UKC

Is the point of a bivvi mat to insulate you from the ground with trapped air (as in Karrimat or Thermarest), or to physically separate you from the ground (picture a very low hammock!)?

Fred

In reply to Fredt:

The body loses heat in three ways; conduction (ie from warm - cold / body - ground), convection, radiation (insulation of the mat)

So to answer a bit of both...

In reply to Fredt:


Not really this because if you were in a low hammock you would still feel cold in cold conditions. That's why the more simple air mattresses don't work well in winter. I wrote a longish blog post on my misadventures with the Alpkit Numo on this: http://lightfromthenorth.blogspot.fi/2014/01/physics-you-can-sleep-on-desig...

Basically if the air can create convection currents below you, you will chill. The air chambers in both closed cell and open cell (thermarests) mats are too small for this to happen.

In reply to mh554:



Well actually the radiation is only reduced in silvered mats old boy!

But the main mechanism is conduction (if on cold ground) or convection (if separated by an air layer). Foam mats (or air mats with foam inside) work best as they highly reduce conduction due to the air layer but also reduce convection due to the open cell foam reducing convection.