We know because we have delivered it at scale. At St Swithun’s House in Kennington, on the southern edge of Oxford, we insulated 267 m² of solid wall externally as the fabric package of an independently designed whole-house plan. That project is the worked example running through this guide.
The outside-in logic
Most of our Oxford work runs the other way. On period terraces and conservation-area frontages the brickwork is the architecture, so insulation goes on the inside, room by room — the approach in our internal wall insulation guide.
St Swithun’s House is the mirror image. A large 1900s detached house, rendered rather than facing brick, with a patchwork thermal envelope built up over a century — original solid brick, a 1950s solid-brick extension, a 2000 cavity-wall extension — and owners who wanted the plasterwork, joinery and room sizes left exactly as they were. Pre-works energy assessment: EPC band D.
That combination makes EWI win. The whole masonry mass ends up inside the insulation, where it stores warmth and steadies internal temperatures. Thermal bridges at floor junctions and internal walls — the persistent weak points of internal insulation — largely disappear, because the layer runs unbroken past them. And the family’s home stays a home: no rooms stripped back, no floor area lost.
What the works actually involve
The insulation itself
At St Swithun’s the walls were wrapped in EPS (expanded polystyrene) boards finished in silicone render, with 90 mm mineral-wool boards priced as the alternative system. The boards were fixed with a wet-and-mechanical system — adhesive bonding plus mechanical anchors, installed to the manufacturer’s guidelines, with adhesion checked so no air gap survives behind the boards to short-circuit the insulation. Over the boards: reinforcing mesh, base coat, then the silicone render finish — breathable, flexible and low-maintenance.
The detailing that decides the outcome
An EWI job is won or lost at its edges. On this project that meant:
- Below the damp-proof course: XPS (extruded polystyrene) insulation carried down past the DPC and across the pitched lower roofs, so the insulation line doesn’t simply stop at the awkward junctions.
- Sills: extended, so rainwater still throws clear of a wall that is now roughly 100 mm thicker.
- Rainwater and drainage: rebuilt around the new wall line — cast-iron pipework replaced, new downpipes, verge trims, and the soil stack re-set.
The roof has to follow the walls
Thicken every wall and the existing eaves no longer oversail them. The contracted answer at St Swithun’s: extend the roof by 150 mm on every elevation — one course of tiles removed all round, treated timber extensions fixed to the rafters, an eaves protection system and breathable felt, new treated battens, and the house’s original tiles re-laid, with the lead valley and apex extended to the new dimensions. The result is a roofline that shelters the insulation for its whole life, in the house’s own tiles.
The scaffold reality
EWI means a full-house scaffold — and the scaffold is the quiet piece of coordination that makes the economics work. At St Swithun’s one structure was designed to serve three work packages at once: the external insulation, a seventeen-item window and door replacement, and the roof extension — rather than scaffolding the house three times. It was designed to the relevant scaffold standards (BS EN 12811 / TG20:13), with edge protection and netting, and inspected throughout under the Work at Height Regulations. If a quotation for EWI doesn’t engage seriously with the scaffold, the roof and the drainage goods, it isn’t a quotation for the whole job.
Windows: position as much as performance
Replacing windows on an EWI project is about where the frame sits as much as what it achieves. All seventeen items at St Swithun’s — replacement double-glazed units, a new front door, new French doors — were detailed for compatibility with the insulation layer that followed, so frames and reveals meet the EWI cleanly instead of being buried or bridged by it, with airtightness tape over the frame-to-wall junctions. The specification set design targets of 1.2 W/m²K whole-window for double glazing (0.7 where triple-glazed) and 1.8 for doors — targets, roughly halving the heat loss of the glazing they replaced.
And the air the house breathed through its leaks
Wrap and seal a leaky house and you must replace accidental draughts with deliberate fresh air. For St Swithun’s we designed a whole-house humidity-controlled ventilation system sized to the actual building — 247 m² of floor area, humidity-sensing inlets and extract units, a quiet central fan running continuously at low power. Critically, the design was coordinated with the insulation programme: redundant extract fans removed and their openings sealed airtight, and every new wall penetration installed before the EWI went on — no holes cut through finished render. The full logic is in our ventilation after insulation guide.
Appearance decisions
EWI changes how the house looks, and the decisions deserve to be made consciously rather than defaulted:
- Render finish and colour. Through-coloured silicone render is the workhorse: breathable, flexible, self-cleaning to a degree. On a house that was already rendered — like St Swithun’s — the street simply sees a crisp new coat.
- Reveals and sills. Deeper reveals are an inevitable consequence of a thicker wall; extended sills and neat insulated returns are what make them look intentional.
- The roofline. A 150 mm eaves extension in the original tiles reads as the house it always was. Skipping it reads as a mistake — and performs like one, with the top of the insulation left exposed to weather.
- What EWI cannot do: preserve facing brickwork. If the brick is the architecture, EWI is the wrong tool, and no render colour fixes that.
The honest summary
EWI is the most transformative single fabric measure available for a rendered solid-wall house — a continuous insulated overcoat that leaves the inside untouched. It is also a whole-building intervention: roof, windows, drainage, ventilation and scaffold all move together, in the right order, under one programme. At St Swithun’s House the insulation, scaffold, window package and an annexe conversion were delivered through 2023 as exactly that kind of coordinated programme — and we make no claim about a post-works rating; what we can point to is the fabric itself: 267 m² of continuous insulation around the solid walls of a house that previously had none.
This article is part of our whole-house retrofit guide. See the full project: St Swithun’s House, Kennington.