So in this ONE SPECIFIC case, a 2.5cu.ft. volume, PLUS 1.2cu.ft. of R13, resulted in an effective Vb of around 3.3cu.ft. Verified twice by careful zplots at 40 and 48Hz. (using 3in. and 1.5in. port tube lengths). A conservative estimate is the "apparent" volume WAS INCREASED by at least half the amount of 'glass added. The major implication of the added volume is that if not accounted for, your Vb will be too large, causing system tuning to be lower than desired...maybe causing low end extension of the bass to suffer. 5 or 10Hz. of box tuning can be significant...if you can trust the sim data. JBL2035 was used in the situation above. Sam