Hi Jacob, The lowest "U" value given to any one product, is that of Aerogel or "frozen smoke" to give it it's descriptive nickname.
It is a material which has the lowest mass of any solid matter, so far produced, and yes is used as an insulation in buildings, so far in tape form for eliminating "cold bridging".
You speak of a large building! and a desire of low cost (very commendable, by the way) although I have no way of knowing the final use for the building, I'm going to take you down a road of, what some might claim, fantasy...
First study the benefits of hempcrete, a form of concrete that can be installed by anyone - fit that is - into shuttering/forms. there is such a plethora of information online about this, as to not warrant posting any of them here...just do the search [The hemp builder] if you are interested.
Straw bale building, is another technique with a longer history than you would think...I know, I know...these almost seem absurd to the ordinary man, but I have grown to know you I think, through your posts, and I believe the thought of ridicule would not put you off...
http://www.strawbalehouse.co.uk/ just skip the eco religious dogma, and learn.
Both these techniques negate the use of insulation, as we mostly think of just now, but they are on the horizon of most new regulation, if you have the ability to think outside the box...bosshogg
Few people are capable of expressing with equanimity opinions which differ from the prejudices of their social environment. Most people are even incapable of forming such opinions.
Albert Einstein 8)