Depends on how thick it is when compressed under your body. Insulation value is proportional to how thick it is, i.e. "dead air" space (as opposed to an air mattress, where the air circulates, moving energy from a hot spot (under your body) to a cold spot (around the edges)). In not too cold summer temperatures I would speculate that a robust, heavy-duty wool blanket would do the job quite nicely. It will also depend on how warm your sleeping bag is: To a certain extent, you can tolerate some heat loss in one spot if you're otherwise warm and snug.


The one and only time I've been sleeping on an air mattress the last 25 years (car camping "the American way" with our American friends, all our own shiny gear back home in Europe) my wife and I used the rubber/synthetic wool carpet from our rental car (toyota raw-4). That was somewhere around 1/4 inch thick. Don't know how cold it was, it may have been as low as the 50's during the coldest night.