The Colder Wall: Why Internal Insulation Is Decided by Vapour, Not R-Value

The Warmth That Rots the Wall

A solid brick or stone wall — no cavity, no modern damp-proof course — loses heat fast, and the obvious fix is to line it on the inside with insulation. It is cheaper than an external system, it leaves the facade untouched, and it can be done one room at a time. Yet internal wall insulation, usually shortened to IWI, is the retrofit most likely to cause hidden damage, and the reason has almost nothing to do with how much insulation you choose. What decides success or failure is not the insulant's thermal value but the way the whole build-up handles water vapour.

Why the wall gets colder, not warmer

Insulation works by keeping heat on the room side. That is exactly the problem. Once a layer sits between the heated room and the masonry, the wall behind it no longer receives that warmth. Through a British or a northern-Italian winter the original brick or stone now runs colder and damper than it ever did before. The point in the build-up where warm indoor air would cool to its dew point — the temperature at which vapour turns back to liquid — shifts inward, out of the room and into the structure itself. Warm, humid household air migrating toward the cold masonry meets that dew point inside the wall and condenses where nobody can see it. This is interstitial condensation, and it is the defining hazard of insulating from the inside.

The joist ends nobody inspects

The most serious consequence is structural, and it stays invisible for years. In most period buildings the timber floor joists, and often the roof purlins, are built into pockets in the external walls. Before insulation those embedded timber ends stayed relatively warm and dry, because heat leaking through the wall dried them out. Line the inside with insulation and the same pocket turns colder and wetter. Timber is generally safe from decay below about twenty percent moisture content; push it above that and fungal rot begins, slowly, at the one point in the structure that carries the floor. Building-science monitoring of interior-insulated mass masonry has repeatedly flagged joist-end decay, and freeze-thaw damage to the outer brick, as the two failure modes that excess moisture causes. Neither announces itself until it is expensive.

Two honest strategies, and the fatal middle

There are only two coherent ways to build an internally insulated solid wall, and they are opposites.

The first is vapour-open. Here the wall is allowed to get damp and then dry again in both directions. Capillary-active woodfibre boards bedded in lime plaster, with no plastic anywhere in the layer, absorb and buffer moisture and release it back as conditions change, holding the interface below the level where mould and rot take hold. This is why conservation-minded retrofits favour woodfibre-and-lime systems on old breathing walls: they work with the wall's original moisture behaviour rather than against it.

The second is vapour-closed. Here a continuous, carefully sealed vapour control layer on the warm side of the insulation stops humid indoor air from ever reaching the cold masonry. Done perfectly, it is safe. The catch is that perfection is required: every socket, pipe and skirting junction is a potential puncture, and a single gap lets a jet of warm moist air into the coldest part of the wall, concentrating the very condensation the layer was meant to prevent.

The real failure case is the middle ground — the quick job. Insulated plasterboard stuck on with dabs of adhesive, the dot-and-dab method, leaves a ribbon of cold air trapped between board and wall. That void is a perfect condensing surface, and it is precisely what current best-practice guidance warns against. Whatever system you choose, the insulation must sit in full, unbroken contact with the masonry.

Thicker is not safer

There is a strong pull to maximise the U-value by piling on depth. The physics pushes the other way. Most of the achievable energy saving arrives within the first sixty millimetres or so of insulation; beyond that the benefit curve flattens. Meanwhile every added centimetre makes the masonry behind it colder still and lifts the condensation risk. Analyses of internally insulated masonry converge on a practical sweet spot of roughly sixty to eighty millimetres, chosen for moisture safety rather than for the best number on paper. Above that band a proper hygrothermal assessment — the dynamic simulation building physicists run, not a single steady-state sum — becomes essential rather than optional.

Detail the junctions, and ventilate

The corollary is that IWI is a system, not a product. Insulating the reveals of window openings, returning the insulation a little way around embedded joists, and sealing the perimeter turn cold bridges from condensation traps into continuous warm surfaces. And because internal insulation almost always tightens a leaky old house, the humidity that used to escape through draughts now has nowhere to go. Controlled ventilation — trickle vents at the very least, mechanical extract or heat-recovery ventilation ideally — belongs to the same moisture strategy, not to a separate afterthought.

Done with that discipline, internal wall insulation is transformative: a cold, unlovable period room becomes genuinely warm without anyone touching the facade. Done as a weekend board-and-dab job, it quietly manufactures the damp it was bought to cure. The wall does not care about the R-value on the label. It cares where the water goes.