Sun, 21 Jun 2026 21:05:45 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310585#Comment_310585 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310585#Comment_310585 Tue, 05 May 2026 20:28:22 +0100 Victorianeco
Specifically, I would like to compare:

1. Cavity Fill Only: Filling the cavity with 80mm expanded polystyrene (EPS) graphite beads while keeping new windows (3G) within the traditional layer.
2. Cavity Fill plus External Wall Insulation (EWI): Performing the above and adding a 120mm EPS board external layer.

While the latter is more thermally efficient, it is also more labour and material intensive. I need to evaluate the payback period based on current material costs and research.

Furthermore, I have concerns regarding the impact of the 120mm EWI on the property's interior. Extending the wall thickness may reduce natural light and affect viewing angles from within the room, particularly at the 30 to 45-degree angles we currently enjoy. There is also the potential loss of the window frame's aesthetic "framing" effect to consider.

What are your current thoughts on these two approaches based on up-to-date material costs and thermal performance research? Please let me know if there are any other factors I should consider at this stage.

Thanks]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310588#Comment_310588 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310588#Comment_310588 Tue, 05 May 2026 22:43:58 +0100 fostertom
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310590#Comment_310590 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310590#Comment_310590 Wed, 06 May 2026 10:54:24 +0100 djh
Firstly the embodied carbon in different types of insulation varies a lot. Consider plastic insulation versus wood fibre, for example.

Then there's the practicalities of operational energy supply. Increasing demand for electrical energy causes all sorts of problems and increased costs. So I feel there's a responsibility to minimise extra demand so far as is reasonable.

And then there's the added questions about airtightness and ventilation that don't go away.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310594#Comment_310594 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310594#Comment_310594 Wed, 06 May 2026 15:09:14 +0100 fostertom 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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310595#Comment_310595 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310595#Comment_310595 Wed, 06 May 2026 18:06:49 +0100 Victorianeco https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310597#Comment_310597 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310597#Comment_310597 Fri, 08 May 2026 12:37:05 +0100 Doubting_Thomas 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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310598#Comment_310598 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310598#Comment_310598 Fri, 08 May 2026 14:02:39 +0100 sgt_woulds

Artificially so, but this is a problem with EPD in general - as we discussed in that earlier thread, EPD assessment doesn't include the total carbon cost of production.

EPS figures do not include the carbon cost of extracting and shipping crude oil to a refinery, the energy used in turning crude into a usable oil for production, the carbon emmited shipping the refined oil to wherever it is to be used, and the transport to the EPS factory.

They only measure the energy used for production of the EPS in the factory which is a very small fraction of the total.

(I can't remember where the quote came from, but I remember reading that half of all emmissions from oil-based fossil fuels comes from shipping the crude around the world for refining before it is ever used for anything useful)

Woodfibre has vastly less carbon input on the production and transport of the timber, but processing the logs into woodfibre products relies on high temperature steam processes that look worse on the EPD assessment compared to EPS.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310601#Comment_310601 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310601#Comment_310601 Fri, 08 May 2026 19:24:50 +0100 Mike1 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):]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310602#Comment_310602 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310602#Comment_310602 Fri, 08 May 2026 19:32:47 +0100 Mike1 Posted By: VictorianecoExtending the wall thickness may reduce natural light and affect viewing angles from within the room, particularly at the 30 to 45-degree angles we currently enjoy. There is also the potential loss of the window frame's aesthetic "framing" effect to consider.
With IWI my walls are around 550 mm thick and I've not found it to be a problem.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310603#Comment_310603 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310603#Comment_310603 Fri, 08 May 2026 20:53:05 +0100 Victorianeco
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310606#Comment_310606 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310606#Comment_310606 Sat, 09 May 2026 17:07:29 +0100 Nick Parsons 2. Calculate R value of 80mm gEPS. 0.08m/0.032 = R = 2.5.
3. Take 'base case' U value (1.5W/m2K). 1/1.5 = R value (0.66)
4. Add the two. 2.5 + 0.66 = 3.16 (R value)
5. 1/3.16 = U value = 0.31. Does not allow for thermal bridges etc.
6. Establish heating regime and set-point temp. Decide whether, for this calc, you will seasonally adjust, or just do a 'winter calc'.
7. Establish external wall area and run the calc with the U value you know you will have, whatever (80mm EPS cavity fill). You will get a W/K figure (having multiplied out the m2).
8. Add the 120 EPS you have referred to. That's 0.12m@ 0.032W/mK, giving an R value of 3.75. Add that to your 'filled cavity' R value of 3.16 and you get R=6.91 (U = 0.145). Calculate the current fuel cost difference between running your heating regime with wall U value 0.145 vs 0.31. Get a quote for EWI and see how many years it would take you to pay back. If that does not matter to you, and you have the money, do it anyway, but not before considering rigid wood-fibre.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310610#Comment_310610 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310610#Comment_310610 Sun, 10 May 2026 09:20:17 +0100 Victorianeco
A lot to consider for sure.

With CWI could I do 3 elevations to begin with as the rear is going to be extended/knocked through or will it hold it's shape when steels are put in? But then I guess I need to move the windows out anyway and seal up any internal gaps

It's a bit of a retrospective nightmare as it had to be done in stages I guess..

Wood fibre would be thicker again though...]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310616#Comment_310616 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310616#Comment_310616 Mon, 11 May 2026 09:33:39 +0100 Nick Parsons
Edit: Re your comment re WF, yes, you'd need 40mm more (so 160mm) to get a U value a tiny bit better than 120 graphite EPS. This is based on lambda values of 0.032W/mK for gEPS and 0.042 for WF. Some WF products claim 0.039. The stuff I used was 0.042.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310678#Comment_310678 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310678#Comment_310678 Sat, 16 May 2026 06:12:46 +0100 Victorianeco
One side of the house is an extension and the installer won't top up the cavity that has existing batts with the beads...

What would you do in this situation?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310679#Comment_310679 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310679#Comment_310679 Sat, 16 May 2026 08:05:21 +0100 tony https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310682#Comment_310682 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310682#Comment_310682 Sat, 16 May 2026 14:20:20 +0100 Victorianeco https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310685#Comment_310685 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310685#Comment_310685 Sun, 17 May 2026 21:50:51 +0100 tony Buy PVA coated beads which will lightly stick together]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310688#Comment_310688 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310688#Comment_310688 Mon, 18 May 2026 06:52:32 +0100 Victorianeco https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310691#Comment_310691 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18429&Focus=310691#Comment_310691 Mon, 18 May 2026 09:14:03 +0100 tony