To be clear, my experience was very positive. The journey to evaluate options was the part that initially has more miss than hit. 

Since I was expecting professional installers, those interactions were fairly confusing, even if eventually explained by ECO4. 

https://youtube.com/shorts/BeIKkFDKYFw?si=U4S-lS3khxqhDt5K

It appears to have been posted by an inspector

News Update from DESNZ. They've decided to extend the ECO4 scheme.

I suspect a large percentage of insulation jobs were carried out under the ECO4 scheme, so perhaps this fits here. 

One thing I can add is that when we were looking to carry out our (self-funded) external wall insulation, we had a few companies that came and that I now understand were doing a lot of ECO4 work. 

Their salespeople really lacked the experience of what was required and showed signs of poor oversight in the big jobs they were working on for local authorities and housing associations, like:

• Generally stated they would only fit 200mm of EPS or rockwool (the cheapest materials)
• They were happy to sell warranty and claim some industry certification but would not get engaged in any discussion on insurance backed ones (yes, there is a niche industry for this work now..)
• Some had special companies they were using for the big work.. And I spoke with some directors that had got so used to it that they could not consider doing work under their original company name 
• Some stated that they would always need to fit permanent low speed fans, to prevent mould. Our thought: we pay our own way, we have fans, why are you trying to add to the job? I now see that the method of leaving a short gap between wall and insulation to prevent damp was too time consuming..
• They were also not able to provide any references for previous work (even if they were clearly doing lots of it) 
• Some confessed they could not be sure of the quality of the work, as they did not know who would assess it 

After this, I contacted some system specifiers/warehouses that would provide materials warranty. They suggested installers. We made both the material and the labour warranties part of the contract and schedule of payment and it worked very well. It helped for a more relaxed process that we had all the details of the inspection regime from system specifiers before we even paid the deposit. 

You raised a point that would never have crossed my mind (lightning!). Still, if we were doing it all over, I’d go with 12 or 15cm (we have drainage and terrace access constraints for even greater thickness).

Building regulations here call for mere 7cm of insulation (they are WAY more strict when it comes to seismic protection thankfully) and this is what developers apply. 90% of original or retrofit insulation projects opt for EPS, simply because it is considered the market standard. It took us some time to find a crew that would apply the rock wool.

Another material used in our part of the world is XPS (Extruded Polystyrene), a stronger material with structural integrity, applied on walkable external roof tops and areas that need to support weight.

Works well.

We have a mechanical ventilation system with heat recovery so we’re not worried about moisture and mould growth. The system was extended to include this extra room.

It will have reduced heat loss compared with no external insulation but was essential for building regulations anyway.

Thank you for sharing. As you can expect, even in South England, with colder and longer winters, whenever rockwool is applied (and this is one of the most common systems for solid wall insulation), the depth is closer to 200mm. At that depth, you start to notice a difference in lighting especially when the windows are not very wide. This means the use of more expensive and better insulating materials can be more appealing. 

We also noticed the insulation made a difference in warmer days. Since the fabric of the building does not warm as quickly, allowing air circulation in the evening is often enough to cool it for the night. 

Location: northern suburb of Athens, Greece. Elevation 400m

We applied a 10cm external rock wool insulation to ground floor and the 1st floor four years ago (the basement was left undone as it is partially buried underground and it is more of a utility rather than living space). We opted for rock wool because of the wildfires we experience in the wider area and because of its permeability properties.
I can’t really comment on the heating savings because we have had very mild winters since doing it. We did experience however considerable relief during the summer. We are able to maintain an internal temperature circa 10 degrees lower than the outside one  (which easily tops 35 at peak noon time), without using any air-conditioning units. We use fans, keep all openings closed during the day and open the windows after sunset. We are lucky to live in an area that drops 8 to 12 degrees lower during the night.

My recommendation to anyone considering external insulation is to install a layer as thick as possible. The delta cost of few extra centimeters in material thickness is a small fraction of the overall cost. 

We mainly had mixed white/grey EPS installed (~100mm wide) in new aerated block based external walls when we had a rear extension built. This was then rendered with a polymer based system that was subsequently painted. I estimate this reduced heating use by 10%

A couple of years ago we had phenolic boards I think 70mm wide finished with k-render installed on all the remaining external solid walls. The execution incl scaffolding took 9 working days including electrics (we had had pipework changes made earlier). System specifier/materials and labour warranties all issued ok. We also had a small section of cavity wall filled. This time I believe the reduction of heating use was 30-40%. We also saved having the walls painted as this was overdue and in fact we probably will never have to do that again. We also left a few wall features untouched as the recent gov schemes do not insist all is done.  

Any bathrooms and similar already had extractor fans, we never had any issues with damp/mould, etc

In the Household Energy Efficiency detailed release, it's stated that at end of 2024 only about 10% of solid wall homes had insulation, about 876,000 properties in Great Britain.

Perhaps  sharing how our own projects insulating went, particularly houses, can help others consider reducing heat lost this way. 

So how did the process go, what materials/methods were used, what was your perception of reduction of energy required for heating and the region/rough location. 

By external wall insulation I mean when it is applied to external walls (i.e. external wall insulation, façade / cladding systems), (i.e. retrofits or new builds where the insulation is placed externally). 

Common External Wall Insulation (EWI) Materials