Insulation, Radiant and Vapor Barriers - Installing Polyiso insulation board in a box truss

I am installing foam board polyiso 3.5" insulation inside a box truss system which will be enclosed by plywood and then sheetrock. It consists of a top cord beam, lower cord beam and then vertical and diagonal members. The top cord is about 3.5 feet above the bottom cord. My plan is to cut the insulation board with a circular saw with a fine tooth veneer blade as there will be triangular pieces. I also plan to cut the insulation board smaller than the cavity leaving at least 1/4" gap all around the edge so that I can put crack sealer type foam insulation as used around windows and doors to seal the edges. I will also put plastic sheeting over either the inside surface on the insulation and then the plywood and drywall, or over the plywood as a vapor barrier, and then the drywall. Any suggestions or advice on cutting the foam board and the above described application? Don't want to reinvent the wheel especially if there is a more pain free way to do it!

A utility knife usually works better than any type of saw with foam board. Just make sure the knife is sharp and make several passes to make the cut instead of trying to do it in just one cut.

IMO it depends on the thickness. For example, 2" foam coated on both sides with vapor barrier can be difficult to get a clean cut on, even if you use a utility blade on both sides before snapping it. A plain ol' wooden handsaw works great and leaves a nice flat edge, although it is messy. But, hey, that's what shop vacs are for! Good luck!