Most homeowners thinking about external wall insulation aren't picturing a scaffold and a spreadsheet, they're picturing a warmer living room and a smaller heating bill. A recent retrofit of a semi-detached house in Northwood, in the London Borough of Hillingdon, is a useful example of how those two things actually connect, and what it looks like when a home's walls are upgraded properly, with the right materials for the building rather than a one-size-fits-all approach.
The project used wood fibre insulation from STEICO alongside a diffusion-open EWI Pro render system, and it's a good case study for anyone with an older, solid-wall home who's wondering whether external wall insulation (EWI) is worth it, and what "doing it properly" actually involves.
Starting With a Plan, Not Just a Product
Before any insulation went anywhere near the walls, the homeowner worked with Furbnow, a whole-house retrofit specialist, to have the property properly surveyed, the existing walls, the heating system, the ventilation, all of it. That survey turned into a tailored Home Energy Plan, which is really just a roadmap: six measures, in the right order, to take the house from an EPC score of 65 (a D rating) up to 96 (an A rating), with an estimated 9% uplift in property value along the way.
It's a good reminder that external wall insulation works best as part of a plan, not a one-off job. Loft insulation needed topping up to 300mm. The walls needed insulating. And crucially, the homeowner ruled out internal wall insulation early on, it would have eaten into room space and created more thermal bridging around floor and ceiling junctions, in favour of external wall insulation, which keeps the internal footprint of the house untouched while wrapping the whole structure in a continuous insulating layer.
Why Wood Fibre, Specifically
This is where the project gets interesting for anyone weighing up different insulation materials. Furbnow recommended STEICO protect L dry, a diffusion-open wood fibre board, paired with breathable EWI Pro primers and lime render.
The reasoning was specific to the building. Older solid-wall homes were built to breathe, moisture generated inside the house (from cooking, showering, simply living in it) needs somewhere to go. Standard, diffusion-closed insulation systems can trap that moisture against the wall, which over time leads to damp and condensation problems. A diffusion-open system, like the STEICO wood fibre board used here, allows that vapour to pass through the wall build-up instead of getting trapped, working with the house rather than against it.
As Joanne from Furbnow put it, the wood fibre board was chosen because it let moisture "escape to the outside," while also helping keep the home warmer in winter and cooler in summer, a nice bit of natural performance that comes from wood fibre's density and thermal mass, which we'll come back to below.
The project team took this seriously enough that internal fitted wardrobes were removed so that the wall behind them could receive the same breathable plaster treatment as the rest of the house, a small detail, but exactly the kind of thing that prevents a hidden damp problem two years down the line.
The Numbers: What 100mm of Wood Fibre Actually Did
It's easy for insulation claims to feel abstract, so it's worth looking at the real figures from this project. Around 100m² of STEICO protect L dry boards, 100mm thick, were installed across the solid walls. The calculated U-value, a measure of how much heat escapes through a wall, where lower is better, improved from 1.7 W/m²K to 0.3 W/m²K.
Walls can account for up to a third of a typical home's total heat loss, so an improvement of that scale has a direct, tangible effect on how much your heating system has to work, and how much it costs to run.
There's a second, ess-talked-about benefit too, keeping the house cooler in summer. Wood fibre is denser than most synthetic insulation materials, which gives it more thermal mass. In practice, that means it absorbs and stores heat during the day and releases it slowly, creating a time lag between the hottest part of the afternoon outside and any temperature rise inside. It's a genuinely useful property as UK summers get warmer, and one that's harder to achieve with lighter insulation materials.
A Lower-Carbon Choice, Not Just a Lower-Bills One
For this particular homeowner, embodied carbon, the carbon cost of manufacturing and transporting a material, as opposed to the carbon saved once it's installed, mattered as much as the end performance.
STEICO wood fibre insulation is made from more than 80% natural, PEFC-certified wood from responsibly managed forestry, and its Environmental Product Declaration shows that STEICO protect L dry actually stores around 160 kg COâ‚‚eq per unit during production, since wood locks away carbon as it grows. That's a genuinely different starting point compared with many synthetic insulation boards.
The render system tells a similar story. EWI Pro's lime render, used alongside the wood fibre insulation, is fired at kiln temperatures around 400°C lower than typical cement-based renders, and is made from NHL lime, marble sand and clay rather than modern cement chemistry, meaning no fly ash, no slag, and no harsh chemical additives. Lime render also continues to absorb CO₂ from the atmosphere for years after it's applied, through a slow curing process called carbonation. It's the kind of detail that won't show up on the outside of the house, but matters if a genuinely low-carbon retrofit is the goal.
Thinking Beyond the Walls
One of the more valuable parts of this project is that it didn't treat the walls in isolation. The retrofit plan also included Mechanical Ventilation and Heat Recovery (MVHR) and looked ahead to solar PV, battery storage, and an eventual switch from a gas combi boiler to an air source heat pump.
That sequencing matters. Improving the fabric of a house first, walls, loft, ventilation, means that when it's time to size a heat pump, it can be specified correctly rather than oversized to compensate for a leaky building. In this case, the measures installed so far are estimated to lift the EPC rating by 10 points, from a D to a C, with an annual saving of around 1,253kg of CO₂, and to help ensure any future heat pump is sized efficiently rather than unnecessarily large (and expensive).
As Neil Statham of Furbnow explained, the approach is about sequencing measures that complement the existing building, rather than picking upgrades in isolation and hoping they work together.
What This Means If You're Considering EWI for an Older Home
If you live in a solid-wall property, common in homes built before the 1930s, and in plenty built since, this project offers a fairly clear set of takeaways:
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A proper survey first. Understanding your walls, heating and ventilation before choosing materials avoids expensive mistakes later.
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Diffusion-open systems suit solid walls. If your home was built to breathe, an insulation and render system that lets moisture pass through, rather than trapping it, tends to be the safer long-term choice.
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Wood fibre offers a genuine two-season benefit. Better winter heat retention alongside real help with summer overheating, thanks to its density and thermal mass.
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The render matters as much as the insulation. A breathable lime finish that works with wood fibre insulation, rather than a standard cementitious render, keeps the whole wall system consistent.
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Fabric first, then everything else. Getting the walls and loft right before specifying heating systems like heat pumps tends to produce a more efficient, better-value result overall.
As STEICO's Technical Director Martin Twamley noted of the project, the value lies in specifying materials that work in harmony with a genuinely low-carbon retrofit, improving comfort for the people living in the house, in both directions, summer and winter, not just on paper.
If you're exploring options for your own solid-wall home, it's worth asking any installer or retrofit specialist whether a diffusion-open wood fibre system might suit your walls better than a standard synthetic board , particularly if damp, condensation, or summer overheating are already concerns.Â
Please contact our technical team for more information and guidance via phone call 0800 133 7072 or email info@ewipro.com
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