How to insulate my shop's ceiling

My "shop" is in the end of a storage shed I had build a few years back. It has a metal roof with no decking and 2x4 trusses. I insulated the walls with fiberglass and put up chipboard for walls but at the time I simply installed some 1/2" pink foam board panels between the 2x4 studs in the ceiling and called it good. I don't heat the shop full time nor do I run an air conditioner although it can get very hot in the summer and very cold in the winter inside this shop. When I want to work in the shop during the winter I have a couple small electric oil filled type heaters I turn on to provide heat which works great. No open flame or red hot heating element to cause a fire. Anyway, over the past several years the foam panels have shifted around to the point where I now have pretty big gaps and I lose heat very quickly so I would like to insulate the ceiling roof better than just pushing these panels back into place. Since it is a metal roof is there anything special I should consider? I usually don't heat the shop for more than a week or so at a time in the coldest weather. Even then I turn the heat down to about 45-50 degrees during the nighttime.
Since I only heat the shop for short periods I'm guessing I wont have to worry too much about condensation so I'm thinking about simply putting fiberglass insulation up between the trusses against the metal roof panels then a plastic sheet to cover the insulation from inside the shop. I may also just cover the whole ceiling with a thin layer of wood panels of some sort like sub-flooring panels.
Is there a better more economical way? Should I consider a foil faced insulation?

I would spray foam it (with closed cell)and be done.

I was thinking a diy spray foam kit, but if the foam panels are working otherwise, seal the edges using expanding foam. It will help hold it in place and leave it where you can easily replace sections if need be. I would avoid doing a combination of batt insulation and a moisture barrier on the inside, the metal will most likely collect condensation from the room and cause some mildew issues if it gets trapped, I think you'll want something like tyvek or something similar that can breath on the inside.

Chuck Thoits said:

I would spray foam it (with closed cell)and be done.

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I have thought about that but assumed it would be too expensive. I'll have to look into this closer.

Darren Wright said:

I was thinking a diy spray foam kit, but if the foam panels are working otherwise, seal the edges using expanding foam. It will help hold it in place and leave it where you can easily replace sections if need be. I would avoid doing a combination of batt insulation and a moisture barrier on the inside, the metal will most likely collect condensation from the room and cause some mildew issues if it gets trapped, I think you'll want something like tyvek or something similar that can breath on the inside.

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I do not want to cause a mildew issue. I'm just thinking I want to hold in as much heat as possible. The metal roof has a built in ridge vent.