Just went on a 3 night, 3-4 day backpacking/camping trip with this bag. Low temps were 0 degrees 1st night, 2 degrees 2nd night, and -6 degrees (Fahrenheit). I used this with the 2" big agnes two track pad. The first night I went to sleep in a thin base layer and woke up a little cold in the early am but was just adjusting to bag/pad and I think the bag was open a little and I had a little condensation on a very thin base layer top I used. The bag was also spacious, almost too much - I have the long being that the regular is for 5'10" and I am 5'11, plus I wanted extra room for clothing. I threw on a mid fleece and was warm the rest of the night. The next night I filled up the bottom of the bag with all my clothing. I slept with the fleece and was very warm (I had to open the bag some when I went to sleep)and slept like a baby. For whatever reason, on the coldest night(-6 degrees) I was the warmest and had to open the bag and adjust draft collar to keep from sweating. I believe there was a slight learning curve in adjusting the zipper, draft collar and filling the bottom of the bag with clothes so there wasn't so much empty space. I believe I would have been comfortable at the -20 rating and could have done even more if I added more clothing. I should have taken off the fleece because I was starting to sweat at times and I did take off my balaclava but when you're snug in a bag at these temps and wake up to hot - You do not want to lose that heat. With the accompanying two track pad rated at -25 I needed no CCF pad in addition. By the third night condensation had built up on the tent and the bag but the DWR coating kept it from soaking the down and losing loft. However if you are out longer at these temps or colder I would suggest a VBL in addition. I also had plenty of space to sleep on my side and found when I took my balaclava off the third night I could still adjust the hood to give me warmth if needed on my side. All in all a great bag.

Hahns is insane, insanely warm! This is the bag you want for those cold expeditions. The Big Agnes Hahns Peak SL Sleeping Bag is rated down to -20 degrees, and still insanely light. Just be sure to pair Hahns up with the Big Agnes Insulated pad, preferably the Big Agnes Dual Core Sleeping Pad and say auf wiedersehen to to Jack's frosty nightmares.

Reading other reviews, there seems to be a some concern about the pad slot system that Big Agnes uses, and I think it might be justified. There's no down fill on the underside of the bag, which seems like a great idea and certainly trims weight, but without the symbiotic Big Agnes pad designed for this bag, you're probably going to be cold in really low temperatures. I measured and cut a regular closed-cell pad to the dimensions of the bag's pad slot; I didn't really feel like shelling out $100 for a sleeping pad. The advantage of the pad slot system that the bag uses is that you get to carry less pad and you definitely won't roll off of it during the night -- these are both big pluses. However, cold air was leaking in through the sides and bottom of the bag, especially if I moved during the night. I was cold, even though it didn't get below 5 degrees F on my test night. I cut my pad very carefully and there weren't any spaces, so I don't think that this was the problem. It seemed like the bag just wasn't insulating well, though I can't imagine that this would be true with the pad made to go with it. Otherwise the bag is great -- well made, lots of loft, and the draft collar is groovy. I'd recommend getting the accompanying pad if you buy this bag, but personally, I'm going to try a different bag.