The blue and yellow foam which is so common and popular has an R Value of about 1.36 with ultralight versions dipping down to 0.66 and 'winter' versions stretching to just above 2. Not as good as I'd have expected.
The Thermarest RidgeRest deluxe has an R value of 3.1 and is available for around $30. Combine this with your air matress this will dramatically increase the insulation value.
http://www.thermarest.com/product_detail.aspx?pID=55&cID=3Apparently Antarctic explorer Ben Saunders uses one with a air matress.
http://www.bensaunders.com/resources/expedition_kit.htmlTo get an idea of the R value of your current air matress you could compare it the thermarests on their website, typically a 2.5 cm thick matress will have about an R value of just slightly less about the same i.e 2.3
A 3.8cm thick matress is R rated at 3.2. You should be able to get a resonable idea of the R rating of your current air matress.
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