Green Building Forum - Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 Thu, 11 Jun 2026 21:45:49 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310788#Comment_310788 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310788#Comment_310788 Thu, 11 Jun 2026 11:42:34 +0100 fostertom
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?]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310789#Comment_310789 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310789#Comment_310789 Thu, 11 Jun 2026 11:52:04 +0100 sgt_woulds
40-100mm of woodfibre to ensure that there won't be any moisture issues.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310790#Comment_310790 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310790#Comment_310790 Thu, 11 Jun 2026 11:53:09 +0100 sgt_woulds Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310791#Comment_310791 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310791#Comment_310791 Thu, 11 Jun 2026 11:54:48 +0100 djh
I agree that if you have a cavity wall, you should insulate the cavity before applying EWI. So the only issue appears to be what to do in high exposure areas?

And the answer to that is standard: add an external drainage plane. A separate layer that rain hits and runs down off the building. They can be made of timber, or render boards or whatever.

As regards the overall premises of your argument, I don't think we're yet at the point where we have sufficient supply of 'green' electricity for it to work at present. Only 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'.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310792#Comment_310792 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310792#Comment_310792 Thu, 11 Jun 2026 11:55:57 +0100 djh 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.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310793#Comment_310793 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310793#Comment_310793 Thu, 11 Jun 2026 11:58:28 +0100 sgt_woulds
https://construction.basf.com/global/en/products/cavipor_reliable_insulation]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310794#Comment_310794 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310794#Comment_310794 Thu, 11 Jun 2026 13:34:46 +0100 fostertom 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.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310795#Comment_310795 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310795#Comment_310795 Thu, 11 Jun 2026 16:43:46 +0100 Peter_in_Hungary
I can agree with that sentiment

but

1 Currently - at least over here - the building regs still require insulation to a high standard for both new build and full retrofit.

2 Insulation to a high standard is still cost effective financially even if not so for carbon footprint and the longer Trump plays games the quicker the ROI on insulation comes back.

So given these two points I don't think that reducing the insulation to take account of the carbon footprint of the energy is going to happen any time soon. Also there will need to be a mindset change to get people off gas combie boilers when for most electricity is seen as the most expensive type of energy and heat pumps still have a bad reputation.

Luckily we don't have any cavity walls here! it's all solid brick, stone or block with EWI for new build or retrofit. (It's almost impossible to meet the building regs for new build without EWI)]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310796#Comment_310796 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310796#Comment_310796 Thu, 11 Jun 2026 16:44:33 +0100 fostertom
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.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310797#Comment_310797 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310797#Comment_310797 Thu, 11 Jun 2026 17:26:52 +0100 djh Posted By: fostertomI 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.Yes. That's a completely different question to your first post!

Posted By: fostertom(rather than thick) will still have all the embodied carbon of transportation, tradesmen's diesel and air-miled lunch
Why 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?

Re Cavipor - look at what the foam is made from.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310798#Comment_310798 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310798#Comment_310798 Thu, 11 Jun 2026 17:42:40 +0100 Peter_in_Hungary 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]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310799#Comment_310799 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310799#Comment_310799 Thu, 11 Jun 2026 18:00:57 +0100 djh Posted By: Peter_in_HungaryOver 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 0.18Out of interest, what are the temperature extremes - winter cold and summer hot?]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310800#Comment_310800 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310800#Comment_310800 Thu, 11 Jun 2026 18:05:06 +0100 fostertom 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.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310801#Comment_310801 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310801#Comment_310801 Thu, 11 Jun 2026 19:03:15 +0100 Peter_in_Hungary Posted By: djhOut of interest, what are the temperature extremes - winter cold and summer hot?
Not much below -18 in winter and nowadays up to 36 in the summer. We had one winter where it didn't go above -10 daytime temp. for a week. That was about 15 - 20 years ago. The beginning of January is the coldest time but recently winter temps aren't much below - 6 where we live and we haven't had a decent snow for about 8 years.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310802#Comment_310802 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310802#Comment_310802 Thu, 11 Jun 2026 19:48:07 +0100 WillInAberdeen
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.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310803#Comment_310803 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310803#Comment_310803 Thu, 11 Jun 2026 20:53:00 +0100 Victorianeco
1960's cavity (80mm) with render finish. Exposure Zone 3 (on paper) but I feel is a Zone 4 in reality

CWI using EPS graphite beads and then just go for triple glazing? Or go all out and go for EWI also... really unsure but we are looking to order up windows with Nordan shortly..

Really need to determine options, but a lot of cold bridges I feel if left as is]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310804#Comment_310804 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310804#Comment_310804 Thu, 11 Jun 2026 21:36:19 +0100 fostertom Posted By: WillInAberdeenEWI is not economically worthwhile even at present fuel pricesAs I've told clients, while suggesting it was 'right thing to do ' - and some did - health,comfort etc. But as the national solution to the existing housing stock, it remains a non-starter - no wonder govts have been so clueless about getting started, apart from the 'low hanging fruits'.

It's almost like an idea ('insulate, insulate') that didn't stack up, wouldn't get going - perhaps just as well.
While another idea that govts were weak on, renewable energy, has taken off, beyond all hopes and expectations, all by itself. Electricity is decarbonising very fast, and no real limit to how far it can go, to supersede almost all fossil-burning.
What govt after govt has determinedly plodded on with at staggering expense, nuclear, will turn out to be a redundant white elephant, which nevertheless contractually has to be paid for at crippling £ per kWh for decades to come, inflating our electricity bills which ought by rights to be in freefall.

Anyway, there looks to be no shortage in view, of climate-harmless renewable energy, for transport, AI farms and whatever comes next.
And for heating/cooling the housing stock without expenditure of climate-harming embodied carbon, instead of tackling it by insulation.

It's time for us green builders to jump ship. Don't worry, there will still be countless projects where traditional 'insulate, insulate' will still make eco-sense.]]> Embodied carbon, EWI, CWI, ASHP, Exposure zone 4 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310805#Comment_310805 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18443&Focus=310805#Comment_310805 Thu, 11 Jun 2026 21:40:13 +0100 fostertom Posted By: djhCavipor - look at what the foam is made fromIndeed - what might that be? Perhaps powdered aluminium, as in Thermalite - something that reacts gassily with the raw material?]]>