It’s a way of insulating a solid wall from the inside using a wood fibre board, finished with breathable lime plaster instead of standard gypsum. Unlike foam-and-plasterboard systems, nothing about it seals the wall up — moisture can still move through it, which matters a lot for older, solid-walled buildings that were built to breathe.

Because it plays to natural fibre’s real strengths, not just breathability for the sake of it:

• • Vapour open: the board and plaster let moisture vapour pass straight through, so the wall keeps drying out rather than having water trapped inside it.

• • Hygroscopic: wood fibre actively absorbs moisture from the air when humidity’s high, and releases it again when the air’s drier, rather than staying inert like foam.

• • Moisture buffering: because of that hygroscopicity, the material buffers spikes in humidity — a shower running or a pan boiling doesn’t just sit in the room air, some of it’s absorbed into the wall and released slowly afterwards, keeping humidity and mould risk more stable.

• • Better indoor air quality: no foams, no fire-retardant chemicals, no plastic vapour barriers off-gassing into the room.

• • Sustainable: made largely from renewable timber industry by-products, with low embodied carbon, and biodegradable at end of life.

• • Suited to traditional construction: built to work with a solid wall’s natural moisture movement.

• • Tried and tested: used across Europe for decades, with a solid track record on historic and traditional buildings specifically.

• • Decrement delay: wood fibre’s density slows and dampens temperature swings, so rooms hold onto winter warmth better and stay noticeably cooler in a hot spell too.

Not automatically — this is the question that gets skipped far too often, and it’s the reason IWI sometimes gets a bad name. Before we so much as order a board, we need to rule out the things that can make it a bad idea, or mean it needs a different approach.

• • Damp: rising or penetrating damp that hasn’t been sorted first — insulating over a damp wall traps the moisture in, it doesn’t fix it.

• • Embedded timber: joist ends, bonded lintels or other timber built into the wall can end up sitting in a colder, damper zone once insulation goes on.

• • Exposure: elevations taking a real battering from wind-driven rain need extra thought.

• • Previous repairs: cement render, hard pointing or gypsum plaster usually need addressing first.

A proper pre-installation survey, not a five-minute look round. We open up small test areas where needed to confirm exactly what the wall’s built from, how thick it is, what state the mortar and existing plaster are in, and where the damp risks actually sit.

It’s modelling how heat and moisture will move through the wall once insulation goes on, specifically to check the risk of interstitial condensation — condensation that forms inside the wall build-up rather than on the surface, so you don’t see it happening until it’s already caused damage. It’s the single biggest factor in whether an IWI job lasts decades or causes problems within a couple of winters.

A Glaser calculation is a simplified, steady-state estimate, and it’s fine for straightforward, lower-risk walls. For anything more complex — solid walls, historic or unusual construction, or renders that aren’t very breathable — we’ll run a full WUFI simulation instead, which models how the wall performs over a whole year rather than a single cold snap.

If the property is listed or in a conservation area, quite possibly — and this can add several weeks to the timeline, so it’s worth starting the conversation early.

Because almost every IWI failure happens at a junction, not in the middle of the wall. The flat, open part of a wall is the easy bit.

Cold can track round the corner from an uninsulated party wall or partition, or up into an uninsulated ceiling, creating a cold spot right at the edge of the insulation — exactly where condensation and mould like to start. So the insulation is returned a short way along the adjoining wall (typically 300–500mm) and up into the ceiling void, to break that bridge properly.

They sit right in the zone most at risk of interstitial condensation once insulation goes on, so they get their own treatment. Depending on condition and access, that might mean:

• • Packing the void around the joist end with offcuts of flexible wood fibre insulation.

• • Parging (rendering) around it with lime mortar to seal out air — lime’s alkalinity also discourages rot and insect activity.

• • Applying a hydrophobic (water-repelling) treatment to the timber.

• • Where a joist end is already showing decay, trimming it back to sound timber and hanging it off the wall on a joist hanger with a small ventilated air gap behind.

Whichever method’s used, the aim is the same: keep the timber dry, and stop warm, moist room air from reaching it.

Full-thickness board doesn’t always fit without blocking light or losing period detailing. In those spots we typically use a thinner IWI board instead — it gives up a bit of thermal performance right at that narrow strip, but still meaningfully cuts the cold bridge without hacking off original detailing.

Board joints and all the junctions above get taped and sealed as part of the job, not as an afterthought. The aim is to stop air — and the moisture it’s carrying — sneaking in behind the insulation, since that’s what causes interstitial condensation even when the wall build-up itself is sound.

It varies room to room depending on what the survey and risk analysis tell us, but here’s a real-world example — the system Unity Lime supply for internal wall insulation with a lime plastered finish, working outward from the existing masonry:

• • Existing masonry wall, laid in lime mortar

• • Levelling coat where needed — Baumit NHL Thermo or RK38 lime plaster

• • 5mm Baumit RK70 N lime bonding coat

• • ISOLAIR MULTI wood fibre insulation board (Pavatex), fixed with adhesive and mechanical fixings

• • 6–8mm Baumit RK70 N lime plaster base coat with reinforcing mesh (Baumit Startex)

• • 3–4mm Baumit RK70 N lime plaster finish coat

Done properly, a system like this typically achieves a U-value of somewhere between 0.60 and 0.28 W/m²K depending on board thickness.

Strip-out back to the substrate, wall prep and a levelling coat if needed, a bonding coat, the wood fibre boards fitted, a lime plaster base coat with reinforcing mesh, a drying pause of roughly 7–10 days, a finish coat, several weeks of curing, and finally decorating with a breathable paint or limewash once it’s fully cured.

Every project’s different depending on room size, how many rooms, and time of year — lime plaster dries more slowly in cold or damp weather — but here’s a realistic range for a single average room:

Doing a whole house room by room means the project runs over several months rather than weeks — not because the crew is on site the whole time, but because each room’s plaster needs to cure before it can be finished and decorated.

• • Dust: strip-out kicks up dust, especially where old plaster comes off. We sheet rooms off, but some dust reaching adjoining areas is normal.

• • Room access: the room being worked on is generally out of action — cleared and inaccessible — for the length of that room’s work.

• • Fixtures and services: skirting, socket covers and sometimes radiators come off and go back on, which can mean short power-downs to that room’s circuit.

• • Smell: lime plaster has a distinctive, slightly damp, earthy smell while it cures — it fades as it dries out.

• • Heating and ventilation: curing rooms shouldn’t be heated or sealed up to force-dry them — you’ll need to leave a room a bit cooler and well-ventilated (not draughty) for a few weeks.

Usually, yes. Most homeowners find it easiest to do one or two rooms at a time so the rest of the house stays usable throughout.

Not until the plaster’s fully cured — commonly 4–6 weeks after the last coat. After that, stick to a breathable paint or limewash. Standard emulsion can seal the surface and trap moisture, which defeats the point of the whole system.