulation in metal stud walls and timber frame constructions. Building acoustic measurements assessed the soundproofing performance of mineral wool, cellulose, wood fiber, and straw in these wall systems, with a primary focus on straw. Key parameters such as density and dynamic stiffness were examined for their influence on the sound reduction index (SRI). Results showed that increased dynamic stiffness of straw raises the mass-spring-mass resonance frequency and enhances mechanical coupling between wall sheets, potentially reducing the SRI by up to 19 dB in double-wall systems. For timber frame walls, the SRI ( R w + C tr ) ranged from 42 to 46 dB, depending on straw’s dynamic stiffness. The study also evaluated the ISO 9052 method for measuring the dynamic stiffness of loose materials like straw. Predicted resonance frequencies, based on measured dynamic stiffness, consistently overestimated observed dips in sound transmission loss, highlighting limitations of the standard. Issues included the required sample load of 7.6 kg over a 0.20 × 0.20 m surface, which significantly influenced dynamic stiffness values. Case studies with varying loads and densities confirmed the impact of sample loading. These findings underscore the need for a new standardized method tailored to loose materials used in wall cavities, distinct from ISO 9052, originally designed for impact sound insulation materials.