Originally Posted by
quasarr
If you can afford it, buying a bag for the summer months will really be worth it. You are not going to be happy lugging around that 10* in June, July, and August in the mid-Atlantic states where the nighttime low is 60-70. You probably won't even be able to sleep in it during July!
And I agree with Malto that you might consider a more versatile system. A 10 degree bag is pretty limited - you will be comfy when it's very cold, and not very comfy the rest of the time! Another option besides a bivy is to have two bags or quilts. For example a 20* bag and a 40-50* quilt. I think this is a great "3-season-plus" option. In spring or fall, use the 20. In summer, use the 50. In the farther reaches of spring and fall, layer both! (the downside is it's a little bulkier than using a bivy, but the upside is the summer use)