Hello - I'm installing a vinyl window - mfg instructions say to drill thru frame about 6" from corners and approx every 16" on the jambs using a 3/8 bit, and 3" #8 screws (shim it and snug fit, not too tight), covering the 3/8 hole later with a plastic plug. I started on the job this weekend.
What they don't say is how far to drill thru the internal chambers in the window - which I believe are intended to provide some kind of structural integrity and / or thermal break in the window construction.
What I did was I drilled thru the inside (exposed) edge only (3/8) and when I hit the first internal chamber wall I stopped, and then just poked a small pilot hole (screw size) from there on, thru any remaining chamber walls, to the shim side.
I drove the screw in until the screw was just snug against the interior chamber wall, and the window (fairly small) seems to be snug in place like that (no foam or other insulation in yet). However it should be no problem to back out the screws one at a time, re drill the remaining chamber(s) , and drive them farther in, if need be.
Are there any issues with poking a hole thru all the interior chambers in the window's design (to push the screw in right to the shim side), or is it good the way it is...?