kiltedinkansas is correct, the excess "bulk" is trimmed out and the area reinforced with a canvas to stiffen and support the inside of the fell. The canvas should be breathable, strong and I use horsehair canvas for strength and durability. The lining should also breathe. but looking you may still sweat a bit depending on what you do. If you lounge around looking good in the beer garden, not so much. If you're an athlete, bandmember or something like that, you're going to sweat in whatever you wear anyway!