We're in the process of remodeling a Morton cold-storage pole barn into a 3.5 season workshop. I'm struggling to figure out the best approach for the ceiling/roof.

The current structure is wooden trusses with wood purlins under steel sheeting. Like this. It has mesh venting at the soffits and at the peak.

The options I'm seeing are:

1) Spray closed-cell foam on the whole underside, sealing up the vents (we're already planning on adding exhaust fans for fumes and such).

2) Attach vapor barrier & steel sheeting to underside of roof trusses, add vent chutes under roof, and blow in a bunch of loose fiberglass.

My concerns are less about ultimate efficiency and more about water management. The roof on the building is 20 years old and has already had leaks found and fixed.

If I spray-foam the underside, water leaking in around a bad fastener goes into the wood purlins and then...where? If it gets warm enough I suppose it could drive vapor back out the same leaky hole, but it seems much more likely to just sit there and rot the purlin (and maybe rust out the roof). It can't easily dry to the outside like shingles could. I'm unlikely to ever notice a problem until it's bad enough to be structurally compromised. OTOH, the spray foam would contribute a not-insignificant amount of strength to the roof itself.

If I put steel sheeting under the trusses and blow in insulation above that, I lose easy access to the top space for wiring, etc. and it's more labor (for me). The costs don't seem to be dramatically different either way.

Input?