If your quilts are in bags in containers covered in an outside shed i wouldn't worry. Remember where it comes from. Its naturally in the outdoor weather all the time. I think down is very resilient. If your worried throw it in the dryer once a month to take out any dampness.JMO
" The mind creates the abyss, the heart crosses it."
“The measure of your life will not be in what you accumulate, but in what you give away.” ~Wayne Dyer