Hi Claire, from what I can see the air mat is an air only bed utilising long thin ribs conjoined to form a mat, air is blown in to inflate them, the SIM has cellular foam inside it that expands drawing most of the air into the cavity for you, then it's finally topped up to the desired pressure.
Dave.
------------- Never argue with an idiot, they will drag you down to their level and beat you on experience.
Mark Twain.
An air mat can be folded into a rectangle and does not have to be rolled. Have been looking at SIMs but they take up a bit more space than a double air bed as they can only be rolled up (or left fully up & flat).
SIMs are more bulky but generally offer superb insulation because they contain foam. They were originally invented for adventurous types who dangle off mountains and sleep on snow and ice. Any bed or mat that contains air is going to be much colder because the air will conduct your body heat away and also creating damp where your natural body moisture condenses on the cold mat. So you end up needing more layers under you.
Having worked this out I prefer a SIM because I don't have to pack as much bedding and there are far fewer problems with condensation too.
I use air mats that have down or artificial insulation in them. They pack up smaller than a sim, lighter than a sim and have the same R Values. The downside is they offer next to no insulation if punctured. A deflated sim will still insulate.
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