"Anyone who has worn a moist T-shirt in chilly weather knows that moisture reduces insulative value."
Here there are two mechanisms for heat loss -
Evaporative, the heat loss occurring when a liquid converts into a gas, and
Conductive, when the water in the soaked insulator facilitates the movement of heat from high to low
Jerry, you are choosing evaporation when you say -
"inside a mat, there's no evaporation - it's enclosed - evaporated water doesn't go away into atmosphere - won't have evaporative cooling"
But be careful...
The upright canister is cooled as liquid goes to vapor. Even thought there is a throttled fuel line that precipitates the pressure drop, the canister per se is a closed system, like an air matt. And it is absorbing heat via direct contact with a "warm body" - the atmosphere in this case.
Water will suck up 540 calories per gram to vaporize. So if one were to Really split hairs, there Could be a heat sink as you cook the interior water liquid into vapor. That heat is still in your pad, but you only get it back when the gas condenses.
Again, as mentioned above by myself and others, is it significant? For most of us, probably not. But it does make for an interesting first year physics discussion.