garage insulation

If the roof is just a simple layer of metal above the structure you WILL have a crap load of condensation if you allow any heat to get to it. At the very least you need to have a layer of osb and some felt so that the moisture runs down the roofing felt. Our barn/house is a truss building with a metal roof. Had nothing but batts under it the first year...damn near like it was raining in here. Had the tin roof taken off, osb and felt put down and we are dry...for now...Spray foam is an option....if you can afford it. Only local spray foam contractor wanted over 10k to do our 1700 square foot roof....Good luck.....