Insulation help for my garage with pics

Actually it does not need a gap between the sheeting and the insulation. It needs a vapor barrier between the sheeting and the insulation.

What happens is that due to heat/cold variation on each side, somewhere in the middle of the insulation the temperature/moisture will be at the dew point. With fibreglass or Roxul insulation, this is where the moisure occurs. The vapor barrier simply protects the moisture from travelling inward and damaging the sheeting. Instead the moisture will wick out the "cold" side as the temperature varies. The venting on the cold side simply allows the excessive moisture buildup to escape from the insulation to outside as opposed to steaming up your rafters (or keeping your insulation wet).

The spray foam is designed to fill in the rafters. It also acts as a vapor barrier, and is designed to not entrap moisture. There is no need for venting in this method, as there is really no moisture build up in the material to evaporate.

You can insulte rafters with roxul or something similar, but you would have more trouble to seal around the vertical boards of the truss, and would have to leave an air gap of a couple of inches between the insulation and the roof sheeting. This small area would have to be vented.

Grant