Mike

A very good point. Since maintaining vapour-permiability will be essential following my current renovation, I'll be inscribing that, together with a few other key points, on a metal plate that will be screwed next to my MVHR unit for posterity - it may be worth considering. In theory I shouldn't need to do that here in France because a mandatory 'house manual' is now required as part of the conveyancing process. It has to include, in detail (product names, technical characteristics, serial numbers, etc.), all the materials and equipment related to insulation, thermal performance, energy use & generation, etc., from initial build (if built after the law came in) through all subsequent modifications. It must also contain instruction manuals, servicing requirements & maintenance details - for example the need to use vapour-permiable paints.

Science is a process. Over time, through repeated experimentation, testing, review and criticism it develops explanations of reality, often based in mathematics, that best fit the currently available evidence. If new evidence is gathered and/or better explanations are developed, then the consensus best explanation of reality eventually changes. Opinions are subjective personal judgements. Individual scientists may hold them, but that doesn't turn opinions into science, nor science into opinions.

If you have a cavity that's certainly the best solution.

I briefly mentioned using cork as a spacer earlier; I'd use continuous strips, not just spacers every 600mm; without that kind of firm support 45x45 would be too flimsy. Personally I'd increase it to 45x75 if I was using timber.

Yes, it will be a big help Installing MVHR, would help too. It keeps internal humidity lower than it would otherwise be, cutting the moisture entering the structure in winter and allowing it to dry out quicker in spring. Making the building airtight should also be a key priority and to accompany that you need an effective ventilation strategy, be it MVHR or otherwise. There are several threads on here about both topics.

Nor me, and £86 million is certainly worthwhile. Just a shame that their parent company isn't British too - their turnover was €17.3 billion (the 4th biggest company in Denmark).

Welcome! There are some exceptions in Part L of the Building Regulations for 'listed buildings, buildings in conservation areas and scheduled monuments' and for 'historic and traditional dwellings'. However they only apply when the work would 'unacceptably alter the dwelling’s character or appearance' - in which case 'the work should comply with standards in this approved document to the extent that it is reasonably practicable'. In theory you might qualify under the first of these, but in practice it's unlikely to help; it's more for when insulating internally and externally are both ruled out for heritage reasons, or double glazing can't be retrofitted to historic windows. In theory the second case wouldn't apply as it's a barn, not an existing dwelling, but the Local Authority may be flexible on that if they want to see it put to use as a house. Don't use PIR. Vapour-permiable options are readily available and advisable - you'll find some topics on this forum. In a renovation there are provisions for when the the full insulation standards can't be met but, from memory, they wouldn't normally apply in a conversion. Is unlikely to be financially viable outside the World's major cities, where the property value /m² of floor area gained exceeds the cost of Aerogel. That's not an argument that will fly; you'll need to use clever design to maximise the space that is available. Sounds like you need to find yourself a local architect experienced in restoring / converting old buildings.

Which is good - but they're owned by Vestas Wind Systems A/S - who are Danish.

It used to, but most of that was closed down. It now holds 12 days maximum. https://www.dw.com/en/why-uk-gas-storage-levels-are-concerningly-low/a-71281361

Last time I checked Denmark was still the World's 2nd largest manufacturer of wind turbines - it was they who developed the technology in the '80s. Of course the UK could have done the same, but why do that when the North Sea was full of oil and gas? Though, in view the current headlines, maybe Denmark should have invested in nuclear weapons instead...

There's best avoided - the problem is that they tend to push moisture-laiden internal air through gaps in the building, leading to condensation within the structure. MEV avoids this risk. Choose either a central central unit, or decentralised dMEV. If you're not going to DIY it, then yes, you need to find a contractor to propose a solution. You can post their proposal here for feedback.

It doesn't, so I'll get myself some more; thanks for the heads-up! Yes, there is a recommendation to use no more than 2 adjacent contactors unless there's a gap between them for cooling. I'm populating the row with alternating breakers & contactors, which should help.

It depends. If the external wall is in good condition, free from penetrating rain and not very exposed, and provided the DPC is doing its job, then it's generally accepted that it's safe to apply internal wall insulation to a solid wall if the resulting wall u-value is at or above 0.5 W/m².K, pretty much irrespective of the method used. However there is significant research such as the Historic Scotland Refurbishment Case Study 4 where they went down to between 0.37 and 0.19 by using vapour-periable materials (important!) without using a vapour barrier, and without any concerns during their 18-month initial study (AFAIK they never published their promised follow-up report); it's worth downloading a copy. Since most of the research is on insulating solid walls I don't recall seeing anything about upgrading insulated cavity walls. Ideally you'd find someone to model the risk with software such as WUFI (though it isn't without problems - best to model the entire building, not just the walls). As mentioned in one of my earlier posts above, remove any gypsum plaster if there's a risk of any condensation against the existing wall - many materials can tolerate some moisture in winter temperatures provided they dry out again quickly enough in spring (so a Glaser analysis using a tool such as Ubakus isn't very useful), but gypsum holds the moisture and turns to slurry. For more reading, see the link to the other thread I posted above. You'll see there that I chose hemp insulation for my own IWI due to its good moisture handling abilities (plus good decrement delay). I wouldn't have been at all comfortable going as low with my u-values using mineral wool, or using a vapour-impermeable build-up. I'd probably choose cork.

That is a lot! Double checked my contactors and they seem to be 0.8 to 1.8 W. Mine claim to be good for 100,000 to 150,000 switching cycles so until your post I wasn't too worried about that - but maybe I should be? The only snubber at the moment is for the circulation pump.

dMVHR can be useful in an apartment or retrofit, but in a new-build house a central MVHR unit normally has the advantage. Only 2 holes to cut through the outside wall, only 2 visible external grilles, no visible boxes on the internal walls, constant-flow-rate fans to counteract the build-up of filter debris, only 1 set of filters to change, no worries about noise from fans or the outside world, many more units - PassiHaus-approved or otherwise - to choose from, the availability of a summer bypass (on good units), a single point of control, pre-heater and/or post-heater* options if needed, often lower installation costs (except in retrofit). *could be added to ducted dMVHR too.