The high cost, narrow processing window, and poor mechanical properties of polyhydroxyalkanoates hamper their use as films for food packaging applications. We report the preparation, characterization, and film blowing of polyhydroxybutyrate-valerate (PHBV)/beer spent grain fibers (BSGF) composites. Beer spent grains are by-products of the beer industry submitted to an acid/caustic treatment and to successive grinding to achieve fibers with typical size of 30 microns. PHBV/BSGF compounds were extruded and successfully subjected to draw down ratios as high as 50. However, processability was lost for BSGF contents above 10 wt%, due to their percolation within the PHBV matrix. Two PHBV/BSGF compounds were processed into blown films. The resulting mechanical, structural, and barrier properties depend on processing parameters and BSGF content. The fully bio-sourced and compostable PHBV based thin films produced under processing conditions scalable to industrial production show gas and water vapor barrier properties attractive for food packaging applications. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers