How do I prepare my walls for IWI? | Can I Just Ask? – Ep.39 

The single most important principle when preparing a wall for internal wall insulation is achieving continuous contact between the back face of the insulation and the wall surface — no voids, no bridged gaps, no variable depths that leave sections unsupported. On a smooth, even wall, preparation can be minimal and the boards can be fixed directly. On undulating masonry — as is typical in older stone or brick buildings — a levelling coat is needed first to create a flat, consistent substrate. The priority before any of that, however, is assessing what’s already there rather than stripping it back out of habit.

The most common instinct when starting a retrofit is to take everything back to bare masonry — and it’s often the wrong call. Existing plaster, if it’s sound and well bonded, can frequently be retained. Stripping it off, sending it to landfill, and replacing it with a clay levelling plaster is costly, potentially damaging to the masonry, and from an embodied carbon perspective, largely pointless if the existing material was doing the same job. Cement render on external faces should similarly not be removed as a default assumption — in many cases it’s providing useful weather protection. The decision on what to keep and what to remove should be informed by hygrothermal modelling of the specific wall, not by a blanket approach. Where existing plaster does need to come off — typically thick coats of bonding or skim gypsum, or situations where years of layered paints and wallpapers make the existing material’s behaviour on the cold side of the insulation too uncertain to rely on — replacing with a known, well-characterised clay plaster gives a reliable, modellable substrate.

One step that applies regardless of what’s retained or removed is a fungicidal wash over the substrate before the insulation goes on. After years of use, mould and fungal spores are present on virtually every wall surface even where they’re not visible to the naked eye. Once insulation is installed, that surface is effectively buried on the cold side of the build-up — in conditions where mould is more likely to be sustained than suppressed. Treating the surface before it’s covered over is a straightforward precaution that removes a risk that would otherwise be sealed in permanently.