 Why does no-one make one?? There are a number of UK and US patents for this type of product. It seams like a good idea, as I think I am right in saying, air is what makes insulated clothing insulated, the down etc is only used as its effective at trapping the air. So surely the lightest insulating material is just air! In which case, a sleeping bag that you just inflate, maybe with some sort of ventilating liner or something, would be a wonder product, no need for a thermarest either! What am I missing, if nothing someone should make one!
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It would be a bit of a blow if you got a puncture.
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 The air needs to be trapped in small spaces to stop the heat leaking away. That's why airbeds are useless in winter.
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 The air needs to be trapped in small spaces to stop the heat leaking away. That's why airbeds are useless in winter. Very true. Which is why Exped, Big Agnes etc started making ones stuffed with down/primaloft presumably.
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 The key is not air, but still air. Otherwise it gets convection currents going in it and they transport the heat nicely away from where you want it. So when you say the down is to trap the air, that really means stopping it moving around. Looking at sleep mats the warmest you can get is probably the inflatble Exped down mat, where again you have down in there to stop the air moving. So you can't get rid of the down or at least something to stop the air moving around. Another approach is the one used in Blizzard survival packs, which are claimed to be armer weight for weight than equivalent down products. They work with cells of air trapped in a foil honeycomb. While popular for survival use, since they're compact, light and waterproof, they haven't taken off for sleeping bag use because of that "waterproof" bit: they're not at all breathable, and that would be a basic problem in any inflatible bag. If it can hold gas under pressure it'll be waterproof, so get very icky and sweaty inside. Pete.
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 I've not had to use mine yet but I think the blizzard bags are meant to let moisture through the inner layer into the honeycomb so not be too bad inside. But then there's also the 'rustle' factor that prevents people from wanting to sleep in them regularly. That, or 'squeak', may also be a factor with something inflatable. I'd imagine too that to get something inflatable with enough air in could end up like an enormous straightjacket! (They do make inflatable splints and MRTs have complete person-size inflatable immobilisers for stretchering folk off the hill). But the main reason, as Pete already said, is the need for still air.
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The new thermarest claims to have 'sorted' the moving air problem in their latest mats. Here's a report from the outdoor awards thingmy.
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T-rests have always used open cell foam as a baffle to stop air moving around. Obviously it can move around a bit to get the thing inflated, but the structure is such that it will prevent excessive problems from convection currents you get in a plain airbed. The trick since the original T-rest has been reducing the amount of foam while not compromising the actual thermal properties too much. As time goes on the lighter models achive their weight primarily by having less foam in them, but moving air wasn't ever much of a problem in older models, they were just relatively heavy. Pete.
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That new T'rest does sound interesting - about the same thickness as an Exped Downmat 7 but scarcely half the weight. I'd be interested to see some info on it's insulation performance, the r-value or whatever tends to get quoted.....
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Well answered, thanks all! Should have realised that one, oh well. Damn that T'rest link, I cant afford a new one, but now I want one! Blocks cold from the ground apparrantly??? Had this argument with my brother the other day (he's a refridgeration engineer), cold does not exist!
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 Cant afford a T'rest? try here Airic's Yes they stop you losing heat to the ground.
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That new T'rest does sound interesting - about the same thickness as an Exped Downmat 7 but scarcely half the weight. I'd be interested to see some info on it's insulation performance, the r-value or whatever tends to get quoted.....
Here's a discussion on the new Thermarest from BPL forums. Some pictures as well. Looks good! Regular length weighs 410g. 2.5" thick. R value is 2.5.
The new Thermarests don't have any foam Pete.
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Blocks cold from the ground apparrantly??? Had this argument with my brother the other day (he's a refridgeration engineer), cold does not exist! Well, okay, if he wants to be pedantic about it then it blocks the heat from leaving you, conducted into the ground. Bottom line is you end up warmer on a mat than not, unless the ground is at least as warm as you. Pete.
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Here's a discussion on the new Thermarest from BPL forums. Some pictures as well. Looks good! Regular length weighs 410g. 2.5" thick. R value is 2.5.
So you get a full length mat for the weight usually associated with a 3/4 length, you get more thickness which, for most people, potentially means more comfort, BUT you only get a thermal performance comparable with the current Prolite 3 series. Definitely some benefits there on the face of it, but imo not a competitor to the downmats then.
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A downmat will be better for winter due to its better insulating value. The new mats sre offering greater comfort for 3 season use. The shorter pads offer good weight savings though.
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My Thermawrap mat weighs 272g. Combined with a Short 260g NeoAir, that would give me a comfortable winter system weighing 532g. Interesting.
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The NeoAir certainly looks tempting -- if the actual weight is as claimed. Damn. I was quite happy with the Montbell UL 90 until I read this.
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The NeoAir certainly looks tempting -- if the actual weight is as claimed. Damn. I was quite happy with the Montbell UL 90 until I read this. Snap! 
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 I wonder what the pack down size is for it. Multi layer foil cell products have had an incredible impact on the building industry, insulation wise. Perhaps Thermerest are using a similar technology.
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It says it packs down as small as a 1 litre bottle. That will probably be for the 'regular' size. The small should be even better?
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