Yes - Embodied Carbon.

Tho, as you say, more thermally efficient i.e. saving carbon released during decades of future operation (Operational Carbon), the carbon released at the outset due to oil drilling, processing, manufacture, transportation at all stages, construction, due to that extra bit of work and material (Embodied Carbon) may exceed the carbon saved during operation, leaving the planet worse off eventually. Or even if the Embodied is only half the Operational saved, it still has same effect on planetary heating, as it's having its effect for twice the average duration of the Operational (area under the curve).

This is becoming more and more apparent, as the 'fuel' consumed during Operation (electricity from renewable source) is progressively 'containing' less and less carbon, so less carbon is saved during Operation as a result of the extra insulation, while the initial Embodied Carbon released gets no such beneficial reduction.

The implication is, GBF's traditional emphasis on super insulation (and all of the green building effort for 50yrs) is almost thrown into reverse - minimise the work and materials that create Embodied Carbon, instead throw more heating at it - as long as that's by well optimised electric heat pump, and as long as national decarbonisation of grid electricity continues to progress.

Yes, I simplified - exaggerated even.
I'll add another 'as long as' to my last sentence - as long as the actual Embodied Carbon is properly assessed and minimised as far as possible, by choice of material and/or product, which hopefully themselves have proper EPDs (Environmental Product Declarations) to quantify their carbon content. No broad assumptions, like I implied above. Farage & co will do that for you, once they get wind of all this.

I think it was WillInAberdeen who recently said that woodfibre insulation, once imported to UK, has unexpectedly high carbon content, to rival EPS, which in turn has considerably less than the other, cellular, plastic insulations.

For newbuild, it's a different story. Starting from scratch, or even starting from the shell of an existing building, there's scope for design that fundamentally minimises Embodied Carbon, even sends it negative, while achieving minimised Operational Carbon, even sending that negative as well.

Note the large chunk of Embodied Carbon that can be avoided, by completely eliminating a heating system, not even ASHP, if Passive House standard is achieved.

Posted By: VictorianecoAny quick way to measure my project or any practical advice?

What information do you have to hand? Can you take area quantities from a 3D model?

It's relatively straightforward to convert an area of insulation coverage to a volume, and then to multiply this by EPD data for each of your insulation choices. This will allow a side by side comparison.

Below is an extract from a spreadsheet I made a few years ago to look at wall construction. My ambition was to make this 'dynamic' so you could swap out different layers from a drop-down list and have the thicknesses change, but keep the u-value static.

Needless to say it got too complex and I parked it, but the principles are sound.

Posted By: VictorianecoI am looking for a comparison of the real-world savings and practical implications for two insulation strategies on a 1960's uninsulated cavity wall property.

While I can't offer a calculation, taking into account the energy costs I'd likely choose option 2 - cavity fill + EWI. I'd try to use products that, for the renovation as a whole, balance out any carbon-heavy materials that can't be easily avoided with natural insulations that have a negative carbon cost (taking sequestered + embodied carbon into account). On this the most recent UK guide that I've come across is this one from Greenspec - https://www.greenspec.co.uk/building-design/embodied-carbon-of-insulation/ - which includes this chart (based on achieving 0.15 W/m²K):

I Ultimately, the decision isn’t just about the embodied energy of the materials, but also the future energy use, the labour involved, and the financial return. Labour cost isn’t the primary factor, but whatever I do needs to make sense from an investment point of view. There’s no absolute right or wrong answer here.

I do know that I need to insulate the cavities, because that will reduce air leakage and improve overall performance. If I don’t insulate them, I’ll end up with thermal bridges across the property, which could create cold spots and potentially lead to damp issues.

What I really need is a simple comparison:

Method A will cost X, with a payback period of Y.

Method B will cost X2, with a payback period of Y2, and running costs reduced by Z.

I’m just wondering if there’s a quick way to estimate or suggest those figures so I can make a more informed decision.