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Can you give any more info on where you want to use the mat. Also, do you want a roll mat (as in a foam mat) or a sleeping mat (all kinds)?
For example:
What temperatures are you going to use it.
Are you bothered about its weight
Are you bothered about its size (compact, three quarter, or full length)
Just a few questions so we can narrow down the suggestions for you
Oarsnpaddle.
"Even the thermarest ought to insulate more than your 2 cm foamy, but of course, it won't if you don't help it the rest of the way and rely solely on the "self inflating" property.The harder it is, the more it insulates as it compress less. As you say, the more you know ..."
I think you may find that the opposite of this is true,which is why there is a trend for foam layer filled 'air mats' lately, to counteract the reduction in efficiency when large air volumes produce thermal currents in the mat which dissipate warmth. The POE ether lite is a good example of this theory, and is much warmer than an ordinary lilo due to having less open space (which is the case with an over inflated thermarest, as the foam inner becomes less efficient). This is not to say that it is a good idea to add a little air (even though this contains moisture, which reduces efficiency), but an over hard mat will be colder in use.
A simple way to add volume to your thermarest is simply to leave the valve open for longer, and let the foam expand to its max size.