I am surprised this idea has not taken off more given that it is (trapped) air which is the insulator in all warm clothing, the more air trapped per unit weight being the yardstick for cross comparison. A product known as Aerovest (http://www.survivalmetrics.com/store/Item/id_aerovest_emergency_survival_vest) was available a few years ago. This provided a waterproof inflatable mylar vest weighing around 2 oz (60 grams). It was a little difficult to inflate (and particularly deflate and the closure and re-use features were not all that they could be – however it could certainly save your life for a very modest weight in your pack/pocket.
This product Xerovest is the current iteration (http://www.xero-gear.com) at US$9 ea. Again it weighs around 60 grams. It can be deflated with a straw and reused.
Klymit (https://gearjunkie.com/pump-it-up-jacket ) started making a more durable one which morphed into Nudown (https://www.nudown.com) a much more high-end inflatable products. These combine a rain shell with an insulated jacket in one lightweight garment.
Of course a garment does not need to be inflatable to provide a degree of protection from the elements. Some folks such as Blizzard have a ‘bubble wrap’ approach for example (http://www.blizzardsurvival.com/) . Then there are those which utilise simple mylar such as in the mylar poncho http://www.coghlans.com/products/emergency-survival-poncho-1390. Lifesystems have some useful gear such as http://www.theultralighthiker.com/lifesystems-thermal-jacket/
You can make your own as illustrated here: http://www.theultralighthiker.com/more-fun-with-sticky-tape-ultralight-mylar-vest/ and here: http://www.theultralighthiker.com/fun-with-sticky-tape-mylar-poncho/
Emergency shelters such as the Blizzard bag: http://www.blizzardsurvival.com/product.php/100/blizzard-survival-bag are also a good idea. Some examples: http://www.theultralighthiker.com/survival-shelter/
Nudown Whitney vest
Blizzard Survival Vest
Coghlans Emergency Poncho