How can I mount an electrical box to exterior foam board?

Question

I am trying to replace an exterior light fixture next to my home's main entry way:

The original light was mounted to a low-profile bracket that was mounted directly to the PVC "box" shown in the picture. The new light fixture isn't deep enough to fit over the low profile bracket, so I cut a hole big enough for an octagonal box. From the foam to the face of the PVC box is 1.5" so the box fits perfectly.

The problem I am facing is that there apparently is no sheathing behind this foam material (can someone tell me what this material is called?) so there is no way for me to securely fasten the junction box.

I've done some test holes to try and find a stud somewhere in this area but I haven't found any. The PVC trim piece is also securely attached to the wall underneath the siding so I can't remove it.

I've also tried a few different types of anchors (mostly rated for drywall) which don't work (they spin freely once sufficient torque is applied).

I'm at a loss, but I'm hoping someone can tell me how to securely attach the electrical box.

Answer 1

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/

Answer 2

Perhaps your test screws were to long and you ran out of thread going past the sheathing? I'd be disappointed to not find any back there.

Speedy's tabbed box seems ideal but if you wanted it able to support considerable weight, you could use a ceiling fan safety brace. However, unlike the tabbed box which is rated for exterior, this one (and likely also the one you're trying to use) is not.