Common wisdom, as I’ve come to understand it, is that if you’re cold, you shouldn’t close your sleeping bag over your head as your breath condenses and you’ll get even colder. However, in my long experience of freezing at night (believe me I am very experienced), this ain’t the way it works. I’m definitely warmer inside, rather than freezing my face off through a little opening.
What say ye, condensation experts?
edit to add: Unless it’s raining heavily, I always sleep under the stars, so that may be a factor to consider. Oh and have only synthetic bags.
No. Think about it for a minute: You use a sleeping bag to keep you warm by trapping your body heat in an envelope. If you breathe into it, your breath, which is moisture ladened, will remain close to your body temperature. In other words, it will remain as vapor and not condense. There is nothing driving the condensation like being near a cold body as happens when you breathe into the tent.
We are also talking about a tiny amount if water when it does condense. Eight hours of breathing releases 150 mL of water. Not all of that is going to condense nor be absorbed by the bag even if you are breathing into it. A significant amount remains as water vapor. How much remains as water vapor is going to depend on the temperature and humidity of the air around and in the tent. I would expect to see much more condensation in the cold mountain conditions I encounter here in Colorado because the temperature differential is greater and the dew point is lower. Our higher altitude can't carry as much water as air down where it is thicker. If I start a night at about 60°F and the temperature drops to 30°F (a common event), the air can't carry the moisture load as well. I camped a lot in other parts of the country where the humidity is higher and I've never noticed that kind of temperature differential.