That "sauna feeling" you mentioned, @drew_carr75, is spot on. I read somewhere that old canvas tents trapped moisture so bad because the material couldn't breathe like modern fabrics. It's no wonder they always felt damp and heavy, even without rain.
Ugh, my grandpa had one of those canvas monsters... I remember waking up in a puddle because the seam over my head would just drip all night. The smell of wet canvas and mildew is stuck in my brain forever. Trying to fold that soggy, heavy thing in the morning was its own special kind of misery. Yeah, modern tents are a straight-up miracle.