That's a good point, John Sawyer... Insultex probably performs best when used as a supplement, between other things. That way, there are no issues of fit or cold intrusion. A sock outside Insultex does something similar. Carrying two yards of Insultex is as useful as carrying a space blanket. They are light weight. Carrying both is a great idea.
"We must, indeed, all hang together or, most assuredly, we shall all hang separately." - Ben Franklin