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Evening All, Currently having some work done on my 1970s house, my builder has suggested we could save a significant amount if we were to use a different cavity wall insulation method to that quoted by my architect, for my external walls. Architect has spec'd Kingspan K106 90mm, leaving a 10mm gap (in the 100mm cavity), my builder had initially suggested we could look at Knauf Dritherm 37 100mm instead, though this would end up needing some additional insulation internally (insulated plasterboard) to achieve required U values for building regs. So our builder has suggested we could look at a different type of Knauf, suggesting there are other thicknesses available. I've taken a look at both the Kingspan K106 and Knauf Drithem 37, there is a difference in the thermal conductivity between them, 0.018 (Kingspan) and 0.037(Knauf). As a complete newcomer to insulation types, I'm a bit confused, is there mineral wool out there that can compete with the U value of Kingspan? Also, with the Kingspan, we'd leave a 10mm gap, but with mineral wool it would take the entire 100mm cavity, are there drawbacks to this? What is it that makes Kingspan a superior product? Is it the U value alone, or is there more to it? I'm not sure whether to fork out the additional to keep with the spec, or look for alternative ways to insulate if the saving is in the 000's? Any ideas, guidance or pointing me in the direction of info would be appreciated. I'm sure there are ways to meet regulations using mineral wool, else why would it be available, but I guess i'm trying to understand the points to consider to help guide why one chooses one over the other. Cheers Ant