The problem with pipe insulation is that solar thermal systems operate at extremely high temperatures, all the time. You can forget most types of plastic and rubber insulation immediately, because they'll just melt off.

You need to find insulation that can withstand constant temperatures of at least 180°F (82°C) throughout the entire system and at least 250°F (121°C) on the pipes running from the collector to the heat exchanger. It doesn't matter too much what the insulation is made of, as long as it can resist those temperatures.

As always, there are special rules to bear in mind, in this case for external and buried pipe runs.

• First, you should never use fiberglass insulation for external or buried pipes. Fiberglass absorbs moisture, becomes saturated and loses its insulation value. It's great for indoor pipe insulation, though.
• Second, if you're considering EPDM and lower-temp rubber insulations, some will work for external and buried pipes, but you still need to check the rating. Otherwise, they will become brittle and wear away over time. This is especially true if they're above ground and exposed to the elements.

Whatever material you choose for external pipes, it's always best to put a jacket around the outside. The environment is very much more hostile than inside a house's walls and your pipes need as much protection as they can get. The sun's heat, winter's cold, UV radiation, insects, birds and other small animals are all potential aggressors, and can all be kept at bay a little longer by a resistant jacket.

Traditionally, aluminum jacketing has the best track record: it's reliable and it lasts a long time. PVC jacketing isn't too bad but it has a tendency to wear out quickly (relative to the very long life of your solar thermal installation), especially on joints and fittings. A modern alternative that looks promising is based around vinyls.