Considering we are now looking at embodied energy and lifecycle cost etc. What would be the optimal EWI thickness to aim for in terms of cost/energy/co2?

Standard 60s build, no insulation currently in cavity.

Windows will be going in (within proposed EWI layer) in the next 2 months, then it will be, CWI (graphite beads).

Thinking of EWI using EPS... Limited by space on the side walls of the house so EPS rather than fibre.

150mm sound okay?

Just then need to consider how to get a good 'stone sill' detail for the windows...

one way to look at it is...

- start by filling the cavity with EPS beads (say 50mm) gives Uval = 0.48 ish
- add 100mm EPS graphite on top gives Uval = 0.20 ish
- add 150mm EPS (rather than 100mm) gives Uval = 0.15 ish
- add 200mm EPS (rather than 150,,) gives Uval = 0.12 ish

I seem to recall seeing Wolfgang Feist (Passivhaus) giving a lecture, which to massively paraphrase him explained that after a lot of empirical data analysis, the Uval for opaque elements should be around 0.15 W/m2K, with lots of lifecycle payback grphs etc. Very convincing.

So, you could take that at face value, or research a bit more and see if that's been revised (as a general benchmark, ignoring that PHPP would do detailed bespoke calcs), in light the elecricity being greener than when I sat in that lecture 15 years ago (or more???).

You'll get lots of different answers, and I'm not sure there would actually be a correct answer that stands the test of time, and vagiaries of personal finances, interest rates, climate change, energy costs, but personally I'd target 150mm, and ask the installer to quote for 120/150/180, and see what the cost diff is. Versus the mobilisation, scaffolding, render, labour, probably not a huge percentage.

Posted By: Peter_in_HungaryIMO the key to the effectiveness of the works will be getting a good quality CWI done without any voids or air flow. Not at all easy as far as I have read about it.

It might be better to do the CWI before the EWI. Easier to install it, and a lot easier to check and rectify any mistakes/voids afterwards if there's no EWI covering the walls.

Yes CWI going in first after the windows

Then the EWI.

I was just advising where the windows will sit in the 'layers'

Heating is ASHP.

I'll get the windows and CWI in then get some prices on 120/150 and 180. There will always be diminishing returns so finding the sweetspot in terms of U-Values v price. 0.15 sounds liek a reasonable target to aim for

Thankyou

Posted By: Peter_in_HungaryIf you are thinking of cost/energy/co2 then the recent (and continuing) greening of the grid throws the long term calcs of co2 for heating up in the air. What will be your fuel for heating?

Exactly. This new calculus of optimal insulation thickness, in which embodied CO2e (the CO2e resulting from mining, manufacturing, transporting and constructing the work) is approximately twice as harmful to climate crisis and ecological collapse, as the subsequent in-use CO2e emssions, depends upon the heating being by ever-lower-carbon electricity. i.e. the building must stop burning fuel (incl biomass) and instead use electric heat pump.

If still burning fuel, then the older calculus prevails, ideally Passive House thinking, in which fuel burning for in-use heating is drastically reduced or eliminated.

It's jaw dropping (in a good way I suppose) how much the world has changed in the last 15 years.

It is no longer a good idea environmentally or financially to put EWI on to a wall that already has CWI.

The emissions of manufacturing, distributing and eventually disposing of the EWI materials, are worse now than the emissions they will ever save, and more expensive.

15 years ago, we would never have imagined we would get to this stage!

The magic ingredients that have changed are the electricity grid getting dramatically cleaner, plus the ability of heat pumps to multiply that benefit.


It is taking some head scratching to get used to this change, as ultra insulation has been a huge part of GBF thinking for many years so is hard to move on from it. Some building folks are getting used to it, others will need more time to let go, and building regs are now hopelessly off the ball, 100 pages about energy and none at all about upfront carbon.




Using the numbers above for CWi versus [CWi + 150mmEWI]:

Additional savings for adding EWI = 0.48 - 0.15 = 0.33 W/m2C

Annual electricity savings: 2500HDD x 24h/d x 0.33 W/m2C ÷ 1000 ÷ CoP 3 = 7kWh per m2

Energy cost saved 7kWh x 25p/unit = £1.75 per m2

£ cost payback time of EWI: £150/m2 ÷ £1.75

= 90years to payback £


CO2 emissions saved from electricity (120g/kWh reducing to net zero by 2050):
(120-0)/2 X 25y x 7kWh = 10 kg lifetime CO2 savings per m2

Lifecycle emissions from EPS, adhesive, render: 20-40 kgCO2 per m2 (depending which EPD you believe)

CO2 payback of EWI: never

For aesthetics though we would like to re-render the property so makes sense to EWI at same point surely and also falls under Building regs to upgrade?

Solar will be being installed also and maybe battery storage

Posted By: WillInAberdeenIt is no longer a good idea environmentally or financially to put EWI on to a wall that already has CWI.

For embodied carbon, maybe there's a new case for using natural IWI? I used hemp insulation in France, and the manufacturer claims that 1 hectare can take up as much as 15 tonnes of CO2 from the atmosphere (much more than a forest in its peak growth phase).

I guess the financial payback depends on what happens to future energy prices, so harder to determine that other than for Building Regs (which, I seem to recall, use current prices).

<blockquote><cite>Posted By: WillInAberdeen</cite>It is no longer a good idea environmentally or financially to put EWI on to a wall that already has CWI.

The emissions of manufacturing, distributing and eventually disposing of the EWI materials, are worse now than the emissions they will ever save, and more expensive.</blockquote>

The latter is true, but therefore rushing to the former, and similar, risks putting into reverse the several other objectives/benefits that drive well considered programmes like PH. Saving nett CO2 is not the only objective. There's also comfort - lack of cold radiation; health due to toxin-removal via MVHR, which is another part of the package likely to be dumped by the same logic; and one other objective I'm maybe not recalling. Also cost/payback, by eliminating heating system, also affecting the 'e' bit of CO2e (and other toxic emissions).

Better perhaps to concentrate instead on the still-unresolved rules of Life Cycle Alalysis (LCA) which still ridiculously doubts whether sequestration of bio-materials can be allowed into the calc, because their eventual fate in 60yrs time can't be guaranteed. Thereby distracting from what could be immediately maximised, to at least certainly impact the shorter-term nett emisions of CO2e, while it's super-crisis tipping-point time.

Also some creative invention into buildings that actually absorb CO2 throughout their in-use lifetime. Edit ... which has got me thinking - that would involve substantially increasing the building's bio-mass (as in weight) during its lifetime - trees growing all over it?
https://ethz.ch/en/news-and-events/eth-news/news/2025/06/a-building-material-that-lives-and-stores-carbon.html
- structural implications. But how about sequestering lots of carbon in the ground under/around it - cellulose like forest or wetland, or perhaps CaCO3? It could be the picturesque mark of an old building, that it gradually elevates, as the ground expands under it! Just in time for rising seas and watercourse flood-risk and/or a defence against groundwater salination.

Posted By: fostertomBetter perhaps to concentrate instead on the still-unresolved rules of Life Cycle Alalysis (LCA) which still ridiculously doubts whether sequestration of bio-materials can be allowed into the calc, because their eventual fate in 60yrs time can't be guaranteed.

That's a little disingenuous - the latest RICS guidance (WLCA 2nd Edition 2024), which is effectively the UK construction industry standard, is clear that biogenic carbon (sequestration) should be reported separately from the carbon used during manufacture.


"The sequestered biogenic carbon in sustainably sourced timber and other biomass must be considered to leave the product system boundary when it is either:

• transferred to another product system through reuse, recycling or recovery as a secondary fuel
• transferred to nature when emitted (as CO2 or methane) through combustion (either incineration or energy recovery processes), degradation in landfill, or emission from anaerobic digestion or composting, or
• transferred to nature if it remains undegraded in landfill after 100 years.

In each case, this must be considered as an emission of biogenic carbon (as CO2 or methane as appropriate) in the module in which it leaves the system."


In other words, the focus is back on the impact of the material at extraction (i.e. now), and no amount of magical accounting can alter this, because the sequestered carbon is effectively 'emitted' anyway.

Crucially though, natural materials such as wood fibre are almost always lower in initial carbon impacts than extractive industry equivalents anyway, without worrying about sequestration.

These are all technologies that exist right now and don't require techno-fix futuristic solutions to passively absorb carbon through living facades etc. They just use low impact materials to save unnecessary heat loss. The key is using them efficiently in the hierarchy of decision making (Build Nothing, Build Less, Build Clever etc.)

Soooo, back to my query.

To EWI or not to EWI, that is the question...

Yes, I'd go along with @Doubting_Thomas. CWI will get you from around 1.5W/m2K to about 0.55 (or possibly as low as 0.4-something with graphite EPS), which deals with the 'steep part of the curve' (the first few mm has the greatest effect. Everything thereafter is diminishing returns). You could stop there, and forget EWI, in which case you have about 0.5-ish, which is 3x as good as before but not earth-shattering, and thermal bridges at every opening where they have been closed with a brick instead of (today's alternative) an insulated cavity closer. You could forget the EWI and do some 'cloaking' IWI at the openings, but as @Doubting_Thomas says, having the scaffolding up and planning rendering anyway takes you a long way towards EWI. Do not, however, underestimate the cost of enabling works (all the things you have to move, extend, tweak, in order to accommodate the EWI.

Go for a system with 3rd-party certification, since your BCO does not *have* to believe you that it 'does what it says on the tin' without certification. Check that any certification is still in date and valid.

Does the whole place need re-rendering? Or will a repair where needed and repaint the house last for the next 20 years? If the latter then perhaps a cost / benefit on the EWI considering all that has been said above.

Of course the sizing of the heat pump comes into the EWI equation.

Posted By: VictorianecoIt doesn't 'need it'

But it's 60 years old and we want to modernise the property to add value also.

So it makes sense

If it is sound then no reason to replace it. Pressure wash and good quality paint should suffice.

If you decide to EWI then if the render is sound then no need to remove it, just pressure wash and EWI over.

Who would you get to do a cost/benefit analysis or similar?

It's only a case at this stage if windows are to go within the original location or in proposed EWI layer then I can look to order up