Sam Proctor of Proctor Group contacted me 2008/05/15 having seen my mention of the Spacetherm product here on Earth Notes. He is in charge of Spacetherm development and offered to chat to me about existing and upcoming products.
(In fact, he's told me about one new product that I'd very much like to try this winter as it would fit in with my general insulation-improvement plans, but it's too new for me to be allowed to describe it in public yet!)
The Spacetherm product at core is a silica-gel-derived aerogel (such as is used by NASA in the Space Shuttle heat-resistant tiles) with an embedded polyester fabric matrix to reduce brittleness.
None of the manufacturing is environmentally unpleasant as he explained it, and in particular, by being able to avoid blowing agents used in other products, there are no harmful leaks/emissions of these agents possible, and thus no ensuing degradation in insulation performance over time. Spacetherm should be good for 50 years without loss of performance, I am told.
The embodied energy is less than most insulation alternatives other than wool.
I asked him about the insulation value of Spacetherm vs (say) blown foam, and Sam suggests that it is up to twice as good, ie a given thickness of Spacetherm will reduce heat loss by as much as twice what foam will, at a cost of maybe 2.5x for the same overall insulation performance. So there's a significant cost premium if you have space for alternatives, but where space is the premium then Spacetherm may well be valuable. (1m^2 of 9mm Spacetherm blanket might cost GBP25, vs 20mm drylining backed by plasterboard at maybe GBP10 for example.)