So I recently made an underquilt for my DIY bridge. No pictures (I know, I know, broke a cardinal rule) but it's not much to look at. It's rectangular (48" by 60") and uses 5osy Climashield from thru-hiker for insulation. For a top quilt you'd call that a 20 degree quilt.
So last night I gave it a try. It was 25 degrees and I had a 20 degree down quilt on top and the 20 degreee climashield underquilt underneath. Definite cold butt syndrome (and shoulder when I slept on my side). Brr. The top insulation was plenty warm (slipped off my socks in fact during the night because I was getting too warm). I ended up putting my Ridgerest between the two layers of my bridge hammock and then I was fine of course.
Now this isn't what I was expecting. I thought a 20 degree quilt above and a 20 degree quilt below would be fine. Instead I felt like I was fine on top but needed more underneath. Now there are several possibilities here.
1. The climashield underquilt is not as warm as the down top quilt.
2. Air leakage between the hammock and the underquilt. Certainly possible. I used clothespins to try to hold the quilt to the sides of the hammock but I'm sure it wasn't perfect.
3. You just need more insulation under you than over you.
Can someone with underquilt experience chime in? I've been a pad user so far so this was my first hang with an underquilt.