Im in Scotland as well and my garage is a pretty standard new build garage, concrete block, standalone from the house. In the winter it gets down to 3-4 degree Celsius and only need the one fan for interval days. I still end up sweating loads and riding in pretty much just tri shorts. My feet do go numb though as its sitting on a concrete floor. I leave all the TR electronics (laptop, monitor etc) in there year round. In the summer it gets to between 13-20 Celsius on the warmest of days so i have 3 fans (need upgrading though). Ive not found humidity to be a problem.