Some thoughts on condensation…
I never had condensation problems until I started going UL. I will explain later.
Since I live in the lower desert and am acclimated to a hot climate, I do very poorly in cold weather. So over the years I have learned (and am still learning) about keeping warm and dry. Some ideas from what I have read over the years and what has worked for me.
First we need to look at the two types of body evaporation; insensitive and sensitive. Insensitive loss is evaporation that we are not aware of; that is vapor lost through breathing and a small amount of perspiration that is natural for us, as the body creates it to keep our skin moist. In an 8 hour period of sleep the average person will lose about 6-7 ounces (200 ml) of moisture through insensitive evaporation and probably around 80% of it is from breathing. That means if the ambient temperature is under 85 F and low humidity, your body is only going to sweat a couple ounces at most.
Sensitive evaporation: Above 85 F or in higher humidity, the body will try to cool itself by producing more evaporation, e.g., sweat. So if you are sleeping in cold weather with low humidity there are only two ways you could get more moisture into the insulation; 1) breathing into the bag or 2) creating an internal climate in the bag that is greater than 85 F. Now there are other dynamics such as dew point, amount of moisture that can pass up and down through the top of the bag. Also, keep in mind that when the body is working, such as walking, running, hiking, etc. it will constantly be producing sensitive sweat depending upon the ambient temperature, humidity, and amount of exertion. How much? Depends. I read somewhere that Alberto Salizar sweated 3.7L per hour in the 1984 Marathon!
For more than 20 years I only had one sleeping bag, a 20 degree rated modified mummy bag. It had no hood. Also was kind of heavy since I bought it in 1971. In cold weather I would wear a thick wool cap and a heavy fleece/nylon shell hat with ear muffs. This stuff was heavy by our standards, but the system kept my head warm and I never wore clothes or insulated garments to sleep in, just base layers. In temperatures below 20F I used either a double wall tent or a single walled pryamid that would actually form a sheet of thin ice on the inside walls. Also, below 20F I started using a vapor barrier liner. At first I used cotton long johns, which were miserable. Then I switched to polypropylene and that was the best base layer inside a VBL, even better than stuff like Calipene. So with this system I never had any moisture in my insulation. If I skipped a shelter, a water proof bivy was used over the bag, and VBL inside the bag for temperatures under 20F. I would be warm before going to bed wearing my insulation layers, I would take them off and get into the bag, and I would be cold at first when I got into the bag with just base layers, but would soon warm up.
When I started to really lighten my gear a few years ago, one of my first purchases was a WM Ultralite sleeping bag. Since it had a hood, I just used it without any other headwear; and as a side sleeper my head would often partially pop-out of the hood and my head would get cold. Unconsciously, I stuck my head inside the bag and wala… a soaking wet foot box. Other times I felt cold and added a puffy jacket and things were even worse. When it gets cold, the body tries to protect critical organs like the brain and stuff inside your torso. It feels the head is too cold, so it cranks up and starts heating the torso… too much moisture, insulation gets damp, you get cold, and it is a vicious cycle. I had similar problems with quilts too, and then Roger Caffin told me to get an insulated balaclava, which fixed me up. When it is cold and your feet and hands get cold, the brain is going to protect the vital organs, so it may not start generating more heat to warm up your hands and feet… we feel cold all over, put insulation on our body and start generating an oven. So, good insulation on head, feet, and hands (or keep hands against your torso) will do the trick, assuming a properly rated bag. When you first get into the bag with only base layers, you will be cold at first but give your body a little time to create a micro-climate that is just under 85 F.
Don’t use VBL unless it is under 20F, and wear only synthetic base layers, wool will make you too clammy. Also in winter, I never assume the weather will be good enough for me to air out my sleeping bag… I work to keep it dry. Lastly, don’t skimp on ground insulation.
This is what has worked for me. Perhaps for people in extremely cold environments who do much more winter camping than I do will have different experiences or will even find fault with my thinking/techniques.