Frozen Butt Syndrome: when your keister gets cold because mother nature is blowing air underneath your hammock.
Regular Hammock
A regular hammock (like the 1/2 Shell No-Net Hammock pictured above) has no insulation. It provides you a great, comfortable place to hang but does very little to protect your body from the drafty air whipping underneath you. We are not just talking about camping on ridge tops or in high winds. Even the slightest breeze can turn a pleasant night out in the woods into an arctic-like experience.
Tent Camping- Sleeping pads
Tent camping combats a cold nights sleep with sleeping pads (no they are not just for comfort!). A proper sleeping pad uses foam insulation to help trap the warmth of your body, keeping the cold frozen ground away from you. Sleeping pads use r-values as a scale of how warm they can keep you. Inflatable sleeping pads, with no insulation, may have an r-value of .7-1 while a foam, four season sleeping pad might have an r-value closer to 10. The higher the rating, the warmer your heinie.
Sleeping pads in the air
Although sleeping pads work great on the ground, they are not meant to be used in the air. This is not to discredit the people out there using this technique. Some swear by it and if it works for you then great! From our experience, even with a sleeping bag added, we find a sleeping pad to be difficult to navigate and not as warm as other methods.
Underquilts to the rescue
Underquilts are exactly what you may think they are. They are a layer of insulation that goes underneath your hammock to keep you warm. They are often stuffed with a similar material to your sleeping bag. A proper underquilt actually sits a few inches underneath your hammock to perform a double act of awesomeness. It keeps the cold air out, yes. More importantly it traps the air warmed by your body, keeping you nice and toasty. Without an underquilt, the air is sucked right out from underneath you.
Fully Integrated Underquilts
You can certainly buy a hammock with a separate underquilt. That is how most people do it. However, this can add complexity to hanging your sleep system. Also, the ends of the underquilt often do not cover the whole hammock and you end up with cold zones at your head and your feet.
Full length integrated underquilts, like Sheltowee’s Shell Zip Hammock (pictured above), offer complete protection from the elements on the back of your body. No need to worry how to hang this under your hammock, everything is built into the hammock system itself and perfectly assembled to keep you warm without the hassle of set up.
In Summary
Hammock camping is inherently different that tent camping. Both have their methods of keeping you warm: sleeping pads for tents, underquilts for hammocks. Full length integrated underquilts provide ease of use and keep your haunches comfy while mother nature does her thing.