I have a very cheap crummy 10$ sleeping bag, and I'm adding a fleece liner to make it more comfortable and increase it's functionality.
Because this bag is such a cheapo piece of junk, the materials used actually ended up costing more than the bag. However, if you are an avid camper and maybe have several really nice bags, then this zip-in liner system can be interchangeable, and possibly extend the life of your higher-end sleeping bag. Its all about better living through doing-it-yourself.
The Liner is designed to zip directly into the edges of the sleeping bag, allowing you to use the installed sleeping bag zipper to vent the bottom if your feet need some air. The liner will also still allow you to zip together 2 identical sleeping bags to make one big one, if needed. The liner can also be removed entirely, and zipped up on its side just like an actual sleeping bag.The other advantage to having a sleeping bag liner is that it can be removed and washed, which keeps the bag itself cleaner.
This instructable is made for a standard adult rectangle bag, but it will probably work with a mummy bag as well, with some modifications.
The sleeping bag used here is Purple on the outside, and yellow on its Inside as a reference.
First, Gather Materials:
1 regular adult rectangle sleeping bag
2 open-end separating zippers, 100-inch length each, preferably with dual pullers.
2.5 to 3 yards of polyester fleece, available from a fabric store
Step 1: Fabric Cutting
Fleece fabric usually comes in a width of 54 to 60 inches. I found some truly hideous material on sale at a fabric store that is a basic 2-sided brushed polyester fleece. Its easy to sew, machine-washable, and the pattern will scare off any marauders.
I started by laying the fabric on top of the bag folded length-wise down the middle, but this is a little bit too narrow to make a good liner when the fabric is folded lengthwise.
So, you can refer to the photo below where I am turning the fabric to fold it width-wise, with the selvage edges at the top and bottom of the bag. (selvage is the term for the white edges with the printed fabric info on it)
To be comfortable, my liner needs to be about 72 inches wide total, and the length will end up being whatever the fabric distance is from selvage edge to selvage edge; in this case, about 60 inches.
I know that 60 inches is not long enough to meet the length of my sleeping bag, so I am measuring from the selvage of my liner, down to the sleeping bag zipper.
I get a distance of about 18 inches.
So, I am going to trim off the excess fleece material that i don't need, and attach it onto the bottom.