Hi extopia,
one disadvantage you dont mention is cost, standard poroton blocks come in at 3.99yoyos each, FBT quoted me 16K for the materials alone. Touchwood is recommending YTONG a polish aircret block which I must enquire into a bit more. I could convince the blockie to work with poroton (father in law) but cost wise is it worth it, a cavity wall built using standard concrete outer leaf, kingspan or other expanded polyurethane? insulation and aircrete block internal leaf can give similar U-value but comes in at about half the price (awaiting full quote) I think what i loose is the breathability
Touchwood, while I can convince my father-in law the benefits of extra-insulation in the form of new blocks, external insulation is a whole different ball game his main arguement, is how the insulation is kept in place, plaster cracking as the insulation degenerates. I'm a bit skeptical on this issues too