The whole under cover/under quilt thing has been an annoyance as a newbie to hangin'. I can see how it all developed as hammocks started as tropical jungle rigs and wound up being used in the middle of a Minnesota winter at -20F. But the idea that you are going to go hammock camping without an under cover or under quilt anywhere in North America in the mountains or anything other than full summer weather is just wrong. Duh.
The annoyance as a newbie comes with the realization that using a pad inside a hammock isn't very comfortable-- or all that warm. I'm a newbie to hammocks but I'm no stranger to camping and I have all kinds of ground insulation, with most of it being next to useless in a hammock, width being the biggest problem, with comfort and stability being close contenders. The rub is that you just dropped a wad on the hammock and tarp and you are only halfway there cost and weight wise. Of course I assumed that when I bought a hammock I would be able to drop my excellent ground insulation and sleeping bag in and be off and running. Kinda, but not really.
IMHO, any of the hammock manufacturers should work this into their product lines more fully. As a newbie, the double-bottom hammocks seem to be the most forgiving as you can just put a foam pad in and be covered for most 3-season camping. Under quilts can be added as the new owner needs or can afford them.
So, after finding hammocks extremely comfortable and wanting to continue the relationship, I found my cold backside and my budget at odds. I looked over the options and applied what I know about staying warm and dry and using what was already in my gear locker.
Toys on hand:
Hennessy Expedition Zip
Grand Trunk Ultralight
Whoopie slings, biners, tree straps
Self inflating pads
The goal was to get some insulation on the bottom of the Hennessy. The first increment is to get through the rest of the summer with 40F and rain coverage. With the Zip model I don't need to worry about bottom access, so that simplifies things right off. I like the under cover and foam arrangement of the Hennessy SuperShelter, but after buying the hammock, a larger tarp (I live in Western Washington --- lots of rain), Amsteel for whoopie slings, biners, and longer tree straps, I didn't have room in the budget for another $125 or so for the SuperShelter or any of the basic under quilts.
I like the undercover concept for the wind and rain protection. It allows using various levels and types of insulation, give full-length coverage and it is simple to deploy.
My first shot at getting bottom coverage was to adapt the Grand Trunk Ultralight as a bottom cover. I had already replaced the stock rope and S hook suspension with a Wild Country carabiner and whoopie slings, so I had this big piece of polyester fabric with channels in the ends. The bare fabric is just 9oz, which I don't think is bad. It was simple enough to string it up on the ends of the Hennessy and tie the sides of the GTUL into the pull-outs on the sides of the Hennessy. I added a foam pad and space blanket. That arrangement is okay, but the fabric on the GTUL is very porous-- great on a hot summer day, but it allows a low of convection loss and isn't water resistant. The foam pad and space blanket were kind of loose, but worked okay; with the Zip model , it is easy to reach below and move things around. Conversion from hammock to UC was simple enough as I could leave the hanging/drawstrings in the channels and slip the carabiners in and out.
But I wanted more, so I went through my gear locker to see what could be used. I found a spinnaker ground cloth I bought years ago and never used. I think it was a Gossamer Gear product at the time and it no longer offered. It is a white spinnaker cloth with a ripstop grid and nearly 100% waterproof. The "nearly" part is why I never got around to using it for a ground cloth. I also had an AMK Heatsheet and some paracord.
AMK HeatSheet: 60"x96", 3oz
Spinnaker ground cloth: 54" x 86", 4.oz
Paracord --- 6'
Newbie "enlightenment" concept: under covers don't need to be weight bearing, so they don't require sewing and can be gathered on the ends. All that is needed is a non-weight bearing hammock.
So I took the end of the spinnaker cloth and coupled it up with the HeatSheet, leaving 1" of the spinnaker cloth on the outboard side and gathered it, making 1" pleats. As I got towards the center I made extra pleats in the HeatSheet as it is 6" wider and let the spinnaker cloth overlap 1" on the outside edge as I did when I started gathering. My idea was that the HeatSheet would be fully inside the spinnaker cloth, protecting it and making more slack inside. I took a 3' section of paracord and whipped the gathered end of the cloth with 4 wraps and finished with an overhand knot on what would be the bottom side and a square knot on the top side, so gathered cloth hangs down, away from the hammock when tied on. I repeated the process for the other end.
The Hennessey came with Delrin snap hooks on braided line and a prussic knot on the suspension so I used them to clip the spinnaker cloth/HeatSheet sandwich under the hammock. The Hennessey has shock cords pull-outs on both sides and I pulled a little bight of the spinnaker cloth through the rings for the shock cords and took a couple half-hitches around the bight with the shock cord. That mated it all together and pulled the undercover out with the hammock. I may lash some mitten hooks to the cloth for the sides and use shock cord for the ends. The side mitten hooks would allow lashing the HeatSheet on the sides and help keep it in place.
So I hopped in and relaxed for a while. I could feel the heat reflected off the HeatSheet and no convection cooling. A foam pad of just about any kind could be added between the spinnaker cloth and the HeatSheet a la SuperShelter.
*The seal on the sides isn't perfect, so some warmth may be lost there. On the plus side, moisture can get out.
*The 86" length on the fabric I'm using doesn't reach the ends of the hammock, so there isn't 100% coverage. Given the cost and weight, that doesn't bother me. It's fine on the warmth issue and only the last 15" or so of the ends are exposed.
*The important concept is to avoid stretching the HeatSheet tight, leaving it loose and lumpy, trapping all the air possible, as well as avoiding damage to the HeatSheet.
*Another option would be to buy enough lightweight fabric to reach the ends of the hammock and just lay the HeatSheet in the under cover, or gather the layers separately and use a short runner between the end of the HeatSheet and under cover-- the idea being to spread the layers out and stabilize them
*Put my extra clothing in the UC.
*Use Polycro plastic for the most lightweight quick-and-dirty version. Polycro has long been used by ultralight hikers for ground cloths. It is the heat-shrink stuff sold for insulating windows. you can buy the extra large Frost King kit from Home Depot for about $6-- over 200" long. It weighs nearly nothing. It wouldn't breath. With this version, it would be interesting to use the double-stick tape that comes with the window kit to glue the HeatSheet inside on the ends (with appropriate slack in the mix). It would be more than tough enough to hold up a foam pad and it would be super compact.
*Use a silnylon poncho in the same way. It would be great to have light shock cords in the bottom hems of the poncho, gathering the ends and making attachment points. The shock cord could be functional for when the poncho is used as rain gear, drawing the back hem of the poncho up under your pack. I see no reason why a "tailored" undercover couldn't be used as a poncho. It actually makes more sense to use it under the hammock, where the hood isn't an issue for leaking and the dimensions aren't a weak rain tarp size, and more than enough for a UC. Dump in a pad, quilt and/or HeatSheet.
* A good poncho size is ~54" x 108", giving enough fabric on the back side to cover your pack. Note the similarity in size to many hammocks.
My 2 cents and thanks for the newbie rant on insulation.