My best suggestion is:
1# foam … Probably should be vented although these boards if not sandwiched tend to somewhat disposible and most I’ve built I haven’t vented since they weren’t meant to have longevity (and I LOVE how these ride). If they are sandwiched I would vent them.
1.5# foam. While I have seen heat related problems with this foam it is somewhat rare and the conditions that caused problems were extreme. I’ve built hundreds of these, maybe even a thousand and I generally don’t vent these or see a need to.
2# foam. I’ve buit thousands. I’ve built thousands because these are fool proof. They don’t leak, they don’t have problems with heat and they almost never delaminate (far better than urethane in that regard). These are easy to build, take less glass and generally hold up the best. Best against breakage and have BY FAR the best strength to weight of any non sandwiched board construction. While sandwich construction gives you some serious advantages, these are the easiest and most fool proof boards you can build without sandwiching … and that includes poopies.
On XPS, I don’t have experience in venting these but I would defer to Mr. Phillips on that one as he is in the same area of SoCal as the primary XPS builer and probably sees enough to judge.