That single paragraph contains the whole discipline. The rest of this article is what “done properly” actually looks like on real Oxfordshire houses.
When IWI is the right answer
Two of our benchmark projects frame the decision.
On Howard Street in East Oxford — a late-Victorian mid-terrace with solid brick walls front and back — external insulation was never on the table. The brickwork is the street. The walls were insulated internally with wood-fibre boards on a lime base coat, as part of a whole-house programme that took the property from EPC E (49) to B (89) on the public register — certificates dated November 2013 and January 2022.
At Orchard House near Banbury, a 1970 stone house in a conservation area, the wall was a hybrid — rubble-stone outer leaf, concrete-block inner leaf, an irregular unfilled cavity of roughly 410 mm. Cavity fill could not be relied on to be continuous in a wall like that, and external insulation would have buried the stone elevations. So the envelope was rebuilt from the inside, room by room.
The pattern: IWI wins when the elevations are protected, characterful or simply not yours to change — and when the walls are solid (or behave as solid, like Orchard House’s irregular cavity). Where the house is rendered, detached and the interiors matter more, external insulation is usually the better tool.
The thickness reality: 30–60 mm, decided room by room
The internet will tell you internal insulation means 100 mm or more. Our project record says otherwise. Across our solid-wall retrofits the working range is 30–60 mm of vapour-open board, specified room by room:
- At Orchard House: wood-fibre boards of 30–60 mm where depth allowed, a deliberate 40 mm profile in the small sitting room to protect floor area, cork with lime plaster in the wet rooms, and calcium-silicate board where condensation risk was highest. The design target for the upgraded walls was around 0.3–0.4 W/m²K — a design target, not a claimed measured result.
- On Howard Street: 80 mm tongue-and-groove wood fibre was specified where the rooms could take it, designed to take the wall from an estimated 2.11 W/m²K to around 0.6 (as-designed values from the project’s performance specification).
- On a five-storey North Oxford Victorian semi on St Margaret’s Road, where floor space was too tight for wood fibre, 30 mm aerogel boards did the same job in around a third of the thickness, and capillary-active calcium-silicate board handled the damp-exposed lower-ground walls.
Moderate thickness is not a compromise. Conservation research consistently finds that moderate internal insulation captures most of the benefit at much lower moisture risk than aggressive build-ups — because the thicker the insulation, the colder and wetter the masonry behind it becomes.
Where IWI is won or lost: reveals, voids and returns
The main wall face is the easy part. Heat and moisture find the places the insulation stops.
Window reveals. If the insulation halts at the edge of each opening, every window becomes a picture frame of cold plaster — and a condensation line. Each reveal receives its own thin insulated return (calcium-silicate board where depth is scarce), so the warm layer wraps the opening and closes onto the frame. At Orchard House the stone-mullion reveals — previously condensing and growing mould — were insulated concurrently with the walls, so reveal, tape, plaster and wall build-up meet as one junction.
The floor void. Where the first-floor structure interrupts the wall, the insulation must continue through the void between the joists — otherwise a cold band runs around the house at every floor level. On Howard Street the specification carried the insulation down through the intermediate floor void and bedded the joist ends in a lime parge coat, so warm indoor air cannot leak past the timber into the masonry.
Partition returns. Where an internal wall meets an external one, the cold bridges around the junction. Our details return the insulation along the partition — at least 400 mm at Orchard House, at least 600 mm on the party walls at St Margaret’s Road — to push the cold line back beyond where condensation could form.
Behind the furniture. At Orchard House, wardrobes standing against external walls received insulation behind them — a wardrobe on a cold wall is a mould incubator.
Fixings. Where radiators or joinery would later hang, structural pattresses (Compacfoam-type blocks) were bonded to the masonry first, and electrical back boxes were specified deep enough for the 60 mm insulation zones — no fixing bridges the warm layer into the cold wall.
The airtightness layer, and why lime does the work
A vapour-open build-up still needs to be airtight — airtight to air leakage, open to vapour diffusion. Those are different things, and confusing them is how internal insulation gets its bad name.
Our build-up runs: masonry stripped back to sound brick or stone (gypsum and cement finishes removed), a lime parge coat of at least 8 mm to level the wall and close air paths, boards bedded in lime adhesive with mechanical fixings and pressed home so no air pocket remains behind them — a void behind an insulation board is a highway for moist air — then mesh-reinforced lime plaster as the continuous, repairable air-control layer, finished in highly vapour-open paint (specified at Sd ≤ 0.1 m). Airtightness tapes seal the reveals, perimeters and every service penetration before plastering.
Every layer — parge, board, plaster, paint — stays vapour-open and capillary-active, so the wall keeps absorbing, buffering and releasing moisture rather than trapping it.
What can go wrong
Honesty is part of the specification. The failure modes we design against:
- Impermeable materials on a breathing wall. Foil-faced foams and cement renders trap moisture in solid masonry. Trapped moisture in a 140-year-old wall means decay, mould and ruined fabric.
- Dabs and voids. Dot-and-dab fixing leaves air channels behind the board where warm, moist room air condenses on cold masonry. Full-perimeter adhesion is non-negotiable — it is written into our specifications and explicitly ruled out dot-and-dab at Orchard House.
- Stopping at the edges. Uninsulated reveals, floor voids and partition junctions concentrate condensation exactly where you cannot see it developing.
- Sealing the house without ventilating it. Without a designed ventilation system, IWI plus new windows trades draughts for condensation — ventilation is planned before the fabric is sealed, never after.
- Ignoring the floor-area cost. IWI takes a few centimetres from each external wall — on Howard Street the certificates record 81 m² before and 76 m² after, a number we put in front of every client before they commit.
The proof it works
The house that answers the sceptics is the Howard Street terrace: solid brick, internally insulated in wood fibre and lime as part of a fabric-first whole-house retrofit — certified on the public energy register at B (89), up from E (49), with the walls recorded as internally insulated, “good”. The same logic at Orchard House ended in a certified EPC B in December 2023.
Internal wall insulation is not a product you buy; it is a set of details you get right, in materials the wall was built to live with. That is the difference between the projects that perform for decades and the ones that quietly fail behind the plaster.
This article is part of our natural insulation materials guide. See the full projects: Howard Street, East Oxford and Orchard House, near Banbury.