Originally Posted by zim
My top was temperate and my butt was balmy. The cold area was my shoulders and upper back.
Well this was not only a poor 1st experience at cold weather hammocking, but it is also quite a variation on how folks are usually cold in a hammock! IOW, usually the 1st place people get cold is their butt and lower back, also popular is cold feet. But your butt was balmy, and your feet were fine! Quite the predicament this is!
Here are then just some general thoughts. If I am reading you right, you were inside your bag and not using it quilt style? Because if quilt style, there is a learning curve to avoid drafts around the neck and shoulders or anywhere if you move in your sleep. But you were warm enough on top, so probably not a factor.
There is quite a range over which people find the base HHSS to be warm, from not at all, to 40 or 30 and a few folks do well quite a bit below 30F. But I remember Tom Hennessy used to call the base system of hammock, space blanket, 1 pad and undercover a 30* system, and that is about how it has always worked for me. If it was a windy 20F with no tarp, you might have really been pushing past the limits of the HHSS even by the standards of those who do very well with it.
EXCEPT, you added the HH bubble pad, which many people have used by itself to 40 or even below. Sense you were previously- after a couple of hours at 20F or lower plus wind chill, only "feeling a bit cool at that point", you would think that adding another 30F worth of protection would push you right into toasty land. But instead your " shoulders and back ended up cold" after a couple more hours.
I don't know. Any chance of the HHSS being installed incorrectly?
Can you determine if there was any condensation? Remember, the key to a condensation free space blanket use is to have it close to your body on top of your pad and keep it warm. Needless to say, condensation occurs on cold surfaces. You mentioned maybe being over dressed. I suppose that could keep a person from warming up the space blanket and maybe lead to condensation? Maybe.
Most helpful tip I know of: If you are plenty warm on top and have insulation you don't need to sleep in, place it ( for ex: a fleece jacket ) under the pad in the UC. Jacket zipped closed so it will form a double layer. This can provide an impressive boost of warmth.
I will be very interested to see if you find an answer to why your butt was warm but your shoulders and back cold.