I know these questions get old, but I haven't seen this specific issue mentioned in the searches.
The other day I made an underquilt that is two layers. The top layer is a vellux blanket cut to size, and the bottom layer is a wal-mart mystery nylon. Maybe DWR. The two long sides, and one short side are sewn together leaving one short side open forming a pocket. This is to make the underquilt milti-weather capable. Just use as is for windy mild weather. Add a ccf pad, leaves, or extra clothes as necessary in colder weather. The open end is made so it doesn't come open once tied under the hammock. My question is, if I were to put a mylar blanket in the pocket, would it cause problems? I'm thinking that since the underquilt isn't a perfect fit, there will be openings for moisture to evaporate, but it will still keep wind from taking away my heat. Any input would really be appreciated since I don't get many opportunities to test out my gear overnight.