This guide explains the materials in plain terms: how wood fibre differs from foam boards, why lime beats gypsum over insulation, where cork and calcium silicate earn their place, and the exact build-ups we have used on real houses — because the right material in the wrong wall is still a failure.
Why old houses need breathable systems
Almost every house built before the 1920s — and plenty after — has solid masonry walls: brick or stone with no cavity, often no damp-proof course, built with lime mortars. These walls were designed, whether their builders used the word or not, around a moisture strategy: the masonry absorbs rain and internal humidity, holds it harmlessly, and releases it again as vapour. The wall breathes, and it stays dry by drying.
Modern impermeable materials break that strategy. Cement renders, plastic paints, foil-faced foam boards and gypsum dot-and-dab all interrupt the wall’s ability to take up and release moisture. Trap moisture in a 140-year-old wall and you get damp patches, frost-damaged brick, rotting joist ends and mould behind the insulation — the classic failure that gives internal wall insulation its bad name. The failure is not internal insulation; it is impermeable internal insulation on a breathing wall.
So the rule that governs every material choice below: on traditional masonry, every layer — parge coat, insulation, plaster, paint — must stay vapour-open and preferably capillary-active, so the wall keeps doing what it has always done. On our St Margaret’s Road project, a five-storey 1880s Victorian semi, the specification spelled it out: the old wall must remain free to dry both outwards and inwards, so moisture is never trapped where it could condense. That is the single decision separating a durable solid-wall retrofit from a slow-motion one.
Wood fibre vs plastic foams
The comparison people expect is thermal performance, and on paper foam wins: rigid PIR insulates roughly twice as well per millimetre as wood fibre (which sits at a thermal conductivity of 0.038–0.04 W/mK in the boards we specify). If insulation value per millimetre were the whole story, every wall would be foam.
It is not the whole story, for three reasons.
Breathability. Wood fibre is vapour-open: water vapour passes through it. It is also capillary-active: liquid moisture wicks along its fibres, spreading out and moving towards surfaces where it can evaporate. If condensation ever forms behind the insulation — and on a cold masonry wall, some winter moisture at the interface is normal — a capillary-active board carries it away to dry. A foil-faced foam board does the opposite: it seals the wall’s inner face, and any moisture in the masonry stays there. On a cavity wall with a damp-proof course that can be acceptable; on solid brick or stone it is a trap.
Moisture buffering. Wood fibre and lime absorb and release humidity from the room air, flattening the daily peaks that showers and cooking produce. The wall becomes part of the house’s moisture management rather than a bystander.
Heat storage. Wood fibre is dense — many times denser than foam — so it stores heat. Rooms warm and cool more gently, and under a roof this density buys decrement delay: the hours it takes summer heat to travel through the build-up. That is why we used wood fibre between the rafters over the bedrooms at Orchard House near Banbury, and 100 mm + 40 mm of it across a whole re-roof in Iffley — a bedroom under a wood-fibre roof stays livable on a hot afternoon in a way a thin foam layer does not match.
Where plastic foams do belong: modern construction, and specific junction duties even on our own jobs — XPS below the damp-proof course and at pitched-roof junctions on an external wall insulation project in Kennington, expanded polystyrene bead as cavity fill in a cleared, genuine cavity, foam glass where a material must be utterly waterproof. Materials are tools, not ideologies. The test is always the wall.
Lime plaster vs gypsum
Every insulated masonry wall on our projects is finished in lime, and the reasons are practical, not romantic:
- Lime is vapour-open — it lets the build-up breathe all the way to the room. Gypsum holds moisture and degrades when damp; cement blocks drying almost entirely.
- Lime is the airtightness layer. A continuous, mesh-reinforced lime plaster over the boards is what stops warm, moist room air leaking into the construction. It is repairable with more lime, forever.
- Lime conditions the surface. Slightly alkaline, it discourages mould; hygroscopic, it buffers room humidity.
- Lime starts the build-up too. Before any board goes on, the stripped masonry is levelled with a lime parge coat — at least 8 mm at Orchard House — which closes the air paths across the rough wall face and gives the boards full contact. A void behind an insulation board is a highway for moist air; the parge coat is how you prevent it.
Lime’s role extends outside as well. On the Iffley project, all four chimneys were repointed in non-hydraulic lime mortar — the soft, breathable mortar the bricks were laid in, which weathers sacrificially and protects the brick, rather than the hard cement pointing that makes brick faces spall. The finishing coat of the logic is paint: everything is decorated in vapour-open paints with a vapour resistance (Sd value) of 0.1 m or less, so the last layer never undoes the first.
Cork, calcium silicate and the specialists
Cork is naturally moisture-tolerant, rot-resistant and insulating — which makes it the right board where humidity is highest. At Orchard House the wet rooms were insulated with cork in a lime-plaster build-up: bathrooms are where wall surfaces meet steam daily, and cork simply does not care.
Calcium-silicate board is the specialist for the hardest square metres: thin, rigid, strongly capillary-active and mould-inhibiting. We used it in two telling places. At Orchard House, the reconstituted-stone mullion surrounds were the coldest details in the house — solid stone bridging straight through the wall, already growing mould at the reveals — and thin calcium-silicate board insulated them within the few millimetres available. At St Margaret’s Road, the lower-ground rooms sit below pavement level, the most moisture-exposed walls in the house; there a full calcium-silicate system (Calsitherm boards with their matching plaster) was chosen precisely because it handles damp conditions that would compromise other build-ups.
Aerogel solves the floor-area problem. Internal insulation costs room width, and in tight spots that matters. At St Margaret’s Road, where 80 mm of wood fibre would not fit, 30 mm aerogel boards on magnesium board delivered comparable performance at roughly a third of the thickness — an exceptional conductivity of about 0.015 W/mK.
Foam glass — cellular glass made of sealed bubbles — is completely impervious to moisture and incompressible. It went under the re-covered lead flat roof at St Margaret’s Road (100 mm), and in aggregate form it formed the insulating base of a rebuilt solid floor on our Howard Street project, under a lime screed with underfloor heating: a fully breathable floor construction to match the walls that meet it.
Where each material belongs
| Material | Belongs | Why |
|---|---|---|
| Wood-fibre board | Solid-wall internal insulation; rafter-level roof build-ups; knee walls | Vapour-open, capillary-active, dense — moisture safety plus summer heat delay |
| Lime (parge, adhesive, plaster) | Every layer of a masonry build-up | Breathable, mould-resistant, forms the continuous airtightness layer |
| Cork (with lime) | Wet rooms | Insulates while shrugging off daily steam and splash |
| Calcium-silicate board | Cold reveals, mullions, basements, damp-prone walls | Thin, actively wicks moisture, inhibits mould |
| Aerogel | Where every centimetre of room counts | Extreme performance at minimal thickness |
| Foam glass | Flat roofs, ground-bearing floors | Waterproof, rot-proof, incompressible |
| Mineral wool | Lofts at joist level; acoustic duty between floors | Cheap, effective and vapour-open where breathability is less critical |
| EPS / XPS / PIR foams | Genuine cleared cavities, below damp-course junctions, modern walls | Right tool where the wall is already moisture-managed by design |
The build-ups we actually use
Specifications are where material talk becomes real. Four from our own projects:
A 1920s solid-brick house, Church Way, Iffley (conservation area). Walls: 60 mm wood-fibre boards finished in lime plaster across 143 m² of external wall — a deliberate, moderate thickness targeting 0.4 W/m²K from an uninsulated 260 mm brick wall, with insulation returns at junctions to blunt cold bridges. Roof: 100 mm + 40 mm wood fibre at rafter level under the re-laid original clay tiles, with an airtightness membrane on the warm side and a ventilated channel above so the roof breathes and drains — a design target of 0.2–0.3 W/m²K from roughly 1.2, without raising the roofline.
A stone house near Banbury (Orchard House). Internal insulation specified room by room, 30–60 mm: wood fibre where depth allowed, a 40 mm profile in the small sitting room to protect floor area, cork with lime in the wet rooms, calcium silicate at the stone reveals — all on a lime parge coat of at least 8 mm, bedded in lime adhesive, pressed home so no air pocket remained behind the boards, finished in mesh-reinforced lime plaster and vapour-open paint.
An 1880s Victorian semi, St Margaret’s Road, North Oxford. 80 mm tongue-and-groove wood-fibre boards (0.038–0.04 W/mK) on stripped brick, finished in breathable lime plasters, across five storeys — with Calsitherm calcium silicate below ground, 30 mm aerogel where space was critical, joist ends bedded in lime parge so warm air cannot leak past them, and party-wall returns insulated at least 600 mm back to kill the cold bridge.
A c. 1900 terrace, Howard Street, East Oxford. 80 mm wood fibre on a lime base coat to the solid walls, a foam-glass-and-lime-screed rebuilt solid floor, vapour-permeable insulation between the suspended floor joists — and the external brickwork prepared to match: impermeable paint and cement pointing stripped back, repointed in lime, so the wall could dry outwards as well as in. The certified outcome sits on the public register: EPC E (49) to B (89).
The common thread: not one universal product, but one universal rule — every layer breathable, every junction continuous, every material matched to its wall. How the materials fit into the wider programme is covered in our whole house retrofit guide; what happens when moisture is handled badly is covered in our damp and mould guide.