I’m fixing up an old uninsulated shed. The roof — built of metal sheeting on plywood, nailed to the 2-by-4 rafters — needs to be insulated. What is the best way to insulate the roof that avoids condensation, which might lead to mould or rot?

I can think of two options:

1. Install batten insulation between the rafters, and then cover this with a vapor barrier.
2. Skip the batten insulation and tack [polyisocyanurate](https://en.wikipedia.org/wiki/Polyisocyanurate) boards onto the underside of the rafters, leaving an airspace between the rafters.

Option 1 risks trapping condensation between the insulation and plywood. Option 2 doesn’t seem to offer very much insulation.

Are there other options to consider?