I think this sounds good.

I would be very wary of anyone claiming their render itself was insulative, but it looks as if this company applies insulation boards and then renders over them. This is an eminently sensible way of doing things and is the norm for new build in the Benelux countries. Indeed it makes our cavity walls look very silly in a lot of ways.

I have no idea about the company from a commercial point of view, however - they may be complete cowboys for all I know - but the way the promotional material is presented on their website seems very sensible. It does look to me as if the person that wrote it knows what they're talking about and what the product can achieve, which is often not the case with insulation companies.

I guess you could price up buying the insulation boards, covering them with a mesh using bolts which also hold the boards in place(I believe this is how it's done, although there may be battens involved; I'm not sure) and rendering it yourself. Of course, whether this is worth it would depend how skilled you were and how you value your time.

You also might well be able to get a local (or Polish!) builder to do it for you cheaper, but I would feel much happier about this if they had done it before as I'm sure there'll be subtleties as with anything constructional.

I have seen people advertise insulative render. They just apply their proprietary render without any boards at all, which I'm sure is pretty useless. (Obviously the material has a resistivity, but it won't be very high.)

Phenolic foam is somewhat better than PIR, although usually slightly dearer as you'd expect. It's a perfectly "conventional" material and was the best insulant you could buy for many years. Nowadays you can get aerogel (as discussed often on this forum), but this is can't be used outdoors, I believe, and is very expensive and hence only suitable for small areas.

Another advantage of phenolic foam is that it's more fire resistant than PIR.

I think a job like this could be DIYed by a competent person - I can't imagine it going disastrously wrong - , but I suspect it might be a case of "if you have to ask".

Fair enough; I wouldn't know.

Slightly off topic, but an interesting observation:

4.5" phenolic foam on 150mm clinker block with render both sides and finish plaster inside has a u value of 0.18.

Seems like that could be an easy, reasonably low-energy way to "build Georgian". You would just mount the windows flush with the masonry openings and then lay the insulation over so that it covered the sash boxes. That would give you the authentic (for a certain period) set back look. I think you might be able to achieve a passivhaus in England with that construction, although I have not done the calculations.

If you wanted thicker insulation, I suppose you could just leave the windows sticking out of the masonry and notch the back of the boards, but that would be fiddlier.

Has anyone ever seen this done?