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That sort of makes sense, though that’s my uneducated take not a comment on your reasons/methodology!
I think there’s probably an art to picking the best solution with space/cost/ease of installation to consider along with actually getting calculations right.
Not had a 40° heatwave in our place yet but I’m happy how it dealt with 30° the other week considering the constraints of the building. We do get serious pull through of air though as 3 outside walls and big windows on the 6th floor with the prevailing winds on one side, can be too much at times.@ectoplasmosis think that’s what my friends have used on their breathable hempcrete house they are designing/building. No membrane and very thick walls with a high resistance to overheating, look forward to seeing it when it’s done.
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Mr_Smyth
Conventional insulation like phenolic is good at keeping heat in - which obviously you want during the winter - but less good at keeping heat out because its heat storage capacity is poor. Wood fibre insulation buffers heat, which is good when temperatures cycle on a daily basis with an afternoon/evening peak, because overheating is both delayed and reduced, and the stored heat can escape outwards during the evening/overnight.
The poor heat storage capacity of conventional insulation is one factor in why you end up with so many new builds that cost nothing to heat but are unbearable in summer.
Bear in mind that while K-value does represent thermal conductivity , U-value represents heat transfer overall as it considers K-value, the thickness of the insulation and other factors.
Not all wood fibre insulation is the same and the Steicoflex 036 we're planning to use has a U-value of 0.36 as the name suggests. You've got to consider the overall buildup of where you're putting the insulation too - e.g. our external wall buildup for the brick walls will contain 140mm of Steicoflex and give an overall U-value of 0.223.
This is all a bit academic though to be honest! There's no way I'd be using foam EWI on an Edwardian house as it's not breathable, plus the type of insulation will have very little impact on overall overheating factors. As per the paper I linked to above, reducing solar gain via shutters/solar control glass will be key.