Bivy sacks add good warmth by allowing for the air around your down or other insulation to be less affected by colder air. A good sleeping pad with a high r-value helps too, like some of the great mats from exped. Other than that I would say to eat and drink hot food and fluids before bed, layer up a bit, and crawl into your bag on a good pad. Should make for a much better night.
Maybe this is a bad idea since nobody else has said it, but here is what I do and I have never had a problem with being cold. I usually have a 15-20 degree bag. Based upon a recommendation from a friend I started throwing a hand warmer pack into the bottom of by bag. I don't wear any extra layers and don't wear socks so that I don't sweat in them. Does a great job. I did just upgrade to a zer degree kelty but i imagine I will keep doing it. It does make it really hard to get out of the bag in the morning because it is so warm but I like it.
I usually dont want to carry enough of them to rely on them every night, but it does work damn well. I sometimes carry a pack of two just in case it gets a little colder then expected for a night or two.
Originally Posted by crumy
I've used a bivy a number of times and love them. Most of the time its during backcountry camping and a few times during the winter. Ill throw my ground pad inside it and sleep right on the snow if needed. They add a lot of warmth because they allow the sleeping bag to further contain the body heat. Make sure it's gortex or some other breathable fabric, otherwise you will get condensation even if the head area is wide open. The bivy I use is REI's basic one. Use one of their 20% off coupons and it's something like $80. I wouldn't sleep in the rain in it on purpose, but I've done it. Most of the time i find it too warm, but its great in cold weather. For the basic one I have, there's no head covering. It's really just a sleeping bag shell. So if running into bad weather I tuck my head under a tree or rock overhang. The rest is waterproof. Sorry, getting off topic. Just saying I've used them a lot in all types of conditions.
Fleece vs bivy? This is the way I look at it. If you were cold outside would you be warmer by throwing on a fleece or a gortex shell? In my experience a shell is much much warmer. It's all about blocking the outside air and containing your own.
Clothes worn in the bag? I go back and forth on this. Not sure why but sometimes it's warmer with longjohns and other times its not. Generally bulky clothes don't work. Air in a bag is good, you just dont want it to circulate out of the bag. I will mention that blood circulation is just as important as other factors. Wearing socks, LJs or other clothes that are a little tight can make you colder than not wearing them at all. It's important to not be too constricted in a bag or any one part of your body. This doesn't happen often, but it does under the right circumstances...usually wool socks that shrunk on me in the dryer and I'm realizing it too late.
Also....it's counter intuitive, but a decent ground pad can make a difference. Thermarests are comfortable, but not great for insulating in cold weather. a foam pad is much warmer. In the winter when its in the teens or lower at night ill use both.
I've noticed the past couple hunting seasons that sleeping on a cot in a tent is colder than the ground. Having that air circulating below you on a cot has an impact when the temps drop. Just thought I would mention it since you didn't state what you sleep on.
I had to go check since others asked about weight. The REI basic bivy is 15oz. Not too bad. I also like the fact you can zip it down a little which makes getting in and out much easier. Nothing worse than having to take a piss in the middle of the night and it takes forever to get in/out your bag during cold temps. Also the zipper is great for regulating heat. More often than not I get too hot vs cold.
Great value for the price and quality. Read the reviews for further info.
I get cold through the night no matter what. I think what happens is sometime in the wee hours of the morning my body just runs out of fuel and can't produce enough heat, and I start getting cold from about my knees down. Since I have to get up through the night to pee anyhow, what I have started doing is keeping my stove at the ready and when I do have to get up I quickly heat up some water (not to boiling, just warm or slightly hot to the touch) and dump it in a nalgene bottle and put it down in the bottom of my sleeping bag. If bears aren't a concern I also try to keep a little snack at the ready too. Between these 2 I can usually make it to time to get up OK.
+1 for the fleece liner. I have not weighed mine but when I was in Boy Scouts we would put them in our bags when snow camping. My mom made one for me that added so much warmth i only use it when it is COLD! It has become the go to present that she makes for everyone in the family. My Wife is now experimenting with making one with a pillow built in for her. I also have found the long underwear makes me a bit warmer but too many clothes does not make a noticeable difference.
I like the hand warmers by my feet sometimes too. Something else that I do is a little tree trimming. I trim off a bunch of pine boughs (soft young ones) and arrange them on the ground with the arch up, cut ends outward-enough to have about 3 to 4 inches of loft. Then I pitch my one man tent over the top of them. Coupled with my waffle pad, this makes for a pretty cushy and warm mattress. What you get in the loft of the boughs is tiny pockets of air that hold heat. This might sound like a lot of work but its worth it. Credit to the 'Survivorman' for this tip.