There are three main factors when choosing a sleeping bag for fastpacking: weight, warmth, and packability. Balancing these three traits will give you a sleeping bag that takes up minimal space, doesn’t weigh much and still keeps you warm. Most fastpacking trips are going to take place from mid spring to early fall. Because of this we are looking at 2 to 3 season sleep systems that pack well and are rated to around freezing or a little higher.
Sleeping Bag Vs. Down Quilt
Fastpacking is about focusing on moving fast with minimal time spent in camp. Because of this we highly prefer ultralight quilts. They might reduce comfort at night some, but they reduce your pack weight and typically pack down much smaller, which for fastpacking is very important.
Building a Sleep System
Something familiar to ultralight backpackers, but maybe not trail runners is creating a sleep system that you use to increase the warmth of your sleeping bag. This is everything you wear or have on the outside of your bag to help keep you warmer. You might use a 40 degree bag, but combined with a light fleece, light down jacket, and even your trail running wind/ rain jacket it your 40 degree bag would be suitable for freezing temperatures. Keep this in mind as you build your fastpacking gear kit.
The Revelation 850DT comes in several temperature rates from 10 degrees to 40 degrees. We recommend the 40-degree version for summer fastpacking as long as you combine it with a light fleece or wind jacket for warmth.The DT stands for Dry Treated so you know the down will help resist water in case you encounter wet conditions. It won't completely resist a deeping soaking, but it will help the down dry out faster and regain its insulating properties. The Revelation 850DT 40 Degree weighs in at a scant 14.32 ounces and can easily be used to fill the small voids in your pack without the use of a stuff sack. With a small compression sack it can pack down to just larger than a softball.
If you don’t think a down quilt will work for you, the Western Mountaineering HighLight will be one of the lightest, most packable sleeping bags you will find. At only 16 ounces it is nearly the same weight as a down quilt, but still has the mummy bag fit. It is made with 850 fill down, comes in three sizes, and is made in the US. The Highlight is a favorite bag of multi day stage racers and crosses over very well to fastpacking. At $340 it is a little expensive, but you get the high quality materials and construction that Western Mountaineering is known for.
Wind Hard Down Quilt - $105
The Wind Hard Down Quilt is our value pick for a fastpacking sleep system. Down quilts and ultralight sleeping bags have traditionally been a very expensive investment. However, with an increase in popularity there are becoming less expensive options that are still great quality and pack down well. The Wind Hard quilt is rated to 32-41 degrees with a stated 850 fill down. In our testing we found it to run a little colder, so we recommend coupling it with a few layers to meet these temperatures. It packs down to smaller than a Nalgene and weighs in at 15.5 ounces. Overall it is a great value in an ultralight package.