A multi-faceted issue:

1. The new thinking, which removes high-standard External Wall Insulation (EWI) from its pedestal as the default/ideal for retrofit, on grounds that the embodied carbon (i.e. the CO2 and CO2-equivalent released to atmosphere) created in 'quarrying', manufacturing, transporting and installing EWI, is likely to exceed, sooner and for longer duration, the in-use carbon it saves over the building's subsequent lifetime.
Couple that with the rapidly declining carbon content of mains electricity, which means that electricity consumed over the building's in-use lifetime, already and increasingly carries little penalty in global heating.
Then, compared to super-insulation hence consuming minimal energy-in-use, the alternative of omitting the super-insulation hence consuming plenty of 'harmless' electric (via heat pump) energy-in-use, can actually be a lot better for planetary climate, both in this emergency-short-term, and over the building's lifetime.

2. Retrofitting the 30% of UK building stock which is still-uninsulated cavity wall, with or without EWI the cavities should be filled (CWI), as it's near-impossible to prevent outside air blowing around in the cavity. Even fitting EWI can't be relied on to seal the multiple cracks, entry points and open-to-loft top of typical cavities - outside air is still liable to bypass the EWI, separated from the interior only by the inner skin. External render can't be relied on either. Even Partial-fill CWI is poor at preventing outside air blowing around in the cavity; it really needs to be full-fill CWI.

3. CWI is cheap to install, with small embodied carbon. So, considering both 1. and 2. above, the new default looks like installing full-fill CWI, without EWI, along with electric (via heat pump) energy-in-use. In which the CWI is an indispensable part, hardly works without.

4. But what about cavity-wall buildings in high-exposure areas? There, CWI is pretty much banned by Building Regs, certainly with traditional 50 wide cavities (as opposed to more recent 100-ish cavities) and certainly in exposure Zone 4 areas (i.e. most of the west/southwest of UK). External render doesn't help.
https://assets.publishing.service.gov.uk/media/606c366ad3bf7f400e10af54/wic-final-report.pdf also gives little hope that a coat of gunk would make the wall more suitable for CWI.
Does anyone know different?

5. Without CWI, a house has nearly zero insulation, if outside air is blowing about only separated from the interior by the inner skin - even if accepting plenty of 'harmless' electric (via heat pump) use. Especially if extending an unfilled cavity-wall house; the existing cavity wall becomes an internal wall, but super-cooled because its cavity is continuous with the still-external parts of the cavity wall adjacent.

6. Similar applies if you've EWI'd your half of a semi-detached house; your unfilled cavity, inside the EWI, will be super-cooled because continuous with next-door's unfilled cavity.

So WTF to do about all this? Any solutions, bright ideas?

Should have said internal airtightness and IWI...

Posted By: sgt_wouldsShould have said internal airtightness and IWI...

And as the old mantra says: build tight and ventilate right! Add MVHR and you'll be fine.

Posted By: djhOnly when it becomes actually possible to supply all affected properties with 'green' electricity without reducing the demand with further insulation would I consider your 'new default'.

That's perfectionist - not there yet, but as rate of renewable energy continues to exceed all predictions, confident we will be soon enough, barring Trumpist takeover. So save that large upfront embodied energy now, when it's emergency-urgent, accept some starting-from-zero/accumulating carbon in-use, anticipating continued electricity decarbonisation, so in-use added carbon will taper off - still quids in, settling for low-hanging fruit.

Posted By: djhadd an external drainage plane

Posted By: sgt_wouldsa thin layer of EWI

(rather than thick) will still have all the embodied carbon of transportation, tradesmen's diesel and air-miled lunch, scaffolding etc, though yes pro rata less of 'quarrying' and manufacture. Plus the embodied energy of rainscreen - would sure add up to same or more embodied carbon as thick EWI.

I should be more specific.

I'm looking for a way, in retrofit, to utilise the 'low hanging fruit' of the minimised-embodied-carbon + ASHP approach:
That is, CWI without added rain-screen/cladding, in Exposure zone 4 (mid Dartmoor), on extg 1950s roughcast rendered brick cavity wall with 50-60mm cavity.

I misread BRegs Part C Table 4, but the end-result is the same:
For 'injected fill *not* UF foam', Full height of wall, 50 cavity, max recommended exposure zone = 3 (no good); for 100 cavity = 4 (OK).

So I'm just on the cusp of acceptability - maybe, as mid Dartmoor is prob zone 4++ !
And the strong cement-based roughcast render is 50yrs old, well adhered but hairline-cracked, many flaking paint layers (ideal for the previous plan, to EWI it).

So I'm asking if anyone has anything to add - to make CWI acceptable in the wall described.
The paper I linked to, about water-repellent exterior treatments, wasn't hopeful, and didn't mention rendered, let alone old, hairline-cracked; only brickwork.

sgt_woulds, your Cavipor link looks fantastic - hydrophobic, vapour permeable, apparently hygroscopic (unfired clay, like cob?) but doesn't say so. This should have implications for Exposure zones, but doesn't say a word about that. Must be very low embodied carbon, although transport ... Do you know if it's actually available? At cost I should think.

PiH - this would just scrape through in UK - BldgRegs Part L1 Section 11 and Table 4.3 - Improved (by CWI) requires 0.55U. Pathetic I know - obviously set so as to not destroy the CWI industry.

Posted By: fostertomPiH - this would just scrape through in UK - BldgRegs Part L1 Section 11 and Table 4.3 - Improved (by CWI) requires 0.55U. Pathetic I know - obviously set so as to not destroy the CWI industry.

Over here both a new build and a total refit require walls to have a max u of 0.24 with a stated ideal target of 0.18

Posted By: djhWhy are you willing to discount the current carboncontent of electricity used to power a heat pump, but not willing to require tradesmen and material suppliers to use electric vans and feed them their lunches from local suppliers?

If only - is there a Register of such ideal tradesmen? Not yet, but soon, yes - that changing landscape already makes EPDs a nightmare, which surely is solvable once they become not a piece of paper but part of a configurable database.

This was in a Zone 4, I think we had a thread on it a few years ago. CWI put in, mouldy, CWI taken out, still mouldy. Is just down the road from GB's biggest onshore windfarm, which should give a clue about the rain being horizontal there.

https://www.bbc.co.uk/news/uk-scotland-glasgow-west-65543897



Why not just put in a big heatpump and leave it at that? Get a quote from heatgeek.com for as-is Vs insulated costs.

EWI is not economically worthwhile even at present fuel prices. I'm doing some DIY woodfibre IWI and even including my labour for free (fwiw!) it doesn't really pay back compared to leaving the money in the bank, though might make house more comfy.

UK government target is Clean Power by 2030, which I think they will miss by one or two years. So any carbon payback has to return within around 5 years, doesn't really work out.