I doubt it has to be a good insulator. The air is travelling up the tube at high speed and there isn't time for it to cool significantly before it emerges.
No, it would have to be a very, very good insulator. You're talking about three miles of tubing exposed to high speed winds and large convective heat losses. Anyone who's taken a course on thermal power systems would know that this is absolutely preposterous.
I'm not saying it's feasible or anything, but he's talking about moving air at over 100 miles per hour. It would only take a couple of minutes to get to the top.
Right, plus it's a 30 metre diameter tube. The bulk of the air moving up the centre would be insulated by the outer layers of air near the inner skin of the tube.
That's not how convection works. It won't be moving quickly in the first place if it cools off. This whole farce of a thread is really tempting me to sink time into calculations to prove that this is absolute bullshit.