The KB1 wont be any hotter than any other piece. Actually seems to be cooler due to the light grey color. One thing to keep in mind with ALL “waterproof/ breathable” fabric is the fact that they are designed to work on the chimney effect. In essence you are pushing warm, moist air next to your body through the pores in the waterproof/ breathable membrane out to cooler, drier air, just like the heat from a fire would go up the chimney. If the conditions outside the membrane are similar to those inside, not as much moisture gets pushed out. Materials with a higher degree of breathe-ability will still function, just not as well. More expensive materials usually mean higher breathable. Sorry for the long winded response, but this is the reason many people buy expensive w/b rainwear and swear that it “leaks”. Whats happening is they arent able to push the warm, moist air inside the jacket out into warm, moist air. Its a science thing.