junction-boxlight-fixture
The junction box for a wall mounted light will be 1/4 in. shy of the finished drywall. The house is 50 yrs old. How do I adjust the depth of the octagonal junction box? Is there a safe way to shim or extend it? I've seen some octagonal box extenders online, but they didn't appear to be adjustable.
That is called fan-fold insulation. Typical for under vinyl siding in older homes.
Very surprising that there is no sheathing, but not unheard of. What I would do is get a round cut-in style fixture box and mount it directly to the vinyl block. This seems like your only option, apart from removing some siding and replacing the vinyl block with a fixture box style piece from Arlington Industries.
or
http://www.aifittings.com/catalog/siding-mounting-blocks/siding-mounting-block/
Strictly speaking, that round metal mounting bracket on the back of the fixture is supposed to be secured to an electrical box.
It can be a shallow box, but it's supposed to be a box. You must protect the wires. You want that new garage to still be there 10 years from now.
I can't really tell from the photo, but the right shallow metal box should fit within that recess on the back of the fixture without you needing to cut a hole in the siding. But if you have to cut a hole, then you have to cut a hole (if you do, rent, borrow or buy a holesaw for your drill big enough for the box).
You could attach the box through to the OSB with screws, although personally, I'd lay a 2x4 flat side against the OSB inside the garage, toenail it into the studs on both sides, and screw the box directly through the OSB into that 2x4. So a stiff breeze or errant basketball doesn't knock your light fixture off the wall. :-)
Drill a hole through the OSB and the 2x4 big enough to feed the cable and to accommodate the clamp that holds the cable in the box. The cable has to be clamped into the box.
Then attach the round mounting bracket that came with the fixture to that box, wire everything up, attach the light fixture to the bracket that you attached to the box. That's the way it's designed.
If it's a metal box, it has to be grounded (screwing the grounded mounting bracket to it will accomplish this).
You also have to secure the cable inside the garage as dictated by your local electrical code.
Finally, I'd use a dose of waterproof silicone or caulk around the box, and then again around the fixture itself after it's mounted, to make sure water doesn't seep inside.
Good luck!
Best Answer
Per Tyson's comment; You should be able to move the box on the threaded nipple (pipe) that seems to be holding it to the hanger bar.
Loosen the nut, in the box, down to the desired height. Then tighten the one outside the box above it. Since you had the ceiling open (at least when this was OP) that shouldn't be to hard. Worse case you may have to back the wires out, remove the box from the nipple, and stack a nut up at the desired height and reassemble.