I have added a spur off of the main A/C system of the house to run cold air to the garage. We live in Houston and heat comes free with every garage down here. :) You can not work in a garage in Houston in the summer without A/C just to remove the humidity if not to lower the temperature. I...
Vinyl siding is good at holding humidity against the building. It almost lasts forever and is like putting another vapor barrier directly against the outside of the building. T1-11 is a natural material and does not act as a vapor barrier.
Although you did not ask about Hardi-Plank it is my...