So this year I've tried something new. I read that the R factor in using cellulose is due to the small trapped air pockets.
So, says I, if I pack the fluff tightly inside the styro, the outide environment won't be a problem.
I've always felt when using fluff, the cold pack was too isolated to lower temp in the box much anyway.
The result shipping this year in 90 deg weather? Not a temp related death yet! Even through late arrival of packages.
Just my results.