The typical filon wall is made up like this:
1/16" filon
1/8" (or metric equivilent) luan ply
foam
1/8" (or metric equivilent) luan ply
Sheet metal (exterior)
vinyl (interior)
I cut a chunk out of the apron to install a hatch, here's the leftovers:
Note the luan on the back of the filon was originally part of the luan on the expanded bead foam - they've been split in this image.
it is a wonder they hold any screws. If I had the issue, I'd do what Joe417 did - remove the vent & add backer wood to give the screws something to hold onto.
