External wall insulation - your experience

Insulation

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Batpred

(@batpred)

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Topic starter 15/10/2025 10:15 pm

Given most of us live in properties built a long time ago, options to insulate walls may be limited, in some cases by insulating them externally.

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

Material	Pros / Key attributes	Limitations / risks / particular use-cases
EPS (Expanded Polystyrene) / Graphite-enhanced EPS	Popular, relatively low cost, good thermal performance per thickness.	Not very vapour permeable; risk of moisture, condensation or trapped moisture if mis-specified. Also flammability and fire-regulations need checks.
Mineral wool (rock wool / stone wool)	More fire-resistant, better vapour permeability, good acoustic performance.	Slightly lower thermal performance per unit thickness compared to foams; requires good detailing to avoid thermal bridging.
Phenolic boards / phenolic foam	Very high thermal performance (i.e. good insulation per thickness).	More expensive; care with fire performance, compatibility with render systems, and installation quality.
Rigid PU / PIR foam boards	High R-value; compact thickness for insulation.	Fire safety / building regulation constraints; edges and joints must be sealed well to avoid heat loss; moisture issues if wrongly sited.
Wood-fibre / wood wool / timber composite boards	More “breathable” / vapour-permeable, more natural / sustainable feel; sometimes used in heritage / retrofit settings.	Lower thermal performance per thickness vs high-performance foams; cost and material availability may be higher.
Hybrid / “eco” materials (hemp, cork, etc.)	In niche / specialist projects, especially where carbon or breathability is key.	Generally more expensive, thicker for same performance, and less standardized.

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Batpred

(@batpred)

Reputable Member Member

Joined: 10 months ago

Posts: 152

Topic starter 15/10/2025 10:53 pm

So our house is in SE England.

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

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