I have purchased flannel sheets at discount pricing and used them as batting for lightweight lap and throw quilts. Where I live it rarely gets cold and these are more than warm enough. Also, they are very easy to take to outdoor events. A queen size flannel sheet will often be enough for 3 laptops.

I have used flannel for batting in summer quilts and really like it. Just be sure to get a good quality flannel and pre-wash it.
I like the idea from SusieQOH of flannel batting and backing.

When I was in chemo. It was very cold. They had to keep getting me the heated blankets. They wouldn't let me use an electric blanket that my cousins bought me to use during chemo

I made a childs quilt and used two layers of flannel for batting and was very happy with it. The flannel was a like new sheet I found at the GW.