Not only does Gortex pass vapour, it passes gasses like Oxygen and Carbon Dioxide. This means that in bad weather, you can completely close-up the Explorer and Observer models without CO2 poisioning. This is due to the area of fabric through which the gas can pass. The more the area of fabric raised above the body, the better the fabric 'breaths'.
We do not recommend using the Combat bivy fully closed due the smaller area of fabric exposed to the user's face. In any bivy bag, should you find yourself feeling stuffy, short of breath you should immediately open the bag to admit oxygen.