Anyone planning on using some district heating between buildings beware of the systems that use pulled through insulation. I think they must stretch it with the pulling process as I've just been servicing the entry/exits of the piping and have discovered that the internal insulation has completely vanished down inside the ducting!

If you are going to use this type then I suggest that you extend the tails so you can shorten the ducting and bend the insulation out over the tail and clamp it on with jubilee clips.

In the picture you can see that the one on the left is devoid of inner insulation. A tape measure pushed down the pipe revealed 4mm and probably still shrinking. All I can do now is restrain the outer one as suggested above and pour something like polystyrene beads down to hopefully fill the void.

Keith, why not wrap a length of Aerogel fleece around it. Either 3mm thick so you can tuck it into the void or maybe you could squeeze in a 6mm thick piece, then just tape it. Thermal performance is around 3 times better than the insulation pictured