I had an experience many years ago with using a curtain PIR in a similar type of installation.
Customer wanted protection above his drop ceiling against someone coming in through the roof. The distance between the hard ceiling and the drop ceiling was about 3 feet. I mounted a Visonic (forget the model #) curtain PIR on it's side. I seem to remember it was rated at
35 feet(?). I couldn't get detection at any range further than about
17-18 feet. I wound up having to use more than one unit above the ceiling, to cover the area that I wanted.
I had to assume that the divergence of the array, between the two ceilings, got too wide with distance to be able to detect the cross section of a body perpendicular to the curtain, at anything further than 17 feet.
When I tested the unit below the ceiling it worked at a greater distance but not at the maximum distance the unit was rated for. I found that when the unit was mounted on it's side and you "stood up" perpendicular to the curtain, the unit wouldn't detect as far away as it would as if the unit were mounted with the unit mounted properly and you walked through the curtain in parallel with the array. I assumed that since the size of the heat source the unit sees is smaller when mounted on it's side, that the range was somewhat shortened.
That was with Visonics very first curtain unit. I'm sure things have changed since then. ( don't have much call for curtain units) But it would be an interesting experiment to see if it still affects the range when used horizontally.
That's what we used to do on large commercial structures. Turn a PIR with a curtain lens 90º so the curtain forms a horizontal plane of detection a foot or so below the skylight. Just be careful to mount the PIR so that sunlight doesn't strike the lens.
A suggestion might be to take some small diameter PVC with some T's and elbows, some joint compound and make a PVC screen. Make sure to run lacing wire for an alarm circuit in a unique pattern through the tubing before sealing. This should make for a nice looking screen. Make sure to UV resistant PVC.
Crash Gord>I haven't covered glass skylights in a long time..what's out there these days? >
As a matter of fact I did consider that but this was in an ice cream shop and the guys former job was in the HVAC and refrigeration business and he had the place colder n an iceberg. And, as I mentioned, it still was reduced range when I experimented with it below the ceiling. Moot point now, but just thought I'd mention my observations just in case anyone wanted to experiment or ran into similar problems in the future.