I do blow back, but like someone who eats lots of sausages, eventually the artery gets clogged.
Drink, blow back, a tiny drop of water stays in the tube and freezes. It doesn't seal the tube. Drink again, blow back again, this time another slight drop catches where that last drop was. Every drink leaves a slight drop of water, until eventually, she clogs up.
I added insulation to the tube, sewn over the top of my shoulder stap (theory was that heat from my body would travel through the shoulder strap and be held by the tube by the added insulation). The tube was fine under the insulation, froze in the small gap where there was less insulation. Friend from Alaksa showed me her tube (that sounds strange). It had closed cell foam wrapped completely around it. I thought about adding fiberfill instead of foam since I like to tuck the mouthpiece in my coat. Fiberfill will compress, allowing the collar of my coat to actually close... Maybe???