I would definitely not insulate the rafter part of your roof. If it is -20C (-5F) outside and 20 above (68F) inside, somewhere in between is the dew point for the moisture inside the building. If you close in both sides of your rafters, it will be trapped in between. This will cause rot and reduce the effectiveness of your insulation. If you are in an area that gets snow, the snow will melt at the top of the roof where it is warmer and freeze at the eave where it is still cold. This will create an ice dam that will cause water to back up and probably leak inside your shop.
Put up vapour barrier on the underside of your ceiling, sheath the ceiling and then insulate. Your shop will be warmer in the winter and cooler in the summer. Make sure you have adequate flow thru ventilation as was pointed out. I have used pink fibreglass on a number of buildings and it works well. I put at least a 4" layer between the ceiling joists and (at least)another 6" crossways over the top of that. I have also just used a blown in cellulose. It is similar in cost, fills the gaps around bracing, and is really fast to do with a couple of helpers. Just be careful not to plug off the airflow at the eaves.
Last edited by Cherryville Chuck; 01-15-2011 at 07:21 PM.