Does blown-in fibreglass hold moisture?

No. Blown-in fibreglass insulation is not absorbent and, if exposed to moisture, will not wick up (pull water through) or hold water. It will dry out and retain its original R-value. Insulation is required to absorb not more than 5% moisture by weight. The ASTM C-553 test showed that insulation used in the blown-in fibreglass absorbed a maximum of 3% moisture by weight.